Atypical Intradermal Smooth Muscle Neoplasms: Clinicopathologic Analysis of 84 Cases and a Reappraisal of Cutaneous "Leiomyosarcoma"

Atypical or mitotically active dermal smooth muscle neoplasms are uncommon lesions, which are most often termed "cutaneous leiomyosarcoma," although preexisting-mostly small series suggest a low risk of aggressive behavior. To further investigate these tumors, 84 cases from consultation and institutional files were analyzed for pathologic and clinical characteristics. There was a striking male-to-female preponderance (4.3:1), with a mean age of 56 years (range, 6 to 82y). Nine patients had a history of malignancies (6 of the skin). Tumors measured 1.3 cm on average and were predominantly located on the trunk (32) and lower extremities (30). Histologically, all tumors were confined to the dermis or showed only very superficial, focal subcutaneous extension. The majority showed an infiltrative growth pattern with fascicles of atypical eosinophilic spindle cells ramifying between dermal collagen fibers. Primary tumors showed a mean mitotic rate of 4.7/10 high-power fields. By the Federation Nationale des Centres de Lutte Contre le Cancer grading system, 97% of primary tumors were grade I lesions, with only 3% showing necrosis. All tumors were immunopositive for smooth muscle actin; 98% expressed desmin, 90% caldesmon, and 45% pankeratin (usually focal). Follow-up in 52 cases (mean, 51 mo) showed no metastases or tumor-related deaths. Eighteen tumors showed local recurrence at a mean interval of 43 months; 12 of the recurrent lesions showed positive margins in the primary excision and 1 showed margins < 0.2 cm. Margin status was not available for the other 5 cases, which recurred locally. Recurrent tumors showed, on average, 13.7 mitoses/10 high-power fields. Of recurrences, 47% were grade I lesions, 35% were grade II, and 18% were grade III, and 28% showed necrosis. The primary excision of tumors, which later recurred, showed no difference in grade, presence of necrosis, or mitotic rate, compared with those that did not recur; there were no discernible clinical differences either. In summary, these tumors, when confined to the dermis or showing only minimal subcutaneous involvement, seem to carry no evident risk of metastasis; hence, the designation "sarcoma" is inappropriate. Margin status is the most important predictor of recurrence. On excision with clear margins, the risk of local recurrence is very low. Hence, we propose the term "atypical intradermal smooth muscle neoplasm" as being more appropriate.