The Maxillofacial Center for Diagnostics & Research

In the Beginning

The first serious suggestions of an association between a benign oral mucosal lesion and the subsequent development of oral malignancy were reported in the mid-nineteenth century. Sir James Paget¹ of London wondered in 1851 about the cancer-producing potential of pipe smoker's palate or "leukokeratosis," and in 1870 he clearly implied that oral "ichthyosis" (white keratotic plaque) was a significant precursor to lingual carcinoma.¹ The latter association was independently advanced in 1877 by Schwimmer² of Budapest, who is usually credited with coining the term "leukoplakia" for white tongue changes seen prior to lingual cancer development in tertiary syphilis. Numerous papers, especially in the German-language literature, fleshed out the relationship between leukoplakia, tertiary syphilis and lingual carcinoma throughout the latter part of the nineteenth century.^3-8

By the time of the classic review by Goodale⁹ in 1900 leukoplakia of the mouth was well established as a precursor to lingual cancer, always based on its close physical association with that cancer and on the fact that it is often seen in the area before the occurrence of the cancer. Unfortunately, even well respected and knowledgeable authorities seemed to have a very poor understanding of the clinical presentations of the premalignancy, as illustrated by the fact that Bloodgood⁹⁹ in 1915 used a drawing of an obvious papilloma to represent leukoplakia (Figure 1).

At one time the term leukoplakia was used for all white keratotic lesions of the mouth and the lesion has had more than 75 names applied to it. It is a term that was also used, until recently, for similar lesions of the larynx, vagina, uterine cervix, and bladder. Today it is defined as a chronic white mucosal macule which cannot be scraped off, cannot be given another specific diagnostic name, and does not typically disappear with removal of known etiologic factors, excepting tobacco.^99,99

A red alteration of the vocal cords adjacent to a carcinoma, reported in 1853, may have been the first example of an erythematous plaque associated with malignancy of the head and neck region,¹⁰ but Queyrat¹¹ of Lebanon? is usually credited with the first description and use of the term "erythroplasia." In 1911 he used the term to describe a red macule of the glans penis in a syphilitic patient, but two French dermatologists, Fournier and Darier,⁹⁹ had already described a red "benign epithelioma" of the penis in 1893. Bowen⁹⁹ appears to be the first to apply the word "precancerous" to these erythematous skin changes. In 1924 Darier¹² first applied the term "erythroplasia of Queyrat" to a lesion of the oral mucosa, and by 1931 Nuguer99 was discussing oral examples of de maladie de Bowen.

Today a variety of oral precancers (Table 1) are rather successfully evaluated and managed as a routine facet of oral health care, despite residual or ongoing controversies of some significance. Ironically, one of the major advances in this field has been the simple recognition that a premalignancy is not guaranteed to eventually transform into cancer. Most, in fact, do so in only a small proportion of cases and a premalignant (precancerous) lesion is now defined as an identifiable, benign tissue or cellular alteration associated with a greater than normal risk of malignant transformation or "degeneration." Much of contemporary oral oncology research is focused on the identification of clinical, histopathologic and molecular biological parameters of oral precancers and early cancers which might aid in the recognition of those lesions with the highest risk of transformation.. Because of the potentially fatal consequences of mistakes relating to premalignant lesions, this work is extremely important, but the state of the art at present leaves much to be desired.

How common are oral precancers?

Prevalence rates

Since the 1950s a number of reports have indicated that at least the most common form of oral premalignancy, leukoplakia, is quite common indeed (Table 2), and that it represents one of the five most common chronic diseases affecting the oral mucosa..^99,99 More recent studies have been especially careful to eliminate frictional keratosis and smokeless tobacco keratosis from their collections of true leukoplakia, and so the prevalence has been reduced somewhat from 3-4% of an adult Western population to 1-2%.^99,99 Even at 1% of the adult population, however, it can be estimated that 1,800,000 Americans are afflicted with this premalignancy.

Incidence rates

No Western study has determined incidence for oral leukoplakia, although in India new lesions have been shown to occur in 0.8/1,000,000 persons annual, compared to an oral cancer incidence rate in the same population of 5/1,000,000. It is not known or assumed that all oral cancers in India arise from leukoplakic lesions, but a large proportion of them probably do so. In the U.S., Bouquot et al⁹⁹ attempted to determine the leukoplakia incidence in the predominantly white community in which the Mayo Clinic resides, but were unable to do so because data from local dentists could not be compiled. The cases diagnosed by physicians provided an annual incidence rate of ___/100,000 population, age adjusted to the U.S. white population, but this should be considered as only the very least possible incidence and the authors presume that the majority of leukoplakic lesions where not being recorded in medical records.

Smokeless tobacco keratosis has a much lower malignant transformation potential than leukoplakia, but in some populations where the habit of chewing tobacco is popular, lesions can be found in up to 9% of males.⁹⁹

Oral lichen planus, a questionable precancer, affects one of every 1,000 adults in Western populations; in certain subpopulations it is seen as often as once per 32 persons.^99,99

No data are available for oral erythroplakia, but incidence investigations of oral/pharyngeal carcinoma in situ, which represents the vast majority of erythroplakias, have determined that only six cases are diagnosed per one million persons each year.

The only other head and neck precancerous lesions for which epidemiologic data are available are laryngeal keratosis (essentially equivalent to leukoplakia) and laryngeal carcinoma in situ. These lesions are diagnosed annually in 5.8 and 0.4 persons per 100,000 population.

Lesions in Oral Cancer Patients

One would expect the that at least some of the oral precancerous lesions would remain intake until the time of formal diagnosis of an associated cancer. Their frequency, then, among the group of oral cancer patients is expected to be much higher. A variety of studies have shown that, in fact, leukoplakia is found adjacent to or near a very large proportion of oral cancers, in some studies as much as 100% of them (Table 4). One population-based investigation has determined that the figure is more like 30% among a group of patients which represent all oral cancer patients in an entire population..⁹⁹

Mashberg et al^99,99 have made a special effort to identify erythroplakic lesions adjacent to or near early oral cancers. According to their thorough review of 220 cancer patients, almost ____% have adjacent erythroplakia. When health care professionals have a high suspicion of oral white and red lesions, as many as 15% of all diagnosed oral cancers will be the preinvasive, carcinoma in situ type⁹⁹

Oral submucous fibrosis is a rare oral precancerous condition in Western cultures but it occurs in as many as four per 1,000 persons in rural communities of the Indian subcontinent, 11/1,000 among chewers of betel quid.⁹⁹

Lesions in Young Persons

Precancers in young persons are almost unheard of. Tomar et al⁹⁹ recently found smokeless tobacco keratosis in 1.5% of 17,027 U.S. teenagers (3% for boys, only 0.1% for girls). Other studies of smaller groups of young people have found somewhat different prevalence rates (Table 3). Prevalence obviously will vary with the popularity of the smokeless tobacco habit, but as a general rule, up to half of daily users will show at least a mild degree of keratosis. Development of keratosis is also influenced by the exact tobacco used, the duration of the habit, the frequency of chewing, and the length of time each day spent in chewing the tobacco.

