Papillary Tumour of the Pineal Region, in a Child With a Germline PTEN Pathogenic Variant, Arisen in the Fourth Ventricle.

We describe a unique case of papillary tumour of the pineal region (PTPR) arising in the fourth ventricle without any demonstrable anatomical continuity with the pineal region, in a 2-year-old male patient harbouring a germline PTEN pathogenic variant. This case arisen in the fourth ventricle shows epigenetic characteristics consistent with PTPR, suggesting the possibility that PTPR develops outside the pineal region. The tumour was composed of epithelioid-looking papillary structures (A, Haematoxylin and eosin [H&E]), intermingled with more densely cellular areas displaying perivascular pseudorosettes (B, H&E). Neoplastic cells resulted immunopositive for GFAP (C), CD56/NCAM (D) and focally positive for cytokeratin 18 (E) and showed dot-like immunostaining for EMA (F). DNA methylation profiling using the Bethesda Classifier version 2.0 demonstrated that the tumour clustered with PTPR-B in the UMAP analysis (G). CNV plot of the tumour showed the isolated loss of chromosome 10 (H).