RARE-06. Expanding the clinical and molecular spectrum of pituitary blastoma

Pituitary blastomas (PitB) are rare DICER1-associated tumors that occur exclusively in children < 2 years of age. They are locally aggressive tumors that are driven by a combination of germline and somatic alterations involving DICER1. Here, we report two patients with pituitary neoplasms that expand the clinical and molecular spectrum of PitB. Patient 1 presented at 10 months with diabetes insipidus and was initially diagnosed with pituitary ependymoblastoma. She received debulking, chemotherapy and focal radiation with complete response achieved, but unfortunately died at the age of 8 years due to cerebral edema. Patient 2 was a survivor of infant leukemia who was treated with chemotherapy and then further chemotherapy with cranial irradiation at relapse. The patient was then diagnosed at the age of 8 years with pituitary CNS-PNET, which was treated with craniospinal irradiation and chemotherapy, and had remained in remission for 6 years. Review of histology in both cases indicates presence of neuroendocrine lobules, primitive cells, and Rathke pouch-like glandular structures, with high Ki67, ACTH and PRAME positivity compatible with PitB. Next-generation sequencing revealed presence of two DICER1 mutations (germline frameshifting and somatic missense) in Patient 1, and one somatic missense DICER1 mutation plus loss of heterozygosity in Patient 2 (no germline alteration). Surprisingly, C19MC amplification was also detected in Patient 1. Methylation profiling confirms clustering among our samples and PitB references, but not ETMR references. MicroRNA array revealed decrease in mature microRNA expression and preferential down-regulation of 5p/3p species in tumor compared to control pituitary tissue. In all, PitBs may present with clinical and molecular characteristics not conforming with the classical descriptions. It might be prudent to consider sequencing for DICER1 alteration in pediatric pituitary tumors to facilitate diagnosis of this increasingly heterogeneous rare entity.