It is not known what the malignant transformation rate is for smokeless tobacco keratosis or even whether or not the cancers developing in the mouths of snuff and chewing tobacco users arise from within a white keratosis or on mucosa adjacent to or distant from the keratosis.

Dysplasia in Leukoplakia

Before looking further into oral leukoplakia it should be emphasized that the diagnosis of leukoplakia is a diagnosis of exclusion which requires the clinician to be so well acquainted with all other white oral lesions as to be able to rule them out prior to using the term leukoplakia for a particular keratosis in a particular patient. It must also be emphasized that the term leukoplakia is a clinical one. Even though biopsy and microscopic evaluation is frequently required in order to identify dysplastic or malignant cells, the presence of such cells does not alter the clinical diagnosis, and leukoplakia is still leukoplakia with or without dysplastic epithelial cells. In retrospect it seems surprising that histology was once considered essential to the diagnosis of this disease in which only one of every four cases is ever biopsied.

Numerous studies

Progression to Malignancy

The concept of oral precancer now clearly assumes that only some of the premalignant lesions will actually transform (transform? degenerate?) into invasive carcinoma. A number of investigations, most reported during the 1960s and 1970s, have followed patients with clinically diagnosed leukoplakias. The ten largest of these studies are listed in Table 3, representing more than 3,300 individuals followed for up to 41 years each through periodic examinations or medical/dental records. Altogether this research represents a remarkable 30,616 person-years (sum of observations times for each individual followed) of total observation time, or to put it another way, 30,616 person-years at risk.

Progression to malignancy occurred in 4-20% of followed patients, some of whom had their lesions removed during the follow-up period. Taken as a whole, the 270 oral cancers developing in the patient cohorts represents an annualized transformation rate of just under 1% each year, that is, the risk of malignancy is one in a hundred during the course of a year. This is considerably higher that the 0.01% anticipated for a Western population.⁹⁹ At such a high annual transformation rate one would expect that should a person only live long enough a leukoplakic lesion would be guaranteed to become malignant.

While this may be the case, the data are skewed toward the early post-diagnosis experience because several of the investigations followed lesions for less than five years, and because even in the longest follow-up studies, the proportion of cases able to be evaluated diminishes with each passing year. Of the 248 patients, for example, followed by Pindborg et al,⁹⁹ fewer than 104 were followed for more than five years and only four were followed for the full ten year period. Likewise, of the 257 patients followed by Silverman et al,⁹⁹ only 118 were followed beyond five years and fewer than 70 were followed more than ten years.

Fortunately, a few of the follow-up investigations have provided enough appropriate data to shed light on the question of when are the most likely times to anticipate malignant transformation. Figure 4 illustrates the cumulative risk of transformation of the four studies with appropriate data and compares them with the cumulative risk of oral cancer development in the general U.S. population, which is relatively representative of European oral cancer incidence rates also. For the most part, the risk of cancer in leukoplakia is greatest during the first two years of its existence, but it definitely does continue throughout the life of the lesion. When one considered that a quarter to a third of oral leukoplakias disappear within several years, it seems logical to presume that the leukoplakias with the longest duration are those with the highest risk of cancer progression.

Case-control studies have strongly implicated smoked tobacco and, to a lesser extent, alcohol abuse in the etiology of oral cancer, but such studies have not further correlated white mucosal plaques with malignant transformation. It is not known, then, whether oral cancers in the smokers identified in epidemiologic studies arise from leukoplakias or from clinically normal mucosa. Various investigations have, however, found leukoplakia adjacent to a third (range: 10-100%) of oral squamous cell carcinomas.

Numerous follow-up studies have established malignant transformation rates for leukoplakia, ranging from 4-20% (excluding clinical subtypes), with an average of 4% overall (Table 2). These figures are somewhat deceptive because follow-up studies exclude those cases with invasive disease at the time of initial diagnosis. The few investigations which have examined the latter feature have found that 7% (range: 0.1%-40%) of oral leukoplakias will have at least one focus of squamous carcinoma in the initial biopsy.

There are so few investigations of the biological behavior of oral leukoplakia, and the lesion itself is so common, that it seems worthwhile to briefly review each in turn. Sturgis and lund⁹⁹ reported the first follow-up study of the disease in a Western population in 1934, after discovering that a remarkable 12% of 143 leukoplakic lesions became malignant during an average follow-up period of five years.

Twenty nine years later Pindborg et al⁹⁹ pointed out the microscopic features of cellular atypia which pointed toward increased risk of malignant transformation. He enhanced this data for a 1968 report of 214 patients followed for one to nine years (average: 3.7 years), finding that 4% went on to malignancy. His was the first report documenting all biological behaviors, including diminution of the lesions. He found, in fact, that 20% of oral leukoplakias disappear without surgical removal and another 18% decrease in size without therapy. Forty-five percent, however, increase in size and 3.3% eventually became malignant, usually within the first year, but also up to five years after diagnosis. Of the 21 cases completely excised at the time of biopsy, only two recurred, three and five years postoperatively.

Kramer⁹⁹ also emphasized the various histopathologic features of prognostic significance, going so far as to attempt a detailed computer analysis which, unfortunately, is little used today. In 1970 he also gave, more significantly, a year-by-year listing of the biological behavior of leukoplakias with and without epithelial dysplasia ⁹⁹

Roed-Peterson⁹⁹ of Copenhagen reported in 1971 on the follow-up of 331 patients with 422 oral leukoplakias. With an average observation time of 4.3 years, he found that nine carcinomas (3%) developed within leukoplakic lesions. Quite significantly, he also found that three cancers (1%) developed at oral sites other than those involved with leukoplakia. He reported, for the first time, a gender difference: 6% of the leukoplakias in females developed malignancy, while only 2% of the males did so. He also was the first the report that a leukoplakic lesion in a nonsmoker was much more dangerous than one in a smoker: 11% compared to only 2%, respectively. He mentioned that those leukoplakias interspersed with red lesions, i.e. erythroleukoplakias, were seven times more likely to become malignant than were those which were purely white lesions (9.1% vs. 1.3%). He was the first to point out that larger leukoplakias are at higher risk than smaller ones, with almost all cancer developing within lesions larger than 5.5 cm.² And finally, he pointed out the fact that leukoplakias of certain oral sites are more serious than those in other oral sites: 44% of his lesions which developed malignancy were located on the lateral border of the tongue, even though less than 2% of all leukoplakias were found at that site.

