Leiomyoma of deep soft tissue. Clinicopathologic analysis of a series.

Leiomyomas of deep soft tissue are extremely rare and, to our knowledge, have never been adequately characterized either histologically or clinically. We report 11 such cases, most often arising in deep subcutis or skeletal muscle. The lesions were equally distributed among males (six cases) and females (five cases); age at presentation ranged from 14 to 62 years (mean, 37 years; median, 35 years). The extremities represented the single most common site (five cases). Characteristic morphologic features included well-defined circumscription with a fibrous pseudocapsule, myxohyaline stromal degeneration, and intersecting fascicles of spindle cells with mostly uniform, round-ended, elongated nuclei and tapering, eosinophilic cytoplasm. Dystrophic calcification was identified in four cases; degenerative nuclear changes, similar to those in ancient schwannoma, were present in three cases. Two neoplasms showed multiple necrobiotic granuloma-like nodules resembling giant rosettes. Mitotic figures were found in only four cases and were scarce, never exceeding one mitotic figure per 50 high-power fields. Immunohistochemically, all cases showed desmin positivity. Clinical follow-up (median, 4.5 years), available for nine cases, revealed no evidence of local recurrence or distant metastasis. As with all smooth-muscle cell neoplasms, meticulous attention to the presence of significant mitotic activity and suppurative tumor necrosis is essential because they suggest potential malignant behavior.