An immunoperoxidase study of S-100 protein in neoplastic cells in serous effusions. Use as a marker for melanoma.

S-100 protein has been demonstrated on histologic sections in a number of neural and nonneural tissues, including a variety of neoplasms. Since pleural or peritoneal effusions are frequently the initial presentation of cancer, a study was undertaken to determine if S-100 protein in exfoliated cancer cells could be used as a marker for melanoma. Cells in 36 serous fluids obtained from 32 patients were retrospectively examined for S-100 protein by the peroxidase-antiperoxidase technique. All samples had been previously studied as Papanicolaou-stained cytology specimens, and 25 samples had been studied by transmission electron microscopy. All benign effusions were negative for S-100 protein. Malignant effusions were negative except for some that contained malignant melanoma cells: two of five pigmented melanomas and both cases of amelanotic melanoma. This study indicates that S-100 protein in malignant cells is a useful marker for malignant melanomas, especially the amelanotic type.