Background: Traditional neurosurgical techniques and/or stereotactic radiotherapy, although effective for solid tumors, can be associated with high morbidity and be relatively ineffective for long-term control of cystic sellar/parasellar tumors. The rationale of our study was to examine the efficacy and safety of stereotactic intracavitary instillation of 90yttrium colloid for the primary treatment of cystic tumors. Methods: As part of a Health Canada approved clinical trial, we have enrolled nine patients (6 females, 3 males; mean age 64, range 43 to 83 years) for treatment of symptomatic and/or enlarging cysts. Ten cystic sellar/parasellar lesions underwent right frontal stereotactic insertion of 90yttrium colloid to deliver a radiation dose of 200 Gy to the cyst wall. Results: Compared with pre-treatment cyst volumes (mean 4.6 cc; range 0.8-16.1 cc), the cysts decreased in size at 3 months (2.6 cc; 0.2-10 cc) with further shrinkage (n=5) at 9 months (1 cc; 0.1-2.7 cc). Of 9 patients with pre-operation visual field defects, 6 showed improvement. The single complication was a delayed (1 month) incomplete CNIII palsy. Conclusions: Our early experience indicates that 90yttrium colloid delivered to a cystic craniopharyngioma provides an efficacious alternative to open surgery for primary treatment of these cystic lesions.