Coordinate transformation after stereotactic frame reapplication in Gamma Knife radiosurgery

In Gamma Knife radiosurgery, the occurrence of reapplying the stereotactic frame leads to re-examination and re-planning. To avoid undergoing invasive second angiography examination for the treatment of vascular lesions, and reduce re-planning time, a mathematical coordinate transformation method using the anatomical information has been developed. The MR or CT images of a human brain before and after frame reapplication were correlated with each other using the Affine transformation. The transformation parameters which minimize the RMS error of the original and transformed coordinates between the images were determined using a genetic algorithm. Three CT image studies of skull phantom and five MR image studies of patients were used for the evaluation. The RMS error in the coordinate transformation of skull phantom and clinical images was 0.3 ± 0.1 mm and 0.6 ± 0.1 mm, respectively. The original treatment plans of patients were converted to new plans using the transformation matrix. For total 9 treatment lesions of 0.2-14.1 cc, 3% and 11% RMS error in the irradiation time and target coverage were found respectively. The deeply-located lesions showed a better RMS error of 3% in the conformity index and similar dose distribution than superficial lesions close to the skull.