Abstract

A healthy 31-year-old man presented to dermatology clinic for a several month history of a tender, draining lesion on his left anterior knee. Physical examination revealed a 1.2 cm subcutaneous nodule overlying the tibia, favored to represent a benign cyst. Ultrasound imaging yielded a similar differential including an epidermoid or ganglion cyst. Histopathologic evaluation of the excision specimen demonstrated a dermal nodule composed of spindle cells arranged in short, intersecting fascicles with smooth muscle differentiation, confirmed by positive smooth muscle actin and caldesmon immunohistochemistry (IHC). Mildly increased cellularity and mild to moderate cytologic atypia were noted. On higher power, increased mitotic activity was appreciated (4 mitoses/10 high power fields) and correlated with an elevated Ki67 proliferation index of 15%. No atypical mitoses or tumor necrosis was identified. The tumor was classified as an atypical intradermal smooth muscle neoplasm (AISMN), and Mohs micrographic surgery was performed to ensure negative margins. This case demonstrated nodular architecture, a rare finding in AISMN which usually shows ill-defined borders. Other differential diagnoses, to include involvement by a subcutaneous or metastatic leiomyosarcoma, were considered. Additional workup with fumarate hydratase and 2-succinocysteine IHC was negative, and clinicopathologic correlation excluded the possibility of leiomyosarcoma. AISMN is a relatively new and uncommon diagnosis that is important to distinguish from leiomyoma and subcutaneous or metastatic leiomyosarcoma as it requires complete removal to avoid local recurrence and portends an excellent prognosis.