Immunohistochemical study of rhabdomyosarcoma. Unexpected staining with S100 protein and cytokeratin.

The immunohistochemical study of 60 cases of rhabdomyosarcomas made it possible to test eight different antibodies currently used in tumour pathology: i.e., antisera to vimentin, desmin, myoglobin, cytokeratin, epithelial membrane antigen, S100 protein, neurofilaments, and leukocyte common antigen. Vimentin was found in 58 cases (97 per cent), desmin in 49 cases (82 per cent), myoglobin in 23 cases (38 per cent), S100 protein in 7 cases (12 per cent), and cytokeratin in 3 cases (5 per cent). Other markers were negative. S100 protein was present in large round tumour cells with abundant eosinophilic cytoplasm (round rhabdomyoblasts), whereas cytokeratin was present in small tumour cells similar to those observed in rhabdoid sarcoma. This unexpected staining should become common knowledge for the correct interpretation of the immunohistochemical study of small cell tumours in the young.