Pleomorphic Leiomyosarcoma Presenting as a Upper Back Epidermal Inclusion Cyst.

Leiomyosarcoma is a common sarcoma of both organs and soft tissues; however, large intradermal tumors are extremely rare. Presented is a pleomorphic leiomyosarcoma in a 64-year-old man, initially considered to be a ruptured epidermal inclusion cyst. The patient had a mildly tender, enlarging soft-tissue mass with a central pore on his right upper back. Incomplete extirpation showed a 5 × 5 cm heterogeneous, predominantly pleomorphic sarcoma with areas of fascicular and storiform spindled cells infiltrating the subcutaneous soft tissue to the underlying fascia and extending upward into the middle and upper dermis with prominent extension into pilosebaceous units. There were small foci with myxoid stroma and large areas of necrosis. CD31 demonstrated thin-walled curvilinear vessels throughout the tumor. The first desmin immunohistochemical stain near areas with myxoid stroma was negative but smooth muscle actin positive. However, desmin positivity was strong and diffuse in the spindled and more pleomorphic areas on 2 additional tissue sections. No rhabdomyoblasts or striated muscle fibers were seen. A diagnosis of pleomorphic leiomyosarcoma was rendered. This case highlights a unique clinical and histological presentation of a leiomyosarcoma initially mistaken to be a ruptured epidermal inclusion cyst, and the need to sometimes apply ancillary immunohistochemical studies to sections from more than one tissue block to accurately differentiate heterogeneous sarcomas with similar histologic features.