Embryonal Rhabdomyosarcoma : Desmin
Immunohistochemistry of Rhabdomyosarcoma (RMS): In the past, the main immunohistochemical markers used to confirm the diagnosis of RMS included desmin, muscle-specific actin (HHF-35), and myoglobin. These markers had problems with either sensitivity or specificity. For. e.g. both desmin and muscle-specific actin are highly sensitive markers of RMS but lack specificity. Myoglobin is highly specific for skeletal muscle tumors but lacks sensitivity. These markers have been largely replaced by myoregulatory proteins such as MyoD1 and myogenin which show both high degree of sensitivity as well as specificity. The next few slides discuss individual markers. DESMIN: Desmin is an intermediate filament protein that is associated with both smooth muscle and skeletal muscle differentiation. Myofibroblasts usually don't express desmin. In the skeletal muscle, desmin is found in the Z bands between myofibrils where it functions as a binding material for the contractile proteins. Desmin is considered to be one of the most sensitive markers of rhabdomyosarcoma (RMS), including most of the poorly-differentiated ones. However, it is not entirely specific. It cannot distinguish between RMS and leiomyosarcoma. It can also be positive in non-myogenic tumors, including Ewing sarcoma, neuroblastoma, desmoplastic small round cell tumor, the blastemal component of Wilms tumor, giant cell tumor of tendon sheath, ossifying fibromyxoid tumor of soft parts, and malignant mesothelioma. Mesenchymal chondrosarcomas commonly express desmin along with focal positivity for myogenin and MyoD1. Angiomatoid fibrous histiocytomas are positive for desmin, CD68, and EMA; however, they lack other muscle markers. Uncommonly, melanomas and schwannomas can be desmin-positive. Therefore, given desmin's lack of specificity, additional markers such as myoregulatory proteins myogenin and MyoD1 are required for accurate diagnosis of tumors with skeletal muscle differentiation.