Widespread invasion of a giant prolactinoma via valveless venous channels: illustrative case.

backgroundApproximately 35% of pituitary adenomas that require surgical intervention are invasive, often extending into the cavernous sinus (CS) via dural wall defects. Given the connections between the CS and multiple valveless venous plexuses in the orbit, face, and clivus, hematological venous spread is plausible, but the pathway by which these tumors locally invade remains uncertain. OBSERVATIONS: A 26-year-old man with right eye proptosis who presented for trauma was incidentally found to have an extensive pituitary mass. The lesion demonstrated invasion into both CSs, the right orbit, the bilateral retroclival space, and the right jugular foramen, with radiographic extension into a dilated right superior ophthalmic vein. Laboratory workup revealed a markedly elevated prolactin level of 18,178 ng/mL (normal: 2.10-17.70 ng/mL), consistent with a giant prolactinoma. He was treated effectively with cabergoline and without surgery. LESSONS: To the authors' knowledge, this is the first documented case illustrating extensive venous-mediated spread of a pituitary adenoma through valveless venous networks involving the CS, orbital, and clival regions, and it provides a plausible mechanism for the enhanced invasive nature of prolactin-secreting tumors. This case underscores the need for thorough imaging and endocrine evaluation before considering the surgical management of suspected invasive pituitary lesions. https://thejns.org/doi/10.3171/CASE25891.