The latest of the dysplasia follow-up studies is the recently reported investigation by Toro and Tinoco of Venezuela.⁹⁹ On 427 cases of severe epithelial dysplasias and carcinoma in situ in 402 adults,

The typical cancer arising from a leukoplakic lesion is diagnosed two to four years after the leukoplakia is diagnosed, but transformation may occur within months or occur after decades.

Leukoplakia is more often associated with progression to cancer when its surface becomes thickened and rough or granular, when its surface becomes verruciform, or when red areas of minimal keratin production are interspersed amongst the background of thickened keratin (erythroleukoplakia, speckled leukoplakia). Figure 1, page 23, depicts these changes visually; appearances toward the right in the drawing are thought to be at higher risk than those toward the left.

Clinical Types of Oral Precancers

Erythroplakia

Erythroplakia is a clinical term for a chronic red mucosal macule which, like leukoplakia, cannot be given another specific diagnostic name and cannot be attributed to traumatic, vascular or inflammatory causes. Such lesions are less common than white precancers but very careful observation will reveal erythroplakia in association with a many early invasive oral carcinomas. Erythroplakia may also be associated with leukoplakia (erythroleukoplakia), and in mixed red and white lesions it is the red portion that is more worrisome than the white. Most cases of erythroplakia are diagnosed on the mucosa of the lateral and ventral tongue, the oral floor and the soft palate.

This lesion has been called "the dangerous oral mucosa" because it typically presents as carcinoma in situ, severe epithelial dysplasia or superficially invasive carcinoma under the microscope. In very high risk settings, such as oral floor lesions in heavy smokers and alcohol abusers, 80% of these red patches already may contain focal areas of microinvasive cancer at the time of initial biopsy.

Follow-up studies are not available for erythroplakia, but its usual microscopic counterpart, carcinoma in situ, has been shown to transform into invasive carcinoma in approximately one of every four cases (Table 3, page 22). This malignant transformation occurs despite the fact that carcinoma in situ lesions are routinely treated by conservative surgical removal or laser ablation. One head and neck site, the larynx, has several investigations which followed untreated carcinoma in situ lesions (Table 4, page 22). Without therapy this disease transforms into invasive carcinoma in 60-90% of the cases within 5-10 years after initial diagnosis.

Smokeless Tobacco Keratosis

Smokeless tobacco keratosis (snuff pouch, snuff dipper's lesion, tobacco pouch) is a chronic white or gray/translucent mucosal macule localized in areas of direct contact with smokeless tobacco. The lesion cannot be scraped off, disappears with cessation of the tobacco habit, and is poorly demarcated from surrounding mucosa. Typically there is a soft, velvety feel to the altered mucosa and further palpation of a tobacco chewer's cheek will usually reveal a distinct "pouch" caused by flaccidity in the chronically stretched muscles in the area of tobacco placement. As tobacco is not in the mouth during a clinical examination the usually stretched mucosa appears folded or fissured. Induration, ulceration and pain are not associated with this lesion but occasional inflammatory erythema may be noted.

It usually takes 5-10 years of tobacco habit for smokeless tobacco keratosis to become apparent, but it may be present after less than a year. Once it occurs, it typically remains unchanged indefinitely unless the daily smokeless tobacco contact time increases, in which case it will gradually become thickened to the point of appearing as a distinctly white, leathery or nodular plaque. In the latter case lesions may become clinically indistinguishable from leukoplakia.

The development of smokeless tobacco keratosis in users is very much dependent on the type of habit popular in a society. Snuff appears to produce more keratoses, for example, than chewing tobacco, and persons who keep their quid in one site are more prone to keratoses than those using multiple sites. Other factors leading to high risk of keratosis include the specific brand of tobacco used, an extended duration of the habit, an excessive daily contact-hours of tobacco on oral mucous membranes, an increased amount of tobacco consumed daily, and a deficiency of beta-keratin or vitamin A.

The malignant transformation potential of smokeless tobacco keratosis has not been determined, but the tobacco habit itself is said to carry a risk four times greater than normal mucosa, based on case-control studies of oral cancer patients. Investigations using large numbers of tobacco chewers have found few if any keratotic lesions with serious dysplasias, although older and smaller investigations have concluded that as many as 16% of biopsied cases show at least mildly dysplastic cells.

Some authorities have attempted to provide a clinical grading scale for smokeless tobacco keratoses according to intensity of whiteness, erythema and fissuring. Unfortunately, none of these grading systems has successfully correlated severe or high-grade lesions with an increased risk of malignancy.

Oral submucous fibrosis

This irreversible precancerous condition is strongly associated with the habit of chewing areca nuts. Affected users experience a burning sensation of the oral mucosa, occasional mucosal ulceration, a peculiar marble-like blanching of the mucosa, and palpable fibrous bands of the buccal mucosa, soft palate and lips. Leukoplakic lesions are commonly seen and oral carcinoma development has been shown to occur in 5% of users during a 15 year period of follow-up. Whether or not these cancers develop more from the leukoplakias than from the nonleukoplakic mucosa is as yet unknown, but presumably the leukoplakias behave in the same precancerous fashion as leukoplakias in persons without submucous fibrosis. It should be emphasized that the increased risk of oral cancer in betel quid chewers is only relevant to those who include tobacco in the quid; without the addition of tobacco the relative risk is insignificant, although all other features of the disease remain the same.

Proliferative verrucous leukoplakia. Proliferative verrucous leukoplakia (PVL) is a very special form of oral precancer. First described under that title in 1985, it is a white mucosal plaque or discoloration which virtually always develops nodular, papillary or verruciform surface projections and which gradually, sometimes rapidly, spreads laterally to encompass a large mucosal area. On an average, 2.6 mucosal sites are affected. Four of every five affected patients are female and the mean age at diagnosis is 62 years (range: 22-89 years). The usual site of female involvement is the buccal mucosa (63% of cases), while the tongue is most frequently affected in men (82% of cases).

Etiologic factors are elusive. Two-thirds of patients do not have a tobacco habit, but there are especially strong associations with human papillomavirus (89%) and candida albicans (50%), a feature which is discussed below in the microbiology section of this paper.

At initial diagnosis, almost half of PVL samples will demonstrate epithelial dysplasia, and few cases are spared this change eventually. More than 70% of affected patients will develop an oral carcinoma during the decade following PVL diagnosis. Thus far no better treatment has been identified than aggressive and frequent surgical interventions and very close follow-up.

