Chapter 121: Neoplasias and Hyperplasias of Muscular and Neural Origin

TUMORS OF SMOOTH MUSCLE

Cutaneous tumors with smooth muscle differentiation Table 121-1) can be categorized according to their microanatomical origin: (a) arrector pili muscles, (b) smooth muscle of genital skin, including scrotum (dartos muscle), vulva, nipple and areolar region, and (c) walls of blood vessels. Each of these sources of smooth muscle can give rise to benign superficial smooth muscle tumors (leiomyomas) as well as their malignant counterparts (leiomyosarcomas).¹ Anecdotally, smooth muscle also has been reported in organoid and blue nevi.² Of note, smooth muscle tumors arising in deep soft tissues (eg, abdominal cavity, pelvis, retroperitoneum), and visceral organs (including uterine tumors) often show overlapping morphologic and immunohistochemical features with cutaneous tumors, but form different clinicopathologic categories because biologic potential, clinical prognosis, and criteria for malignancy depend primarily on location.^3,4 Histologically, the constituent cells of smooth muscle lesions are spindled and contain abundant brightly eosinophilic cytoplasm as well as blunt-ended (cigar-shaped) nuclei. Smooth muscle antigens, including α-smooth muscle actin (α-SMA), muscle-specific actin, desmin, and h-caldesmon are readily identified by immunohistochemistry. Atypical intradermal smooth muscle neoplasms and leiomyosarcomas often at least focally express epithelial markers, and sporadic cases may express S-100 protein.^5,6 Figure 121-1 provides an overview of the diagnosis of cutaneous tumors with smooth muscle differentiation. The spectrum of cutaneous smooth muscle lesions includes leiomyomas, leiomyosarcomas, and smooth muscle hamartomas.