This presentation details the pathological investigation of a rare soft tissue tumor in a 17-year-old female—a round blue cell tumor presenting as a slow-growing, reddish-blue nodule on the thigh that had been present for four years. Initial clinical suspicion leaned toward a vascular anomaly (hemangioma) due to imaging findings suggesting vascularity and necrotic spaces. Pathologically, the tumor exhibited sheets of monotonous round blue cells with scattered mitoses and focal necrosis, suggesting a diagnosis associated with gene fusions/translocations rather than random aneuploidy. The differential diagnosis included Ewing sarcoma and Angiomatoid Fibrous Histiocytoma (AFH). Immunohistochemistry showed strong, diffuse CD99 positivity, favoring Ewing sarcoma. Further molecular testing (FISH) confirmed the diagnosis as Ewing sarcoma, specifically the common $EWSR1-FLI1$ fusion, while ruling out the $EWSR1-NONO$ or $EWSR1-ATF1$ fusions associated with AFH. The speaker highlights the unusual presentation: primary cutaneous/superficial Ewing sarcoma, which is often more indolent than deep-seated cases, and the positive patient interaction following diagnosis.
Case Study: Diagnosing a Rare Superficial Round Blue Cell Tumor
0:00 Case Introduction: Discussion of a slow-growing, soft, reddish-blue nodule on a 17-year-old female's thigh, present for at least four years. Initial imaging suggested a hemangioma or vascular malformation.
0:30 Histopathology: Examination reveals sheets of uniform, monotonous round blue cells with scattered mitoses and areas of pseudovascular spaces (blood-filled spaces due to tumor disintegration) and focal necrosis.
1:28 Malignancy Clue: The lack of significant nuclear pleomorphism suggests the tumor is likely associated with a gene translocation or fusion.
3:04 Differential Diagnosis: The pathologist considered Ewing sarcoma and Angiomatoid Fibrous Histiocytoma (AFH) due to the histology and blood-filled spaces.
3:51 CD99 Immunostain: Strong, diffuse (4+) membranous staining for CD99, highly sensitive for Ewing sarcoma, points toward that diagnosis.
5:22 EMA/Desmin Stains: EMA was positive, but Desmin was negative. The pathologist remained torn between Ewing sarcoma and AFH.
5:48 Morphological Conflict: The four-year history and skin location are unusual for rapidly growing Ewing sarcoma, favoring AFH morphology, though the overall morphology leaned toward Ewing.
6:02 Molecular Testing Approach: The pathologist consulted with a colleague and opted for FISH testing for $EWS/FLI1$, which is rearranged in both Ewing sarcoma and AFH.
6:55 Confirmed Diagnosis: FISH was positive for rearrangement. Subsequent testing confirmed the $EWS-FLI1$ fusion (most common in Ewing sarcoma) and ruled out the $EWS-NONO$ and $EWS-ATF1$ fusions associated with AFH.
7:16 Cutaneous Ewing Sarcoma: The case is noted as an excellent example of primary cutaneous/superficial Ewing sarcoma.
7:24 Positive Patient Outcome: The patient and mother met with the pathologist; the patient underwent a shorter course of chemotherapy and is currently alive and well with no recurrence.
8:24 Mimics to Remember: The speaker warns that cutaneous Ewing sarcoma can sometimes be mistaken for Merkel cell carcinoma due to potential positivity for keratin (25% of cases) and neuroendocrine markers.