This video details a complex soft tissue sarcoma case identified in the thigh of a young adult. The mass exhibited an unusual, highly sclerotic pattern characterized by dense collagen deposition and hyperchromatic spindle cells arranged in loose fascicles, cords, chains, and striking micro-alveolar or pseudovascular structures. After considering and ruling out vascular lesions (like Epithelioid Hemangioendothelioma), immunohistochemistry confirmed the diagnosis as Sclerosing Rhabdomyosarcoma, a rare variant currently grouped with Spindle Cell Rhabdomyosarcoma. The tumor stained positively for Desmin and MyoD1 (focally Myogenin). The speaker highlights the importance of recognizing this deceptive morphology, noting that molecular studies typically separate these tumors into two prognostic groups: infantile/pediatric cases associated with gene fusions (better prognosis) and adolescent/adult cases carrying MYOD1 mutations (worse prognosis).
Sclerosing Rhabdomyosarcoma: A Pathological Case Study
0:00 Initial Presentation: The case involves a mass in the thigh of a young adult, exhibiting an unusual pattern characterized by hyperchromatic spindle cells and abundant, dense sclerotic collagen.
0:45 Unique Morphology: Tumor cells are arranged in loose fascicles, cords, chains, and small nests that often mimic micro-alveolar or pseudovascular structures. Focal necrosis may be present (0:47).
1:39 Ruling Out Vascular Lesions: The pseudovascular structures lead to a differential diagnosis including vascular tumors (e.g., Epithelioid Hemangioendothelioma), but the tumor is negative for vascular markers (0:09).
2:25 Diagnosis Confirmed: The tumor stained positive for Desmin and MyoD1, and focally positive for Myogenin. Diagnosis: Sclerosing Rhabdomyosarcoma (SRMS), currently grouped with Spindle Cell Rhabdomyosarcoma (SCRMS).
3:00 Sarcoma Workup Importance: The speaker stresses that due to the non-typical appearance of rhabdomyosarcoma variants, Desmin should be included in the initial immunohistochemistry panel for unclassified sarcomas.
3:44 Morphology Spectrum: SRMS features sclerosis and pseudovascular channels. SCRMS (the other end of the spectrum) features fascicles of spindle cells, often mimicking Leiomyosarcoma or Fibrosarcoma (herringbone pattern).
4:08 Clinical Context: These variants should be considered in the differential, especially in children, and are notably common in the paratesticular region in kids (5:16).
Infantile/Pediatric: Associated with gene fusions (e.g., VGL2, SRF, NCOA2). Generally have a relatively good prognosis (4:50).
Adolescent/Adult: Associated with MYOD1 mutations (not translocations). Tend to have a worse prognosis (5:07).
5:25 Key Immunostains: While Desmin and Myogenin are standard, MyoD1 is often the preferred and more consistently positive stain for the Spindle Cell/Sclerosing Rhabdomyosarcoma group, as Myogenin is frequently only focally positive (5:48).
6:20 Differential Challenge: The extremely dense sclerotic collagen can occasionally mimic osteoid, complicating the differential with tumors like osteosarcoma.