Pituitary neuroendocrine tumor with transsellar anastomosis concomitant with internal carotid artery agenesis: illustrative case.

backgroundEndoscopic transsphenoidal surgery is the standard treatment for pituitary neuroendocrine tumors (PitNETs); however, preserving pituitary function by maintaining adequate vascular supply remains challenging. Unilateral internal carotid artery (ICA) agenesis with transsellar anastomosis is rare and has not been reported in association with PitNETs. OBSERVATIONS: A 46-year-old woman with hyperprolactinemia (970 ng/mL) and a growing sellar tumor underwent endoscopic transsphenoidal resection after intolerance to bromocriptine. Preoperative imaging revealed right ICA agenesis, confirmed by the absence of the carotid canal. Digital subtraction angiography (DSA) showed a transsellar anastomosis arising from the left supraclinoid ICA, crossing anterior to the pituitary stalk. Gross-total resection was achieved, and this collateral structure was preserved by maintaining the arachnoid plane. Postoperatively, vision and prolactin levels normalized without endocrine deficits. Histological examination confirmed a lactotroph PitNET. LESSONS: This first reported case of PitNET associated with unilateral ICA agenesis highlights the critical role of DSA in identifying transsellar anastomosis as the sole vascular supply to the pituitary gland. Precise preservation of the arachnoid plane enabled total resection with intact endocrine function. https://thejns.org/doi/10.3171/CASE25976.