Lesion size following Gamma Knife treatment for functional disorders

In this study we investigated the reproducibility and consistency of the size of radiosurgical lesions produced for functional disorders. The T1 gadolinium-enhanced magnetic resonance (MR) images of 56 patients treated for parkinsonism, pain, or other functional diseases were used to measure 140 lesion sizes at various times after radiosurgical treatment (1-26 months, mean: 11.3 months). Only the 4-mm collimator was used to create the lesions. The maximum dose ranged from 110 to 180 Gy (mean: 145 Gy). In 42 cases (78%), one isocenter was used to create the lesion. Thirteen lesions (20%) were created with two isocenters and in 1 case, three isocenters were used. Lesions were detectable on MR images as early as 30 days after treatment. The maximum lesion volume was reached after 6-12 months and ranged from nondetectable to more than 4,000 mm3. Larger lesion volumes were strongly associated with the use of more than one isocenter. In addition, maximum doses of 160 Gy or more increased the likelihood of producing lesions larger than expected. It is therefore concluded that the use of the Gamma Knife for the treatment of functional disorders is safest when single-isocenter shots with the 4-mm collimator and a maximum dose of less than 160 Gy are used.