Tumor-to-tumor metastasis of colorectal adenocarcinoma within a gonadotroph pituitary adenoma

Pituitary metastases are uncommon, and most cases originate from breast or lung cancer. Tumor-to-tumor metastasis, in which one neoplasm metastasizes into another, is particularly rare in the sellar region. We report a 66-year-old man with metastatic sigmoid colon adenocarcinoma treated with modified FOLFOX6 (5-fluorouracil, L-leucovorin, and oxaliplatin) plus panitumumab; extracranial disease was controlled, whereas a sellar mass initially suspected as a pituitary adenoma slowly enlarged over 2 years. Endocrine testing showed anterior hypopituitarism without arginine vasopressin deficiency. Endoscopic transsphenoidal surgery was performed for continued enlargement and diagnostic uncertainty. Histopathology demonstrated a gonadotroph pituitary adenoma infiltrated by colorectal adenocarcinoma, with extensive intermingling of the 2 components rather than discrete nests, favoring tumor-to-tumor metastasis over a collision tumor. The metastatic component consisted of viable tumor cells without necrosis or convincing treatment effect despite prolonged chemotherapy. This case underscores that metastatic tumor within a pituitary adenoma may enlarge despite favorable systemic therapy outcomes. Metastasis should therefore remain a key differential diagnosis for a sellar mass radiologically suggestive of a pituitary adenoma.