Rare intraparenchymal choroid plexus carcinoma resembling atypical teratoid/rhabdoid tumor diagnosed by immunostaining for INI1 protein.

The authors present the case of a rare extraventricular, intraparenchymal choroid plexus carcinoma (CPC). This 6-year-old girl presented to the emergency department with a 1-week history of headaches, nausea, and vomiting. Imaging studies revealed an intraaxial cystic and solid mass located in the right frontal lobe with central nodular enhancement and minimally enhancing cyst walls. Gross-total resection was accomplished via craniotomy without complications. The initial pathological diagnosis was atypical teratoid/rhabdoid tumor (AT/RT); however, immunostaining for INI1 protein (using the BAF47/SNF5 antibody) showed retention of nuclear staining in the tumor cells, resulting in a change in the diagnosis to CPC. There was no evidence of recurrence at the last follow-up 2.5 years after treatment, which supports the diagnosis of CPC over AT/RT. This case emphasizes the importance of immunostaining for INI1 protein for distinguishing CPC from AT/RT in cases with atypical or indeterminate features.