Recurring oral melanotic macule

Oral melanoma is a rare cancer, but it has been estimated that one-third are preceded by an apparently innocuous mucosal melanosis. A recent example of recurring oral melanotic macule has been reported to have eventuated after seven years in melanoma of the affected mucosa. Not one of the six early biopsies demonstrated histopathologic characteristics of melanoma, they were classic examples of oral melanotic macule. DNA flow cytometry, however, showed obvious aneuploidy of cells in the early biopsies, as well as in the final melanoma. This suggests that histologically benign-appearing oral melanoses may actually be the radial growth phase of oral melanoma in some instances. It certainly adds a new perspective to melanotic mucosal discolorations.

Histopathologic Features of Oral Precancers

Experience has taught us that certain cellular and tissue alterations are associated with malignancy and premalignancy. Altered cells appear to be more primitive than normal and so these changes are presumed to be examples of immature or inappropriate differentiation, although pathologists typically refer to them as dysplasia or atypia. Animal models for oral carcinogenesis have reliably demonstrated that normal epithelium passes through stages of more and more severe dysplasia prior to the onset of invasive neoplasia. As a general rule, fewer than 20% of oral leukoplakias will demonstrate dysplasia, 8% will have severe dysplasia. On the other hand, erythroplakias show severe changes more than 90% of the time.

Cellular changes in dysplasia. Specific alterations of individual epithelial cells are important in the determination of epithelial dysplasia (Figure 2, page 23). Cells and nuclei take on a more primitive appearance, similar to those of basal cells with enlarged nuclei (nuclear hyperplasia), dark-staining nuclei (hyperchromatism), enlarged, often eosinophilic nucleoli (prominent nucleoli), and with an increased nuclear-to-cytoplasmic ratio. These cells also appear to be crowded more closely together than normal keratinocytes (increased cellular density). Such changes are not exclusive to carcinogenesis, as they may be seen in reactive epithelium or epithelium influenced by a variety of systemic alterations. Flow cytometry and image cytometry add significantly to the pathologist's ability to assess nuclear changes associated with eventual cancer development.

There is often an increase in mitotic activity in dysplastic epithelium, but this also is seen in many reactive lesions. Enlarged, tripolar or star-shaped mitotic figures (abnormal mitoses), however, are much more indicative of precancerous changes. Abnormal mitosis may also be defined as mitotic figures found in unusual locations above the basal cell layer.

A key alteration of dysplastic epithelial cells is variation in the shape of the cells and nuclei. This pleomorphism is unusual outside of cancers and precancers. Premature production of keratin below the surface layer is another important alteration, but it is much more commonly seen in oral carcinomas than in oral premalignancies. This dyskeratosis may be represented by individually keratinized cells or by tight concentric rings of flattened keratinocytes (epithelial pearls). The pathologist must be careful not to misinterpret a keratin-filled surface indentation cut tangentially as true intraepithelial keratosis. Not all dyskeratosis is related to malignancy or premalignancy. Individually keratinized cells, for example, are also characteristic of hereditary benign intraepithelial dyskeratosis (Witkop disease).

Cellular necrosis and loss of cellular cohesiveness (acantholysis) are major signs of poorly differentiated carcinoma but are extremely rare in the epithelial dysplasia of oral precancer. When present, these features must be distinguished from intercellular edema, intraepithelial inflammatory cells and degenerating cells with pyknotic nuclei and vacuolated cytoplasm. Virus-induced koilocytes may be seen but are not necessary to the diagnosis, nor does their appearance appear to be of prognostic significance.

When dysplasia is seen in epithelium which otherwise has the microscopic features of lichen planus (liquifactive degeneration of basal cells, saw-toothed rete processes, hyperkeratosis, subepithelial band of inflammatory/immune cells, etc.), the lichen planus features are essentially ignored and the lesion is graded according to the below-mentioned criteria for epithelial dysplasia, although the term lichenoid dysplasia may be applied to the case.

Tissue (morphological) changes

Many oral precancers show excess surface keratin (hyperkeratosis, hyperparakeratosis, hyperorthokeratosis) and most show hyperplasia of the spindle cell layer (acanthosis), but both changes are common to a number of mucosal lesions without a cancer transformation potential and neither is necessary to the diagnosis of dysplasia.

Smokeless tobacco keratosis is characterized by a somewhat unique intracellular vacuolization or "edema" of superficial layers, perhaps interspersed with streaks of parakeratinized cells. This change most likely results from a low-grade chemical burn from the alkaline tobacco used. It has been referred to as surface etching and has no implications one way or another relative to the risk of malignant transformation.

Basal cell hyperplasia, as discussed above, is of major importance to the diagnosis as well as to the grading of dysplasia. Other tissue changes, however, are important features of oral epithelial dysplasia, especially certain alterations of the rete processes.

Extremely elongated rete processes with minimal cellular atypia are of little concern, as they are characteristic of a variety of hyperplastic conditions, including papillomavirus infections, frictional keratosis, psoriasis and pseudoepitheliomatous hyperplasia. Rete processes with a bulbous enlargement of the lowermost region (drop-shaped rete processes), however, are worrisome regardless of their size, especially if secondary projections or nodules are seen to arise from the basal layer and branch at indifferent angles into the lamina propria and connective tissue papillae. There is no physiological explanation for secondary nodules extending laterally from a rete peg of the oral mucosa. To many, these alterations are ominous enough to alter the histopathologic grading of a lesion to a higher level.

Dysplastic epithelium may be atrophic as well as acanthotic and some experts believe that atrophic forms have a higher risk of malignant transformation. Atrophic epithelium often lacks rete processes and may be ulcerated, thereby mimicking a traumatic or inflammatory lesion with thin, regenerating epithelium creeping in from the margins. Regenerating epithelium, fortunately, often has granulation tissue beneath it to distinguish it from precancerous dysplasia in atrophic epithelium. The very nature of thin, atrophic epithelium presents a diagnostic grading dilemma because basilar hyperplasia very quickly extends to the surface. There are no standards for this situation but Speight et al (1996) have recommended that these be regarded as severe dysplasias. Leukoplakias of the vermilion border of the lower lip in actinic cheilitis (actinic cheilosis, farmer's lip, sailor's lip) are especially prone to these difficulties.

An alarming morphological alteration of dysplastic epithelium is loss of stratification (loss of polarization) due to an apparent inability to properly differentiate and mature from basal cells to prickle cells to flattened keratinocytes. Cells high in the epithelium have the same immature appearance as those in the basal layers. This feature is especially pronounced in severe epithelial dysplasia and carcinoma in situ.

