Sclerosing rhabdomyosarcoma: a clinicopathologic and immunohistochemical study of five cases.

We report 5 cases of sclerosing rhabdomyosarcoma. The patients included 4 adults and 1 adolescent. In the 5 cases, 3 tumors occurred in the head and neck region and 2 in an extremity. Histologically, all 5 tumors were characterized by the presence of abundant extracellular hyaline matrix, mimicking osteoid or chondroid tissue. They were composed mostly of primitive small round cells that displayed diverse growth patterns. In 2 cases, focal areas suggestive of spindle cell rhabdomyosarcoma were present. Typical features of embryonal or alveolar rhabdomyosarcoma were not noted; however, rare strap rhabdomyoblasts were identified in 1 case. Immunohistochemically, all 5 cases showed diffuse immunoreactivity for MyoD1, with varied expression of myogenin, desmin, muscle-specific antigen, and α-smooth muscle actin. All patients underwent surgery, combined with adjuvant radiation therapy or chemotherapy. Of 4 cases with follow-up, recurrence was found in 2. Sclerosing rhabdomyosarcoma represents a special variant of rhabdomyosarcoma and is possibly related to embryonal rhabdomyosarcoma. Rhabdomyosarcoma is the most common malignant soft tissue tumor of childhood and adolescence.1 Rhabdomyosarcoma is relatively rare in adults older than 45 years.2 Based on the morphologic features and molecular analysis, the current World Health Organization classification categorizes rhabdomyosarcoma into 3 main subtypes: embryonal (encompassing the botryoid, spindle cell, and anaplastic variants), alveolar (including the solid variant), and pleomorphic.3 Although embryonal rhabdomyosarcoma and alveolar rhabdomyosarcoma typically affect children younger than 15 years, they can be occasionally encountered in adults.4 In comparison, pleomorphic rhabdomyosarcoma arises almost exclusively in adults,5 with very few cases reported in children.6 In 2000, Mentzel and Katenkamp7 reported a peculiar type of adult rhabdomyosarcoma characterized by prominent hyaline sclerosis and a pseudovascular growth pattern. They called it sclerosing, pseudovascular rhabdomyosarcoma. In 2002, Folpe et al8 described 4 additional cases with similar features, and they proposed to designate the lesion as sclerosing rhabdomyosarcoma. Because of the presence of heavily hyalinized collagenous matrix and occasional pseudovascular growth pattern, sclerosing rhabdomyosarcoma is frequently misdiagnosed as chondrosarcoma, osteosarcoma, or angiosarcoma at initial evaluation. The relationship between sclerosing rhabdomyosarcoma and the conventional rhabdomyosarcomas remains uncertain, although cytogenetic studies suggest a link with embryonal rhabdomyosarcoma.9 To increase the recognition of this rare entity, we describe 5 new cases of sclerosing rhabdomyosarcoma.