Expression of protein gene product 9.5 in epithelioid and conventional malignant peripheral nerve sheath tumors

Context.-Due to the frequent lack of S100 protein expression in malignant p eripheral nerve sheath tumors (MPNSTs), especially the epithelioid variant, these tumors are difficult to diagnose without the aid of electron microsc opy or a clinical history of neurofibromatosis. Methods.-Protein gene product 9.5 (PGP9.5), a broad neural marker, is expre ssed in nerve fibers and neurons of both the peripheral and central nervous systems. We compared its expression to that of S100 protein in 16 cases of MPNST. As controls, 6 monophasic synovial sarcomas, 9 leiomyosarcomas, and 5 dermatofibrosarcoma protuberans were included. Results.-Expression of PGP9.5 was seen in 15 MPNSTs, with 3 to 4+ positivit y in the majority of the cases. Ten cases, 2 epithelioid and 8 conventional MPNSTs, were reactive with PGP9.5, but were negative for S100 protein. Fiv e cases were immunoreactive for both S100 protein and PGP9.5. One case was negative for PGP9.5 but demonstrated focal S100 protein positivity. Express ion of PGP9.5 was seen in 4 of 6 synovial sarcomas, 3 of 9 leiomyosarcomas, and none of 5 dermatofibrosarcoma protuberans. Conclusion.-Although PGP9.5 is not a specific marker for MPNST, it is a mor e sensitive marker than S100 protein (94% vs 38%). When there is a lack of S100 protein expression and a broad panel of immunostains, such as cytokera tin, epithelial membrane antigen, and smooth muscle actin, yields only foca l or equivocal staining, PGP9.5 is a useful diagnostic adjunct in confirmin g the neural origin of a spindle cell sarcoma.