Life-Threatening Radiation Necrosis After Complete Response to Pembrolizumab in a Patient With a Metastatic Silent PIT1 Pituitary Neuroendocrine Tumor (PitNET)

: Metastatic pituitary neuroendocrine tumors (metPitNETs) are rare neoplasms with limited therapeutic options. Temozolomide is the first-line therapy, but primary or secondary resistance frequently occurs. Immune checkpoint inhibitors (ICIs) are emerging as a promising second-line option; however, clinical experience remains limited. We report the long-term follow-up of a 62-year-old male patient who received pembrolizumab (PBZ) treatment for a silent metPitNET derived from the PIT1 lineage after multiple surgical and radiation therapies and temozolomide failure. PBZ was proposed based on extensive PD-L1 expression by tumor cells. A remarkable clinical, radiologic, and metabolic response was soon observed, progressively leading to complete disease remission after 21 months of treatment, with moderate immune-related adverse events. However, an unexpected rapid neurological deterioration occurred, due to the progression of a pseudotumoral temporal radionecrosis surrounded by an impressive vasogenic oedema, requiring emergency neurosurgery 7 weeks after PBZ withdrawal. The temporal mass had progressively developed on a previous small temporal metastasis treated through stereotactic radiosurgery, the corresponding area was hypometabolic at 18FDG PET-CT imaging, and histopathologic examination confirmed extensive radionecrosis and the absence of residual tumor cells. This is the first documented complete response to ICI in a PIT1-derived metPitNET. However, this remarkable response was complicated by the severe evolution of a brain radionecrosis, probably favoured by long-term PBZ. This case underscores the need for multidisciplinary expertise to differentiate treatment effects from neoplastic progression and to carefully follow-up the patients for potentially severe late treatment-related complications. It also questions the optimal duration of treatment in responsive cases.