Rhabdomyosarcoma (RMS), the most common soft tissue sarcoma of childhood, displays a variety of histologic patterns. Immunohistochemistry is used extensively to distinguish RMS from its mimics. Myogenin and MyoD1, myogenic transcriptional regulatory proteins expressed early in skeletal muscle differentiation, are considered sensitive and specific markers for RMS and are more specific than desmin and muscle-specific actin and more sensitive than myoglobin. Previous studies have focused on expression of myogenin and MyoD1 in small round cell tumors. This study assesses myogenin and MyoD1 in rhabdomyosarcoma subtypes and spindle cell tumors considered in the differential diagnosis of RMS. Formalin-fixed, paraffin-embedded archival tissue from 32 RMS, 107 non-RMS, and 11 benign skeletal muscle samples was stained for myogenin and MyoD1 with standard immunohistochemical techniques. Nuclear positivity was scored on a three-tiered scale. All RMSs expressed myogenin. Alveolar RMS (ARMS) showed strong nuclear staining, especially in tumor cells lining fibrous septae and perivascular regions. In cases with a subtle alveolar architecture on routinely stained sections, myogenin highlighted and enhanced visualization of the alveolar morphologic pattern. Embryonal RMSs (ERMSs) were more variable in myogenin staining pattern and intensity. No cases of nodular fasciitis, malignant fibrous histiocytoma, malignant peripheral nerve sheath tumor, inflammatory myofibroblastic tumor, myofibrosarcoma, leiomyoma, leiomyosarcoma, or alveolar soft part sarcoma stained for myogenin. Focal nuclear reactivity was seen in desmoid (2 of 10), infantile myofibromatosis (2 of 10), synovial sarcoma (1 of 10), and infantile fibrosarcoma (2 of 10). Non-neoplastic skeletal muscle fiber nuclei stained positively for myogenin in both tumor-associated samples (25 of 40) and benign skeletal muscle samples (5 of 11). Although all RMSs were immunoreactive for MyoD1, cytoplasmic and nonspecific background staining and reactivity of nonmyoid tissues hindered its practical utility in paraffin-embedded samples in this study. Although myogenin is a highly sensitive and specific marker for RMS, it is rarely seen in other spindle cell soft tissue tumors. As previously reported, ARMS stained more strongly than ERMS. In contrast to previous studies, rare non-RMS (7 of 107) displayed focal nuclear reactivity, and entrapped atrophic or regenerative skeletal muscle fibers also stained positively. Although these are potential pitfalls in the interpretation of myogenin, careful attention to morphology and other features, to the relative paucity of myogenin-positive nuclei in non-RMS, and to the presence of entrapped muscle fibers should prevent incorrect interpretation. Because the extent of myogenin expression in RMS is much greater than in non-RMS, it is a very useful marker when interpreted in the context of other clinicopathologic data.
