Leiomyosarcoma
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Fascicles of interlacing spindled cells.

Leiomyosarcoma is a rare malignancy of the oral and pharyngeal region.  It arises from smooth muscle cells, especially those found in blood vessel walls, and from undifferentiated mesenchymal cells.  Often, the tumor completely obliterates its origin in blood vessel walls. The epithelioid variant, called malignant leiomyoblastoma or epithelioid leiomyosarcoma, is most prevalent in the gastrointestinal and genitourinary tracts and has rarely been reported in oral or pharyngeal locations.

Leiomyosarcoma is typically present in the mouth as a painless, lobulated, fixed mass of the submucosal tissues in a middle-aged or older individual.  It is exceedingly rare in children and often has a rubbery or semi-firm consistency to palpation. Lesions are usually less than 2 cm. in diameter at diagnosis and are slow-growing, but secondary ulceration of the mucosal surface has been reported.

Leiomyosarcoma is composed of fascicles of interlacing spindle-shaped cells with abundant eosinophilic cytoplasm and moderately large, blunt-ended nuclei, often with mild atypia. Cellularity and cellular differentiation can vary considerably between tumors and between different areas of the same tumor. The well-differentiated lesion shows the spindled cells streaming or interweaving in fascicles in a fashion similar to that seen in leiomyoma. Nuclear palisading may be seen in several areas of the tumor, as may ischemic areas of stromal fibrosis and hyalinization.

Eosinophilic myofibrils are occasionally noted but are much more readily discerned with the Masson trichrome stain (bright pink) or the PTAH stain (deep blue-purple). Increased mitotic activity is commonly seen, as are hyperchromatic nuclei. The presence of mitoses is valuable for separating benign from malignant smooth muscle neoplasms, and numbers as low as two per 10 high power fields have been associated with metastasis.

Lesional cells in poorly differentiated leiomyosarcoma are less elongated, more fusiform or rounded, enlarged, and more pleomorphic. Hyperchromatism of nuclei is often pronounced and numerous normal and abnormal mitotic figures are scattered throughout the lesion. Focal areas may contain giant cells with multiple, pleomorphic, even bizarre nuclei. The stroma is typically sparse, but cellular streaming is usually far less regular than in the low grade lesions, although the fascicles may be as uniform as those of well-differentiated lesions. There occasionally are perinuclear vacuoles, presumably from dissolving glycogen. Hemorrhage, focal necrosis, increased vascularity and focal myxoid change are not uncommon features of poorly differentiated lesions.

Epithelioid leiomyosarcoma demonstrates numerous rounded epithelioid cells with either eosinophilic or clear cytoplasm . These cells seldom display obvious myoblastic differentiation and are easily demonstrated with the PAS (periodic acid-Schiff)-diastase reaction; electron microscopy will usually show the classic features of leiomyoblasts.⁴¹³ Pleomorphism may be minimal or extreme and mitotic activity is often the key to determining whether or not a smooth muscle tumor is malignant: lesions with 5 or more mitotic figures per 10 high-power fields should definitely be considered malignant, but those with a lesser number are also probable sarcomas.

As with other leiomyosarcomas, the epithelioid variant has glycogen granules demonstrated with PAS staining, and the cells appear bright-red with the Masson’s trichrome staining of intracellular myofibrils. Longitudinal striations of myofibrils may be seen within lesional cells with the PTAH stain and reticulin staining of well-differentiated lesions will demonstrate a delicate meshwork of reticulin fibers surrounding individual tumor cells (or clusters of tumor cells, in the case of epithelioid lesions). Well differentiated lesions are likely to be immunoreactive for desmin, alpha smooth muscle actin and muscle-specific actin (Table 4).

Radical surgery is the treatment of choice for leiomyosarcoma, with adjunctive chemotherapy or radiotherapy used occasionally. The prognosis is poor, with numerous recurrences and distant metastases.  The exact location of the tumor, especially relating to the surgeon’s ability to adequately remove it, appears to be almost as important in determining the prognosis as tumor size at diagnosis. Overall five-year survival is approximately 35-50%.

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