The pathologist must pay particular attention to the appearance of the epithelial cells at the lateral surgical margins, and the presence or absence of dysplasia should be mentioned in the histopathologic description of the lesion. The presence of squamous metaplasia of the excretory ducts of the minor salivary glands should also be mentioned, especially when cellular atypia is evident. Many treatments for oral precancers remove or destroy only the most superficial portions of the submucosal tissues, thus leaving behind the salivary glands and their ducts.

Grading of dysplasia

Most pathologists grade oral epithelial dysplasia according to a combination of cellular and tissue changes. Low-grade lesions are considered to be less "severe" than high-grade lesions but this assessment is a subjective one and it is not unusual for different pathologists to place emphasis on different aspects of dysplastic alteration, thereby arriving at somewhat different diagnostic conclusions. Regardless of individual emphasis, however, certain oral precancer grading criteria are in general use:

• the cellular atypia or dysplasia is similar to that seen in squamous cell carcinoma;
• there is no evidence of invasion into underlying stroma (the diagnosis would then change to carcinoma)
• the epithelium with the greatest proportion of atypical cells has the greatest risk of being or becoming a carcinoma;

• the epithelium with the most extreme atypia of cells has the greatest risk of being or becoming a carcinoma;
• the final grading or diagnosis should be based on the most severely involved area of change, even if that area includes no more than a few rete processes.

Oral epithelial dysplasia is subdivided into three prognostically significant categories, mild, moderate and severe. Mild (grade I) dysplasia demonstrates proliferation of atypical or immature basal cells above the parabasal region but not extending beyond the lower third of the epithelium. Moderate (grade II) dysplasia demonstrates a similar proliferation into the middle one-third of the epithelial cross-section. The term severe (grade III) dysplasia is reserved for abnormal proliferation from the basal layer into the upper third of the epithelium. Carcinoma in situ, thought by some to be a premalignancy and by others to be an actual preinvasive malignancy, requires top-to-bottom change with undifferentiated, primitive cells from the basal layer to the topmost layer, although most authorities accept a thin layer of surface keratin with abrupt transition into the underlying atypical cells. The grading of dysplastic oral epithelium must also take into consideration the degree of cellular atypia, as described above, and those lesions with marked alteration are elevated into a higher grade level.

It is difficult to determine the amount of basal cell hyperplasia in oral precancers because of transverse sectioning of tissue samples and because of the natural undulation of the inferior margin of squamous epithelium. It may be helpful to make this determination from the top of the connective tissue papillae because of the great variability of rete process length in acanthomatous epithelium, but there is no established standard and in many cases the basal hyperplasia is confined to the rete processes themselves.

Correlation of dysplasia with eventual malignant transformation

While the microscopic features of dysplasia are now relatively well established, few investigations have actually followed lesions with specific dysplastic grades or changes in order to determine their natural history. This is, of course, made quite difficult by the following facts:

• the biopsy procedure itself has removed the cells upon which the diagnosis is based;
• the more severe dysplasias are typically removed or destroyed prior to follow-up;
• the grading of dysplasias is an extremely subjective pursuit and there is often poor correlation between pathologists, even between very experienced pathologists.

Keeping the above caveats in mind, as a general rule it seems to be accepted that the biological behavior of severe epithelial dysplasia and carcinoma in situ are identical, or so similar as to make the distinction mote. Investigations have found that 20-35% of severely dysplastic lesions develop carcinoma, which is similar to the figures for carcinoma in situ, as was previously mention (Table 3, page 22). At the opposite end of the spectrum, mild epithelial dysplasias so seldom eventuate in carcinoma, and are so similar to reactive epithelial changes, that few pathologists consider them a serious threat or recommend complete removal of the associated white lesion.

One of the great ironies of the world of oral precancers is the apparent fact that the correlation between the clinical appearance of leukoplakia and cancer transformation is relatively good while any microscopic changes less than severe are not particularly helpful in determining prognosis. Recent investigators have even concluded that they were better able to correlate the presence of papillomavirus with the clinical appearance of oral precancers than with the microscopic grade of dysplasia. Much is yet to be learned. Perhaps the addition of molecular biologic parameters to the mix will shortly provide better prognostic indicators.

Microbiology of Oral Precancers

Several microorganisms are associated with proliferative epithelial lesions. The 70+ types of human papillomaviruses (HPVs) are often suspected of playing an important role in the development or malignant transformation of oral precancers, in particular the high-risk subtypes HPV-16 and HPV-18, which are associated with cervical and upper aerodigestive tract carcinoma (up to 90% and 54% of cases, respectively). The case for an etiologic role for this virus is strengthened by the fact that HPV E6 protein is known to bind to and inactivate the p53 tumor suppressor gene, possibly allowing chromosomal instability and subsequent neoplastic growth. HPV-16 has also been shown by McCance and colleagues (1988) to produce obviously dysplastic epithelial cells in differentiating tissue cultures which are otherwise sterile. HPV-31, HPV-33 and HPV-35 have also been associated with oral precancers and cancers, although the association is not as strong as that for HPV-16/18.

High-risk HPVs are found in 15-42% of leukoplakias, in 50% of erythroplakias, in 50-100% of oral squamous cell carcinomas, and in up to 10% of normal oral mucosa. Verruciform leukoplakias are especially prone to contain viruses, with 89% of PVL specimens having recently demonstrated positivity for HPV 16 or HPV-18. There seems to be a distinct predilection for HPV-associated leukoplakias to occur on the buccal mucosa, while lesions of the ventral tongue and oral floor are much less likely to contain the virus. There also seems to be a confusing lack of a positive correlation between HPV presence and the severity of the histopathologic grade of epithelial dysplasia, a feature also of some studies of HPV in oral carcinoma. The male gender predilection seen in oral carcinomas with HPV has not yet been demonstrated in oral precancers, nor has HPV been associated with younger oral precancer patients, as it is with oral carcinoma patients.

The prognostic significance of HPV presence in oral precancers is yet to be determined by large follow-up investigations, but the 1996 study of 49 lesions (10 erythroplakias, 39 leukoplakias) followed by Nielsen et al for an average of 6.3 years found HPV in all three cases which eventuated in malignant transformation. Two of the transformed cases were initially erythroplakias and one was a verrucous leukoplakia. It may be significant that this Danish study, using four different sophisticated laboratory techniques, were unable to demonstrate HPV in any of its 20 control subjects. Survival from oral carcinoma does not appear to be associated with the presence or lack of HPV.

