Stereotactic dose planning system used in Leksell Gamma Knife model-B: EGS4 Monte Carlo versus GafChromic films MD-55

The accuracy of the dose planning system (Leksell GammaPlan) used in Gamma Knife (model-B) radiosurgery was verified using both the GafChromic films M D-55 (improved) and the EGS4 Monte Carlo technique, The Monte Carlo techniq ue was employed to calculate the dose distribution along the x-, y- and z-a xes when a single shot with opening of all 201 sources was delivered at the centre of a simulated water phantom with a diameter of 160 mm. Collimator helmets with different size of the Gamma Knife unit were verified. Good con sistency (typical discrepancy less than 2%) was obtained between the result s of Monte Carlo and GammaPlan. Small discrepancies, however, were obtained by GafChromic films. Discrepancies, as great as 10% when using the 4 mm co llimator helmet, at the low percentage isodose curve along the z-axis of th e measurement results were probably due to the small energy dependency of t he GafChromic films. Significant discrepancies were not observed along x- a nd y-axes because such small discrepancies were easily over-washed by other gamma beams coming from the x- and y-directions. Similar results showing t he discrepancies between the GafChromic films and GammaPlan were obtained w hen using the 8, 14 and 18 mm collimator helmets. However, the discrepancie s along the z-axis became smaller as scattering effect increased when using larger collimator sizes. We suggest that the Monte Carlo technique should also be applied in stereotactic dose planning system verification as it is an ideal and reliable computational technique. (C) 2000 Elsevier Science Lt d. All rights reserved.