Paratesticular rhabdomyoma: a morphologically distinct sclerosing variant

Extracardiac rhabdomyomas, which currently are classified into fetal, adult, and genital types, are rare. We have identified 7 cases of a distinct morphologic variant of rhabdomyoma that affects mainly young men in the paratesticular region, seen in consultation between 2001 and 2011. The 7 male patients were adults (median age 24 y) and presented with tumors in paratesticular soft tissue (4 left-sided, 3 right-sided). Grossly, the median tumor size was 4.5 cm (range, 2.0 to 12 cm), and lesions were well circumscribed with a uniform tan-white cut surface. Microscopically, these rhabdomyomas were characterized by bundles of large well-differentiated skeletal muscle cells with copious eosinophilic cytoplasm that were variably round, polygonal, and occasionally strap shaped. The tumor cells were set in a dense hyalinized collagenous stroma, often with adjacent prominent lymphoplasmacytic aggregates. Tumor cells had round, occasionally vesicular, nuclei (sometimes binucleate or multinucleate) with small or inconspicuous nucleoli. All tumors lacked nuclear atypia and necrosis. Mitotic activity was virtually absent, although 1 tumor showed a count of 1 per 50 HPF. All tumors were diffusely positive for desmin, 4/4 were diffusely positive for fast myosin, and 1/1 examined was positive for myf-4. All patients were treated by local excision (5 with positive margins). Four patients with known follow-up data had no evidence of tumor recurrence or disease progression (median follow-up time 8.5 mo). The clinical course as determined thus far is benign, similar to other types of rhabdomyoma. However, this rare paratesticular subset of rhabdomyomas appears to be morphologically distinct from rhabdomyomas at other locations and appears to represent a separate variant.