MR imaging of pituitary adenomas after gamma knife stereotactic radiosurgery

OBJECTIVE. The purpose of this study was to evaluate the response of pituit ary adenomas to radiosurgery as manifested by changes in size and appearanc e on serial MR imaging MATERIALS AND METHODS. Over a mean follow-up period of 36 months, changes i n 44 pituitary adenomas were assessed on 147 enhanced MR imaging studies. P rior surgery had been performed in 36 tumors (82%). RESULTS. At the time of radiosurgery. mean tumor volume was 5.9 +/- 0.8 cm( 3) (mean diameter, 2.2 cm). The mean reduction in volume at last follow-up was 41% (+/-5%, p < 0.001), and a decrease in tumor volume of 25-100% was o bserved in 34 tumors (77%). Mean reduction in tumor volume at 6 months afte r radiosurgery was 9% (p = 0.095); at 1 year, 24% (p < 0.001); at 2 years, 34% (p < 0.001); at 3 years, 41% (p < 0.001); and at 4 years, 50% (p = 0.00 8). Six months after radiosurgery a slight and transient increase in size w as observed in 21 % of tumors. During follow-up, neither decreased contrast enhancement nor cyst development was associated with changes in tumor volu me. CONCLUSION. Tumor control was observed for most pituitary adenomas after ra diosurgery and occurred gradually over a period of several years. A small i ncrease in tumor size might be observed in the first 6 months after radiosu rgery. In most cases, reductions in tumor size were not accompanied by a ch ange in contrast enhancement or cyst formation.