Background: Due to high risk of recurrence and complications associated with microsurgical resection or aspiration of enlarging cystic sellar lesions, stereotactic intracavitary irradiation with 90Yttrium (90Yt) has been proposed as an alternative to mitigate these challenges. Long-term efficacy data for this procedure is lacking. Methods: We conducted a Health Canada approved, single-centre, prospective cohort study to assess the effect of 90Yt therapy on cyst volume, visual field (VF) testing and endocrine function. All patients underwent right frontal stereotactic insertion of 90Yt colloid (200 Gy cyst wall). Results: 22 patients [mean age = 63.5 (± 15.4) years] received 90Yt therapy. Mean follow-up was 52 (5-113) months. Cysts included: craniopharyngioma (86%), Rathke’s cleft cyst (9%) and cystic prolactinoma (5%). Mean cyst volume reduction was 77% at 12-months (5.2 ± 4.5 cc to 1.2 ± 1.5 cc; p = 0.002). Rate of pre- and post-op hormonal dysfunction was 59% and 68%, respectively (p = 0.50). Pre-operative VF deficits were found in 68% of patients, of which 32% normalized and 36% remained stable (p = 0.180). Worsened vision was detected in 9% of patients post-operatively. Conclusions: 90Yttrium therapy significantly reduces sellar cyst size without having a deleterious effect on vision and endocrine function.