Pituitary adenomas are tumors that occur in the pituitary gland. Most pituitary tumors are benign, approximately 35% are invasive and just 0.1% to 0.2% are carcinomas.[1] Pituitary adenomas represent from 10% to 25% of all intracranial neoplasms and the estimated prevalence rate in the general population is approximately 17%.

Reference
Wiki: Non-functioning pituitary tumours