Fine-needle aspiration cytology of pseudosarcomatous lesions of soft tissue.

Pseudosarcomatous lesions are benign neoplasms of the musculoskeletal system that are likely to be misdiagnosed as malignant, based on clinical and histologic features. These include soft-tissue "tumors" considered reactive or reparative lesions such as nodular fasciitis and myositis ossificans. Also included in the "pseudosarcoma" category are benign neoplasms which show "pseudoanaplastic" cytologic atypia. The latter include lipoma, leiomyoma, angiomyolipoma, and benign peripheral nerve-sheath tumors. These neoplasms, particularly the reparative processes and the nerve sheath tumors, are increasingly being subjected to initial diagnosis by fine-needle aspiration cytology. Even by conventional cytology this group of lesions represents a well-known pitfall for the diagnostic pathologist. We review some cytologic features: repair-like change, cohesion of cellular fragments, and presence of "normal" elements in the aspirate, which may help the cytopathologist avoid misdiagnosis of these notoriously difficult entities.