This presentation details the pathological diagnosis of a rare case involving a soft tissue round blue cell tumor in a 17-year-old female, which had been misdiagnosed as a vascular malformation for four years. The tumor presented as a soft red-blue nodule and exhibited sheets of monotonous round blue cells with scattered mitoses and areas of necrosis. The pathologist initially considered Ewing sarcoma or Angiomatoid Fibrous Histiocytoma (AFH) due to the morphology, including pseudovascular spaces. Immunohistochemistry showed strong, diffuse CD99 positivity (suggesting Ewing sarcoma) but EMA positivity and desmin negativity (aligning partly with AFH). Molecular testing (FISH) confirmed the presence of the $EWS/FLI1$ fusion, the hallmark of Ewing sarcoma, while ruling out the $EWSR1/CREB1$ and $EWSR1/ATF1$ fusions characteristic of AFH. The case is highlighted as an unusual, superficial/cutaneous presentation of Ewing sarcoma, which in this context behaved indolently, leading to successful treatment with a short course of chemotherapy and a positive patient outcome. The speaker emphasizes the importance of integrating morphology, immunohistochemistry, and molecular testing to distinguish among uniform round blue cell tumors, noting that some can mimic other skin cancers like Merkel cell carcinoma due to aberrant marker expression.
Summary of the Pathological Case Study: Superficial Ewing Sarcoma
0:00 Presentation: Case review of a soft tissue round blue cell tumor excised from a 17-year-old female; a nodule present for at least four years, previously suspected to be a vascular malformation or hemangioma.
0:01:13 Morphology: Gross pathology showed sheets of monotonous round blue cells with scattered mitoses and areas of discohesion/necrosis.
0:02:36 Diagnostic Clue: The uniformity (lack of pleomorphism) strongly suggested a tumor associated with a gene translocation or fusion.
0:03:04 Differential Diagnosis: Initial considerations included Ewing sarcoma and Angiomatoid Fibrous Histiocytoma (AFH), the latter suggested by the presence of blood-filled spaces.
0:03:51 CD99 Immunohistochemistry: Results were strongly and diffusely positive (4+), highly sensitive for Ewing sarcoma, prompting further molecular investigation.
0:05:22 EMA/Desmin Staining: EMA was positive, Desmin was negative, which complicated the distinction, as AFH can co-express EMA and Desmin.
0:06:02 Molecular Approach: FISH testing was performed, which rearranges in both Ewing sarcoma and AFH.
0:06:55 Final Diagnosis: Testing confirmed the $EWS$ fused with $FLI1$ (the most common Ewing sarcoma fusion), while ruling out the fusions seen in AFH ($EWSR1/CREB1$ and $EWSR1/ATF1$).
0:07:16 Case Significance: This is presented as an excellent example of a primary cutaneous or superficial Ewing sarcoma, which often behaves more indolently than deep-seated tumors.
0:07:24 Patient Interaction: The pathologist met the patient and her mother to discuss the pathology, highlighting positive interaction between pathologists and rare cancer patients.
0:08:09 Outcome: The patient was treated with a shorter course of chemotherapy and, at the time of recording, was alive and well with no recurrence.
0:08:51 Mimicry: Pathologist warns that this tumor type can aberrantly express markers like Keratin and neuroendocrine markers, potentially mimicking Merkel cell carcinoma in older adults.