Atypical intradermal smooth muscle neoplasms (formerly cutaneous leiomyosarcomas): case series, immunohistochemical profile and review of the literature.

Atypical intradermal smooth muscle neoplasms (AISMN, formerly known as cutaneous leiomyosarcomas) are uncommon neoplasms, which seem to be remarkable for their excellent prognosis in contrast to their deeper counterparts. The rarity of AISMN has posed a challenge for characterizing the morphologic spectrum, immunohistochemical staining pattern, and behavior. In this study we evaluated the histologic and immunohistochemical features of 20 cases of AISMN. Clinical follow-up was available on 19 out of 20 patients and ranged from 1 to 124 months with an average of 35 months and a median of 20 months with a male predominance (male to female ratio was 2.3:1). Our data show a wide variation in differentiation and atypical features. Among these, the presence of mitotic figures is diagnostically valuable in rendering the final diagnosis. A broad panel of immunohistochemical stains revealed that smooth muscle actin and muscle specific actin, when used in combination, identified smooth muscle differentiation in 100% of the cases. With some caveats, CD34, S100, and CK 5/6 were helpful in ruling out other important cutaneous spindle cell neoplasms. Significantly, loss of phosphatase and tensin homolog (PTEN) staining was seen in the majority of our cases (80%), supporting a role for PTEN loss in the etiology of these lesions. Logistic regression analysis revealed that positive margin status was helpful for predicting recurrence (100% sensitivity and 94% specificity). We conclude that AISMN can have significant morphologic variation and overlap with other spindle cell neoplasms of the skin and that a limited panel of key immunohistochemical stains should be used to distinguish this lesion. The different surgical measures such as wide excision versus Mohs procedure showed a similar clinical outcome. Although the significance of frequent PTEN loss supports a molecular mechanism of tumor genesis, the diagnostic utility of the stain remains to be determined.