Pseudosarcomatous lesions of the soft tissues reported as sarcoma during a 6-year period (1958-1963).

Pseudosarcomatous lesion of the soft tissues is a term used in the present study for various soft tissue lesions and tumours easily clinically or histologically, or both, misinterpreted as sarcoma. Eighty-one cases, that is to say 10 per cent of all tumours classified and reported to the Swedish Cancer Registry as malignant soft tissue tumours during the 6-year period studied (1958-1963), were reclassified as pseudosarcomatous lesions of the soft tissues. Forty-seven cases were classified as pseudoarcomatous proliferative lesions of the soft tissue with or without bone formation; 38 cases of nodular fasciitis, 1 of proliferative fasciitis and 8 of proliferative myositis. In 3 of these cases there were mixed forms of proliferative fasciitis and proliferative myositis with areas compatible with the diagnosis of nodular fasciitis evident in all cases. Twenty-two cases of atypical fibroxanthomas of the skin were next in frequency, followed by 7 ancient neurilemmomas, 2 spindle cell lipomas, 1 pseudomalignant osseous tumour of the soft tissues, 1 pigmented villonodular synovitis and 1 juvenile xanthogranuloma. An attempt is made to explain the reasons for these erroneous diagnoses of sarcoma and it is stressed that for these lesions the conventional histological criteria for malignancy are not valid. The awareness and knowledge of the existence of these particular entities are therefore considered mandatory for an accurate diagnosis.