This video presents a detailed pathology case discussion concerning a high-grade cutaneous malignancy in a 75-year-old man presenting with an ulcerated scalp nodule. Initially, the tumor was considered an Atypical Fibroxanthoma (AFX) or Pleomorphic Dermal Sarcoma due to its pleomorphic nature and CD10 positivity. However, the sheets of cells displayed a highly specific "rhabdoid" cytology, characterized by crescent-shaped, eccentric nuclei displaced by dense, homogeneous pink cytoplasm. Driven by this morphology, additional immunohistochemical analysis revealed strong, diffuse positivity for Desmin and nuclear expression of Myogenin (Myf4). This confirmed the diagnosis of primary cutaneous Epithelioid Rhabdomyosarcoma. The case serves as an important teaching point regarding the differential diagnosis of high-grade pleomorphic cutaneous sarcomas, underscoring the necessity of using broad screening markers, including Desmin, to identify this extremely rare but highly aggressive tumor, which led to rapid metastasis in this patient.
Pathology Case Discussion: Epithelioid Rhabdomyosarcoma
0:00 Patient Presentation: The case involves a 75-year-old man with an ulcerated erythematous nodule on the scalp. Initial staining showed CD10 positivity.
0:55 Cytology: The tumor cells display a unique appearance, described as having "crescent-shaped" nuclei pushed to the periphery by a large blob of dense, homogeneous pink (eosinophilic) cytoplasm.
2:47 Morphology Terminology: This cellular pattern is described as having rhabdoid or plasmacytoid morphology, indicating an eccentric nucleus displaced by cytoplasmic inclusions.
3:08 Initial Differential Diagnosis: The tumor was initially considered to be in the Atypical Fibroxanthoma (AFX) / Pleomorphic Dermal Sarcoma (PDS) spectrum, especially due to the CD10 expression. Other common sarcoma mimics (keratin-positive, S100-positive) were ruled out early.
5:12 Key Differential (Plasmacytoma): The speaker stresses the importance of considering plasma cell myeloma (Plasmacytoma) in the differential for rhabdoid/plasmacytoid morphology, noting that cutaneous myelomas can look very pleomorphic and often do not express CD45.
6:32 Desmin Positivity: The tumor demonstrated blazing, strong, diffuse positivity for Desmin. This finding indicated intermediate filament (Desmin) buildup, confirming a myogenic lineage.
7:43 Confirmatory Stains: To confirm skeletal muscle differentiation, Myogenin (Myf4) was positive with strong nuclear expression. MyoD1 was also positive, albeit focal, which is common.
8:39 Final Diagnosis: Based on the morphology and immunohistochemistry, the final diagnosis was Epithelioid Rhabdomyosarcoma (a subtype often associated with rhabdoid features).
8:49 Clinical Significance: Rhabdomyosarcoma is extremely rare as a primary cutaneous tumor, especially in older adults, but it retains a highly malignant potential.
9:53 Prognosis: This case was highly aggressive; the patient developed multiple lung metastases within six months of diagnosis.
9:28 Key Takeaway (Screening Panel): The speaker recommends including Desmin and ERG/CD31 (vascular marker) in the initial immunohistochemical screening panel for high-grade pleomorphic spindle cell tumors in the skin (AFX differential) to avoid missing rare but aggressive entities like rhabdomyosarcoma and solid angiosarcoma.
10:28 Desmin Caveat: While focal Desmin expression might be seen in AFX/PDS due to non-specific myofibroblastic differentiation, rhabdomyosarcomas typically show strong, diffuse staining.