A case report of CIC‐rearranged undifferentiated small round cell sarcoma in the cerebrum

CIC‐rearranged undifferentiated small round cell sarcoma (CIC‐rearranged USRCS) is a recently established type of Ewing‐like small round cell sarcomas, characterized by CIC gene rearrangement, most commonly CIC‐DUX4 fusion. This report presents the second case of CIC‐rearranged USRCS arising primarily in the cerebrum. A 64‐year‐old otherwise healthy woman presented with a 1 × 1 cm sized hemorrhagic subcortical tumor in the left temporo‐parietal lobe. The tumor repeatedly recurred, and the patient underwent three surgeries, chemotherapy with doxorubicin and ifosfamide, and radiotherapy, as well as gamma knife surgery. Systemic examination revealed no other extracranial masses. Imprint cytology revealed small to moderate‐sized round‐to‐ovoid tumor cells with mild pleomorphism and variations in size and shape. The nuclei contained finely granular chromatin, and some had easily‐recognizable nucleoli. The tumor exhibited a mainly cytoplasmic pattern of CD99 immunostaining, rather than a diffuse membranous pattern. The tumor also exhibited diffuse positivity for calretinin and p16, as well as partial positivity for WT1 (nuclear and cytoplasmic staining pattern) and D2‐40. FISH assessment showed CIC split signals. In conclusion, CIC‐rearranged USRCSs can occur primarily in the cerebrum. It would be impossible to diagnose them through cytology alone, but cytology would be useful to rule out other small round cell brain tumors including gliomas, lymphomas, carcinomas, and germinoma. Immunohistochemical analysis including tests for CD99, calretinin, and WT1 would help to suggest CIC‐rearranged USRCSs and distinguish them from Ewing sarcomas. Additionally, immunohistochemistry for p16 might be useful in the diagnosis. Diagn. Cytopathol. 2016;44:828–832. © 2016 Wiley Periodicals, Inc.