Herpes simplex virus (HSV), especially type 2 HSV, was once considered to be the cause of a large proportion of cancers of the uterine cervix, and it has been suggested that it plays a role in oral carcinoma. Epidemiologic evidence now suggests that it may be no more than a common companion infection in persons with HPV infections and that the latter virus plays a much more important carcinogenic role. Currently, the evidence to prove a causal relationship between HSV and oral precancers or cancers is insufficient, but continued research may eventually show a relationship.

Candida albicans is the other microorganism with a major association with oral precancers. It has been found in up to half of biopsies from verruciform/nodular leukoplakias and erythroleukoplakias. Investigations using fungal cultures instead of biopsy samples have found the microorganism in essentially all examined cases of verruciform or nodular leukoplakia. Some authors prefer to include this group in a separate leukoplakia subtype called candida leukoplakia, chronic hyperplastic candidiasis, or candida hyperplasia. There were early indications that three times as many candida leukoplakia cases developed carcinoma than leukoplakias without candida (30% and 10%, respectively), and case reports certainly associate chronic hyperplastic candidiasis with dysplastic cells and carcinoma development. More recent investigations, however, have found that the same proportion of cases go on to malignancy whether or not candida is present in the epithelium.

It may be that the fungus is merely growing in an altered epithelium, epithelium with a strong susceptibility to transform into carcinoma. It might also be that the fungus is producing the keratosis and dysplasia, perhaps in concert with tobacco smoke or another carcinogen. Laboratory experiments have shown that the yeast is capable of actually producing keratotic plaques on the tongues of rats, and certain strains of candida have been shown to produce carcinogenic nitrosamines. It has also been shown that at least some candida leukoplakias diminish in size or become less irregular in appearance when treated with antifungal therapy. Some oral pathologists are now recommending a course of medication prior to biopsy of a suspected candida leukoplakia in order to reduce the abnormal histopathologic appearances of epithelial cells in the biopsied tissue.

Molecular Biology of Oral Precancers

Investigations at the level of cellular biology continue to provide exciting and much needed insight into oral premalignancy. Tumor suppressor genes which function as negative regulators of cellular proliferation have received the most recent attention. When inactivated by deletion or mutation, unrestrained cellular growth ensues. The suppressor gene most frequently altered in carcinomas of the upper aerodigestive tract is the p53 gene, located on chromosome 17p, but doc-1 (deleted in oral cancer-1) and BRCA1 gene mutations have also been seen. Tissue samples are necessary for proper study of these genes, although serum levels for some of their associated proteins are now being evaluated.

Results from several studies suggest that the p53 gene in its normal state is capable or arresting the cell cycle after DNA damage to that cell. Mutated p53 genes not only loose this ability but tend to accumulate within the nucleus because they have a longer half-life than the normal or wild-type genes. The resulting p53 overexpression or mutation has been demonstrated more frequently (43%-93% of cases) in oral mucosal carcinoma cells than in any other human cancer, and its occurrence in oral dysplasias and microscopically normal mucosa adjacent to head and neck carcinomas suggest that its alteration is an event which occurs early in carcinogenesis. Some investigators have evaluated the next step in the process, i.e. the interaction of p53 proteins with various other cellular proteins and viral oncoproteins, for example, finding that 36% of oral carcinomas and 19%of oral dysplasias demonstrate complexes formed by the binding of p53 with heat shock proteins.

Alterations of the p53 state are thought to be prognostically significance in a number of human malignancies. Only a few precancer studies, however, have used cases from patients who eventually developed head and neck malignancy. These showed a correlation of p53-positive immunostaining with increasing severity of dysplasia (10% of control cases were positive, as were 50% of hyperkeratoses without dysplasia, 67% of low-grade dysplasias, 85% of high-grade lesions, and 89% of invasive carcinomas). It also appears that as the severity of epithelial dysplasia increases in precancers the proportion of immunoreactive cells increases. Interestingly, there seems to be no strong correlation between p53 positivity and markers for cell proliferation rates, such as Ki-67, at least for carcinoma in situ lesions. Less surprising is the fact that routine markers for normally differentiating oral epithelium, such as cytokeratin-4, are lacking in carcinoma in situ.

Proto-oncogenes are genes that normally promote cell growth. Alteration of the structure or function of a proto-oncogene converts it to an oncogene, which in turn produces an abnormal protein or oncoprotein. In cancers of the head and neck region, overexpression of the p21 ras oncoprotein has been observed more frequently than any other, but this has not been evaluated in precancers of that region, nor is there any apparent prognostic significance to its presence.

Growth factors necessary for normal growth and development may also play a role in oral cancer and precancer. Fibroblast grown factors (FGF) are widely distributed in normal and neoplastic tissues. They are involved in angiogenesis and wound healing, and have also been shown to be mitogenic for keratinocytes. Almost all oral carcinomas are immunoreactive to FGF and oral carcinoma cells in culture are capable of expressing FGF. Biopsy samples of oral dysplastic lesions have also demonstrated positive focal staining which becomes stronger with increasing immaturity or severity of the dysplastic cells. The significance these findings is yet to be determined but they suggest that overexpression in mucosal lesions might reflect malignant change.

Epidermal growth factor receptor (EGFR) is the protein of the proto-oncogene c-erb. Its expression has been correlated somewhat with an increased rate of recurrence in some cancers of the head and neck region, but study results vary and the final significance of EGFR has yet to be determined. Thus far, investigators have not evaluated this aspect of oral precancers.

The proliferative activity of a tumor may reflect its biological potential and can be measured in various ways, including the immunohistochemical demonstration of proliferation markers such as proliferating cell nuclear antigen (PCNA) or the nonhistone nuclear protein, Ki67 (MIB 1). Local recurrence has been associated with a decreased PCNA expression or increased Ki67 expression in head and neck cancers, but evaluation of these events in oral precancers has not yet been reported.

Flow cytometry is a readily available means by which to assess proliferative and tumor activity, especially aberrations such as DNA aneuploidy and nondiploidy. In laryngeal precancers, aneuploidy and nondiploidy is significantly more frequent in lesions which precede to laryngeal carcinoma. As with p53 mutation, DNA alterations are considered to be early changes of carcinogenesis, although one does not necessarily precede the other. Also, p53 immunoreactivity and DNA aberration show a high degree of complementarity, with almost all precancers proven to eventuate in invasive carcinoma displaying one or the other alteration. These two alterations appear to hold the greatest hope of becoming important and practical tests for the evaluation of highest-risk oral precancer, but at the present time neither is quite a feasible for routine practice.

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Epidemiology

Mehta FS, Pindborg JJ, Bhonsle RB, Sinor PN. Incidence of oral leukoplakia among 20,358 Indian villagers in a 7-year period. Brit J Cancer 1976; 33:549-554.

