This pathology case review focuses on a mass located in the thigh of a young adult, presenting with an unusual and challenging morphology. The tumor is characterized by hyperchromatic spindle cells arranged in cords, loose fascicles, or micro-alveolar/pseudovascular structures, embedded within abundant, dense sclerotic collagen. Initial differential diagnoses included vascular tumors, but subsequent immunostaining revealed positivity for Desmin, MyoD1, and focally for Myogenin, confirming the diagnosis of Sclerosing Rhabdomyosarcoma (SRS), which is currently grouped with Spindle Cell Rhabdomyosarcoma (SCRMS). The video details the critical differences in presentation and genetics for this sarcoma variant, noting that adult cases are often driven by MyoD1 mutations (associated with a worse prognosis), contrasting with infantile cases, which are fusion-driven and typically carry a better prognosis.
Sclerosing Rhabdomyosarcoma: A Pathological Case Review
0:08 Unusual Morphology: The mass, located in the thigh, exhibits an unusual pattern characterized by hyperchromatic spindle cells and abundant, dense sclerotic collagen.
0:21 Cellular Arrangement: Tumor cells are arranged in loose fascicles, cords, chains (elongated rows), and, in other areas, in small nests or micro-alveolar/pseudovascular structures, mimicking vessels.
0:47 Key Histological Features: The tumor showed focal necrosis and significant cellular atypia, although the hypocellular nature of the dense collagen made malignancy less obvious at low power.
1:51 Differential Diagnosis Exclusion: Vascular tumors (like Epithelial Hemangioendothelioma) were considered but ruled out, as all vascular markers were negative.
2:25 Immunostaining Results: The tumor was positive for Desmin and MyoD1, and focally positive for Myogenin.
2:35 Final Diagnosis: Sclerosing Rhabdomyosarcoma (SRS), a rare and atypical variant grouped with Spindle Cell Rhabdomyosarcoma (SCRMS). This diagnosis requires inclusion in the workup panel as its appearance deviates significantly from typical rhabdomyosarcoma.
3:37 SRS vs. SCRMS Features: Sclerosing forms feature dense collagen, cords, chains, and pseudovascular channels. Spindle cell forms (SCRMS) feature fascicles resembling leiomyosarcoma or the herringbone pattern of fibrosarcoma.
Infantile/Pediatric Form: Characterized by gene fusions (e.g., VGL2, SRF, NCOA2, TAF1). Tends to have a relatively good prognosis and is common in paratesticular sites.
Adolescent/Adult Form: Characterized by MyoD1 mutations (not fusions). Tends to have a worse prognosis. Common sites include the head/neck and extremities.
5:27 Immunostain Caveats: While Desmin and Myogenin are standard rhabdomyosarcoma markers, MyoD1 is often the preferred and better stain for the spindle cell/sclerosing variant, as Myogenin is frequently only focally positive.
6:20 Differential Challenge: The dense sclerotic collagen can be so extreme that it may be mistaken for osteoid production, further complicating the diagnosis.