[Spindle cell rhabdomyosarcoma in adults: a new entity in the spectrum of malignant mesenchymal tumors of soft tissues].

Rhabdomyosarcomas (RMS) are rare soft tissue sarcomas showing a skeletal muscle differentiation. Histologically, embryonal, alveolar and pleomorphic subtypes are distinguished. RMS represent one of the most frequent sarcomas in children and adolescents, and in this age group embryonal and alveolar subtypes predominate. RMS in adults is extremely rare, and the pleomorphic subtype is most frequently seen. Spindle cell and sclerosing RMS in adults are very rare and, unlike the morphologically comparable tumors in children and adolescents, clinically aggressive neoplasms, which need to be distinguished from spindle cell malignant melanoma, spindle cell sarcomatoid carcinoma and from a number of spindle cell sarcomas of varying lines of differentiation. Spindle cell and sclerosing RMS in adults arise predominantly in the head and neck region and on the extremities, and are composed of atypical spindled cells and scattered rhabdomyoblasts. Since spindle cell RMS may contain areas of lower cellularity with abundant sclerosing stroma, and sclerosing RMS may show focal areas of increased cellularity, it is most likely that both forms represent a morphological spectrum of a single RMS variant.