Bouquot JE, Gorlin RJ. Leukoplakia, lichen planus and other oral keratoses in 23,616 white Americans over the age of 35 years. Oral Surg Oral Med Oral Pathol 1986; 61:373-381.

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Clinical Characteristics & Follow-up Studies

Paget J. Cancer following ichthyosis of the tongue. Trans Clin Soc London 1870; 3:88.

Schwimmer E. Die idiopathischen Schleimhautplauques der Mundhohle (Leukoplakia buccalis). Arch Dermat Syph 1877; 9:611-570.

Sturgis SH, Lund CC. Leukoplakia buccalis and keratosis labialis. N Eng J Med 1934; 210:996-1006.

Pindborg JJ, Renstrup G, Poulsen HE, Silverman S. Jr. Studies in oral leukoplakias. V. Clinical and histological signs of malignancy. Acta Odont Scand 1963; 21:407-414.

Pindborg JJ, Jolst O, Renstrup G, Roed-Petersen B. Studies in oral leukoplakia. J Amer Dent Assoc 1968; 76:767-771.

Einhorn J, Wersall J. Incidence of oral carcinoma in patients with leukoplakia of the oral mucosa. Cancer 1967; 0:2189-2193.

Roed-Petersen, B. Cancer development in oral leukoplakia: follow-up of 331 patients. J Dent Res 1971; 50:711.

Mincer HH, Coleman SA, Hopkins KP. Observations on the clinical characteristics of oral lesions showing histologic epithelial dysplasia. Oral Surg Oral Med Oral Pathol 1972; 33: 389-399.

Shafer WG, Waldron CA. Erythroplakia of the oral cavity. Cancer 1975; 36:1021-1028.

Banoczy J, Csiba A. Occurrence of epithelial dysplasia in oral leukoplakia: analysis and follow-up study of 120 cases. Oral Surg Oral Med Oral Pathol 1976; 42:766-774.

Banoczy J. Follow-up studies in oral leukoplakia. J Max Fac Surg 1977; 5:69-75.

Pindborg JJ, Deftary DK, Mehta FS. A follow-up study of sixty-one oral dysplastic precancerous lesions in Indian villagers. Oral Surg Oral Med Oral Pathol 1977; 43: 383-390.

WHO Collaborating Centre for Oral Precancerous Lesions. Definitions of leukoplakia and related lesions: an aid to studies on oral precancers. Oral Surg Oral Med Oral Pathol 1978; 46:518-539.

Maerker R, Burkhardt A. Klinik oraler Leukoplakien und Prakanzerosen. Retrospektive Studie an 200 Patienten. Dtsch Z Mund Kiefer GesichtsChir 1978; 2:206-211.

Kramer IRH, El-Lablan N, Lee KW. The clinical features and risk of malignant transformation in sublingual keratosis. Br Dent J 1978; 144:171-180.Gupta PC, Mehta FS, Daftery DK et al. Incidence rates of oral cancer and natural history of oral precancerous lesions in a 10-year follow-up study of Indian villagers. Community Dent Oral Epidemiol 1980; 8:287-333.

Baric JM, Alman JE, et al. Influence of cigarette, pipe, and cigar smoking, removable partial dentures, and age on oral leukoplakia. Oral Surg Oral Med Oral Pathol 1982; 54:424-429.

Roed-Petersen B. Effect on oral leukoplakia of reducing or ceasing tobacco smoking. Acta Dermato-Venereol 1982; 62:164-167.

Silverman S, Gorsky M, Lozada F. Oral leukoplakia and malignant transformation. A follow-up study of 257 patients. Cancer 1984; 53:563-568.

Amagasa T, et al. A study of the clinical characteristics and treatment of oral carcinoma in situ. Oral Surg Oral Med Oral Pathol 1985; 60:50-55.

Murti PR, Bhonsle RB, Pindborg JJ, et al. Malignant transformation rate in oral submucous fibrosis over a 17-year period. Community Dent Oral Epidemiol 1985; 13:340-341.

Hansen LS, Olson JA, Silverman S. Proliferative verrucous leukoplakia. Oral Surg Oral Med Oral Pathol 1985; 60:285-298.

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Bouquot J, Weiland L, Ballard D, Kurland L. Leukoplakia of the mouth and pharynx in Rochester, MN, 1935-1984; incidence, clinical features and follow-up of 463 patients from a relatively unbiased patient pool. J Oral Path 1988; 17:436.

Bouquot JE, Weiland LH, Kurland LT. Leukoplakia and carcinoma in situ synchronously associated with invasive oral/pharyngeal carcinoma in Rochester, Minnesota, 1935-1984. Oral Surg 1988; 65:199-207.

Mashberg A, Samit AM. Early detection, diagnosis, and management of oral and oropharyngeal cancer. CA 1989; 39:67-88.

Bouquot JE, Gnepp DR. Laryngeal precancer--a review of the literature, commentary and comparison with oral leukoplakia. Head Neck 1991; 13: 488-497.

Lumerman H, Freedman P, Kerpel S. Oral epithelial dysplasia and the development of invasive squamous cell carcinoma. Oral Surg Oral Med Oral Pathol 1995; 79:321-329.

Zakrzewska JM, Lopes V, Speight P, Hopper C. Proliferative verrucous leukoplakia: a report of ten cases. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 1996; 82:396-401.

Murrah VA, Perez LM. Oral lichen planus: parameters affecting accurate diagnosis and effective management. Pract Perio Aesth Dent 1997; 9:613-622.

Rossie K, Guggenheimer J. Oral candidiasis: clinical manifestations, diagnosis, and treatment. Pract Perio Aesth Dent 1997; 9:635-642.

Silverman S Jr, Gorsky M. Proliferative verrucous leukoplakia: a follow-up study of 54 cases. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 1997; 84:154-157.

Histopathology (Microscopic Characteristics)

Shedd DP, Hukill PB, Kigerman MM, et al. A clinicopathologic study of oral carcinoma in situ. Am J Surg 1963; 106:791-796.

Kramer IRH, Lucas RB, El-Labban N, et al. A computer-aided study on the tissue changes in oral keratoses and lichen planus, and an analysis of case groupings by subjective and objective criteria. Br J Cancer 1970; 29:408-426.

Kramer IRH. Carcinoma in situ of the oral mucosa. Int Dent J 1973; 23:94-98.

Mincer H, Coleman SA, Hopkins KP. Observations on the clinical characteristics of oral lesions showing histologic epithelial dysplasia. Oral Surg Oral Med Oral Pathol 1972; 33: 389-392.

Shafer WG. Oral carcinoma in situ. Oral Surg Oral Med Oral Pathol 1975; 39:227-238.

