A pituitary metastasis, an adenoma and potential hypophysitis: A case report of tumour to tumour metastasis in the pituitary.

Tumour to tumour metastasis is a rare event, especially in the pituitary. Metastases to pituitary adenomas most commonly occurs in late stage disease, commonly presenting with visual field defects and adenohypophyseal dysfunction. The most frequent primary cancers are lung, breast and renal carcinoma which deposit most commonly in prolactinomas, somatotropinomas, gonadotropinomas. In nearly 40% of cases, sellar symptoms are the harbinger to the diagnosis of primary malignancy. The abnormal vascularity and growth promoting microenvironment of pituitary adenomas may encourage metastatic seeding and proliferation of these "collision tumours". Here, we present a case of a breast carcinoma metastasis to a pituitary null-cell adenoma in the setting of immunotherapy. Infundibular thickening in the setting of immunotherapy is often ascribed to hypophysitis, but our case highlights that metastatic spread should be part of the differential diagnosis.