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Case Reports

Abstract

Atypical intradermal smooth muscle neoplasms (AISMN), formerly known as cutaneous leiomyosarcomas, are uncommon tumors. Unlike their deeper counterparts, AISMNs exhibit an excellent prognosis. Their rarity has made it challenging to characterize their morphologic spectrum, immunohistochemical staining pattern, and behavior. 

We present a rare case of AISMN in a 71-year-old female with a single small nodule on the lower eyelid. The lesion appears white-tan, slightly verrucous, and measures 1.3 x 1.3 x 0.5 cm. Histology reveals highly cellular spindle cells arranged in a fascicular pattern, confined to the dermis without extension into subcutaneous tissue. The cells exhibit spindle shape, plump cigar-shaped nuclei, and eosinophilic cytoplasm with pleomorphism, marked cytologic atypia, and hyperchromatic nuclei. Mitotic activity is observed at 15 per 10 high-power fields (HPF), including atypical mitotic figures. Tumor necrosis is not identified. Immunohistochemical staining demonstrates strong positivity for caldesmon, patchy positivity for desmin, and negativity for pancytokeratin, S100, SOX10, and Melan A. These findings are consistent with a smooth muscle neoplasm.  

According to the 5th edition of the WHO Classification of Skin Tumors, AISMN is the preferred terminology for smooth muscle tumors with cytologic atypia limited to the dermis. Clinicians and pathologists should recognize AISMN as a differential diagnosis due to its limited metastatic potential and excellent prognosis following complete excision.