Shafer WG, Waldron CA. Erythroplakia of the oral cavity. Cancer 1975; 36:1021-1028.

Waldron CA, Shafer WG. Leukoplakia revisited: a clinicopathologic study of 3256 oral leukoplakias. Cancer 1975; 36:1386-1392.

Kramer IRH. Basic histopathological features of oral premalignant lesions. Mackenzie IC, Dabelsteen E, Squier CA (editors). Oral premalignancy. Iowa City: University of Iowa Press, 1980.

Shear M, Pindborg JJ. Verrucous hyperplasia of the oral mucosa. Cancer 1980; 46:1855-1862.

Wright A, Shear M. Epithelial dysplasia immediately adjacent to oral squamous cell carcinoma. J Oral Pathol 1985; 14:559-564.

Abdel-Salam M, Mayall BH, Chew KL, et al. Prediction of malignant transformation in oral epithelial lesions by image cytometry. Cancer 1988; 62:1981-1987.

Kaugers GE, Burns JC, Gunsolley JC. Epithelial dysplasia of the oral cavity and lips. Cancer 1988; 62:2166-2170.

Crissman JD, Zarbo RJ. Dysplasia, in situ carcinoma, and progression to invasive squamous cell carcinoma of the upper aerodigestive tract. Am J Surg Pathol 1989; 13(suppl):5-16.

Kaugars GE, Mehailescu WL, Gunsolley JC. Smokeless tobacco use and oral epithelial dysplasia. Cancer 1989; 64:1527-1530.

Eisenberg E, Krutchkoff DJ. Lichenoid lesions of oral mucosa: diagnostic criteria and their importance in the alleged relationship to oral cancer. Oral Surg Oral Med Oral Pathol 1992; 73:699-704.

Regezi JA, Zarbo RJ, Regev E, et al. p53 protein expression in sequential biopsies of oral dysplasias and in in situ carcinomas. J Oral Pathol Med 1995; 24:189-22.

Speight PM, Farthing PM, Bouquot JE. The pathology of oral cancer and precancer. Curr Diag Path 1996; 3:165-177.

Microbiology

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Molecular Biology

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Murrah V. Human papillomavirus in oral leukoplakia. Proceedings, annual meeting, American Academy of Oral Pathology; Portland, Maine; May, 1994.

Xu X-C, Ro JY, Lee JS, et al. Differential expression of nuclear retinoid receptors in normal, premalignant, and malignant head and neck tissues. Cancer Res 1994; 54:3580-3587.

Regezi JA, Zarbo RJ, Regev E, et al. p53 protein expression in sequential biopsies of oral dysplasias and in in situ carcinomas. J Oral Pathol Med 1995; 24:189-22.

Ankathil R, Mathew A, Joseph F, Nair MK. Is oral cancer susceptibility inherited? Report of five oral cancer families. Oral Oncol, Eur J Cancer 1996; 32B:63-67.

Kaur J, Srivastava A, Ralhan R. p53-HSP70 complexes in oral dysplasia and cancer: potential prognostic implications. Oral Oncol, Eur J Cancer 1996; 32B:45-49.

Pillai KR, Remani P, Kannan S, et al. Lectin histochemistry of oral premalignant and malignant lesions: correlation of JFL and PNA binding pattern with tumour progression. Oral Oncol, Eur J Cancer 1996; 32B:32-37.

Kannan R, Bijur GN, Mallery SR, et al. Transforming growth factor-alpha overexpression in proliferative verrucous leukoplakia and oral squamous cell carcinoma: an immunohistochemical study. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 1996; 82:69-74.

Kaugers GA, Silverman S Jr, Lovas JGL et al. Use of antioxidant supplements in the treatment of human oral leukoplakia: review of the literature and current studies. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 1996; 81:5-14.

Kropveld A, van Mansfeld ADM, Nabben N, et al. Discordance of p53 status in matched primary tumours and metastases in head and neck squamous cell carcinoma patients. Oral Oncol, Eur J Cancer 1996; 32B:388-393.

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Table 1: Precancerous lesions of the oral, pharyngeal and laryngeal mucosa, clinical terms only (modified from

Speight PM, Farthing PM, Bouquot JE. Curr Diag Path 1996; 3:165-177).

                 # reverse smoking: smoking with the lit end of the cigarette in one's mouth

Table 2: Malignant transformation rates (%) of the largest leukoplakia follow-up studies from Europe and the United States, ranked by sample size (modified from Bouquot JE, Whitaker SB. Quint Internat 1994; 25:133-140).

        * includes 13 cases from the pharynx.
       ** weighed for different sample sizes; many studies include a disproportional sampling of high risk lesions; the overall malignant
             transformation rate is usually considered to be 4-6% when all lesions are assessed.

Table 3: Malignant transformation rates (%) for oral carcinoma in situ and/or severe epithelial dysplasia, ranked by sample size. Lesions appeared clinically as red, white or combined red and white macules, i.e. not all were pure erythroplakias. Cancers not arising from the site of the precancer are excluded (modified from Bouquot JE, Ephros H. Prac Perio Aesth Dent 1995; 7:59-68).

    # sum total of time which lesions were collectively followed (mean follow-up times are in parentheses)
    @ the only population-based study; represents middle-class whites
@@ includes only carcinoma in situ cases
    * assumes an average follow-up time of 3 years (range=1-8 years); data not available from the report
   ** statistically weighed for different follow-up time periods and sample sizes

Table 4: Malignant transformation rates (%) for laryngeal carcinoma in situ, as determined by all English language follow-up studies published to date, ranked by sample size (modified from Bouquot JE, Gnepp DR. Head Neck 1991; 13:488-497).

    * includes only cancers developing in the anatomic area of the previously recorded dysplastic lesions
  ** includes only untreated cases of carcinoma in situ and severe epithelial dysplasia
*** weighed for different sample sizes, excludes untreated cases
    # includes all epithelial dysplasias: mild, moderate and severe dysplasia, and carcinoma in situ
  ## includes only severe epithelial dysplasias and carcinomas in situ

Figure 1: Leukoplakia clinical subtypes. This composite drawing represents the various clinical appearances of oral leukoplakia, with anticipated underlying histopathologic changes. A lesion has increasing malignant transformation potential as its appearance approaches that toward the right in the drawing. (Used with permission from Neville BW, Damm DD, Allen CM, Bouquot JE. Oral and maxillofacial pathology. Philadelphia: Saunders; 1995:285).

Figure 2: Epithelial dysplasia, cellular changes. This artist's drawing shows the various cellular alterations usually associated with dysplastic cells of oral precancers. (Used with permission from Bouquot JE, Ephros H. Prac Perio Aesth Dent 1995; 7:59-68).