In this study, we examined patients with functioning pituitary adenoma that underwent Gamma Knife radiosurgery (GKS). In particular, we assessed the effects of GKS on the growth and endocrinological response of the functioning pituitary adenoma. Forty-two cases of functioning pituitary adenoma treated with GKS were analyzed. The mean follow-up duration was 42.5 months (range 6-98), and the mean tumor volume was 1.4 cm3 (range 0.1-3.8). Multiple isocenters, ranging from 1 to 6 in number (mean 2.7), were used. The tumor margin was covered by an isodose ranging from 50 to 90%. The margin dose was 18 to 40 Gy (mean 28.5) and the maximum dose varied from 35 to 80 Gy (mean 54.1). Tumor growth was controlled in 96.9% of the cases and tumor shrinkage occurred in 40.6% of the cases. Hormonal response was observed in 35 of the 42 (83.3%) patients after GKS, with a mean duration of 6.8 months. Sixteen of the 42 (38.1%) patients showed hormonal normalization, with a mean duration of 21 months. In our multivariate analysis, high integral dosage (p=0.005) and maximum dosage (p=0.001) correlated significantly with hormonal normalization. For patients with functioning pituitary adenoma, GKS can be effective in controlling tumor growth and inducing hormonal normalization, especially if patients are reluctant to undergo surgical resection, or are not able to undergo microsurgery under general anesthesia. It appears that early hormonal normalization can be induced by high maximum dosage (at least 50 Gy) and broad coverage of the target volume within the isodose curve, while keeping the maximum dose to the visual pathways below 9 Gy.
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