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Yen-Lin Huang



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    EP1.09 - Pathology (ID 199)

    • Event: WCLC 2019
    • Type: E-Poster Viewing in the Exhibit Hall
    • Track: Pathology
    • Presentations: 1
    • Now Available
    • Moderators:
    • Coordinates: 9/08/2019, 08:00 - 18:00, Exhibit Hall
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      EP1.09-10 - A Diagnostic Pitfall in Posterior Mediastinal Tumor: Expression of CD117 in Atypical Ewing Sarcoma Masquerading as Classic Seminoma (Now Available) (ID 981)

      08:00 - 18:00  |  Presenting Author(s): Yen-Lin Huang

      • Abstract
      • Slides

      Background

      Ewing sarcoma is a rare malignancy and occurs most commonly in metaphyseal-diaphyseal portion of long bones in children or young adults. Atypical Ewing sarcoma refers to unusual alterations at cell morphology and immunoprofile associated with atypical clinical presentation mimicking carcinomas, lymphomas, or other sarcomas, but still harbors EWSR1 rearrangement. We recently encountered a case of atypical Ewing sarcoma involving the posterior mediastinum, which was initially misdiagnosed as seminoma based in part on CD117 expression by immunohistochemistry.

      Method

      This 11-year-old boy without any systemic disease has been suffered from intermittent chest pain for months. CT scan showed a huge left thoracic tumor (7 cm, Fig 1A), centralized in the posterior-medial portion of mediastinum with adjacent rib destruction. The radiological findings were compatible with malignant neurogenic tumor or germ cell tumor. Microscopically, it showed tumor cell nests with oval nuclei, conspicuous nucleoli and abundant clear cytoplasm (Fig 1B). Immunohistochemically, the tumor cells revealed strong positivity for CD117 (Fig 1C). Based on the above findings, seminoma was diagnosed. A carboplatin-based regimen for malignant germ cell tumor, JEB (carboplatin, etoposide and bleomycin), was given and he finished two cycles. The tumor had initially decreased in size (4.5 cm) for the first two months but progressed again (7 cm) in the third month. Therefore, VATS biopsy was performed.0409-v2-01.jpg

      Result

      Microscopically, it showed compact tumor cells in sheets, lobules or focal cord-like pattern with frequent necrosis, similar to the tumor cells seen in previous biopsy regarding both growth pattern and cell morphology. However, it also contained cells with high N/C ratio, finely dispersed chromatin (Fig 1D) and brisk mitotic activity (up to 24 mitoses per 10 HPFs) which were different from the cells of classic seminoma and prompted the consideration of atypical Ewing sarcoma and other small blue round tumors. After thorough immunohistochemical study, the tumor cells were positive for CD99 and NXK2.2 (Fig 1E), but also focally positive CD117. Fluorescent in situ hybridization demonstrated unbalanced EWSR1 rearrangement (Fig 1F), confirming the diagnosis of atypical Ewing sarcoma.

      Conclusion

      Atypical Ewing sarcoma could be present as a huge mediastinal mass with only partial rib destruction as well as strong CD117 expression, giving a false impression of primary mediastinal tumor and misdiagnosed as seminoma. Pathologists should always keep in mind this rare but lethal disease in the differential diagnosis. IHC of CD99 and NKX2.2 or even molecular test for EWSR1 rearrangement would help pathologists make the right diagnosis.

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