Immunophenotypic diagnosis of leiomyosarcomas and rhabdomyosarcomas with monoclonal antibodies to muscle-specific actin and desmin in formalin-fixed tissue.

Two fibrillary proteins, muscle-specific actin (MSA) and desmin, are found only in cells of smooth and skeletal muscle lineages. Among the monoclonal antibodies (MAbs) to these antigens which we have tested, we found several to be reactive in formalin-fixed, paraffin-embedded sections. This finding widened the possibility of using these MAbs in routine diagnostic surgical pathology for the immunodiagnosis of rhabdomyosarcomas (RMS) and leiomyosarcomas (LMS). We therefore conducted a comparative study of three such MAbs which are available commercially and which we applied to paraffin-embedded, formalin-fixed tissues from 15 patients with RMS and 19 patients with LMS. The case selection criteria included typical light-microscopic appearances as well as immunoreactivity with at least one of the MAbs. MSA was detected in all cases of RMS and LMS, whereas desmin was reactive in 12 of 13 RMS and 10 of the 19 LMS. (Desmin antigenicity was judged to be lost in two RMS, since the vascular smooth-muscle tissue present in the specimens failed to react with these antibodies.) In LMS, desmin tended to show focal positivity, whereas the MSA in the same specimens was diffusely positive. These results demonstrate the utility of MAbs for confirmation of the muscle lineage of LMS and RMS in formalin-fixed, paraffin-embedded tissue. The results also indicate that, with the MAbs tested, the antigenicity of MSA is preserved more consistently than that of desmin in formalin-fixed, paraffin-embedded tissue, and that MSA is a more sensitive marker for the detection of muscle differentiation than is desmin, especially in LMS.