Immunohistochemistry remains the current ancillary method of choice in the pathologic evaluation of small blue round-cell
tumors. In at least 20% of cases of
rhabdomyosarcoma (RMS), it is considered an essential factor in the final and/or differential diagnosis of the
malignancy. Newer immunostains (antimyogenin, MyoD1) generated against intranuclear myogenic
transcription factors offer pathologists the best hope for improving the sensitivity and specificity of RMS diagnosis. A large series of RMS (956) were studied consecutively from the intergroup
rhabdomyosarcoma study and children's oncology group files, along with multiple other malignant, benign or reactive lesions. A panel of
antibodies to muscle-related
antigens (
myogenin, MyoD1,
desmin, muscle-specific actin) was studied using
formalin-fixed,
paraffin-embedded tissue, an
avidin-
biotin/
peroxidase complex immunohistochemical technique,
antigen retrieval technique as appropriate, and automated immunostaining.
Myogenin and MyoD1 were equally sensitive (positive for 97% of RMS cases), with both also showing similar specificity (90% vs. 91% of cases) for the diagnosis of RMS.
Myogenin and MyoD1 staining were sometimes intact in areas of coagulative
tumor necrosis, but negated by B5 fixation. Isolated, rare benign
myogenin-positive nuclei were seen infrequently in reactive lymph nodes. Specifically, both
myogenin and MyoD1 had significantly greater extent of expression for alveolar RMS (ARMS) than embryonal RMS (ERMS) (both with P < 0.001). Similarly, both
myogenin (P = 0.001) and MyoD1 (P < 0.001) had significantly higher expression for ARMS than RMS, not otherwise specified (NOS). They were never expressed in undifferentiated
sarcomas; however, reactive or regenerative myocytes did show expression. Immunostains against intranuclear myogenic
transcription factors are, at present, the best available markers for confirming the diagnosis of RMS. Their differential expression in reactive myogenic lesions, variability in ARMS versus ERMS, and absence in undifferentiated
sarcomas suggest new biologic questions to be explored in future studies.