Evaluation of Organ Specific peripheral dose for Gamma knife 4C based on Monte Carlo

Introduction: Stereotactic Gamma Knife radiosurgery has been widely used for treating brain tumors. The scattered radiation outside of treatment field (peripheral dose) can induce the secondary cancer to specific organ. This paper investigated the absorbed dose to eyes, thyroid, heart, lung, breast and colon using a Monte Carlo technique for Mird phantom. We also study the effect of depth of tumor and helmet size on the peripheral dose. Materials and Methods: In this work, gamma knife 4C was simulated as 201 surfaces disc source modelling. Based on this model, parallel beams of uniformly distributed method emitted form 201 disk sources that correspond to inner aperture for helmet. The photon beams converged at the isocenter of gamma knife. The coordinates of centers of these discs as planes to be determined using Matlab software. The 201 disc sources simulated for 4, 8, 14 and 18 mm helmet and also various tumor depth using mcnpx. Results: Large differences were observed between the organs close to target volume such as eyes and thyroid respect to organ far from target volume. The eyes received the maximum dose and the left eye dose was more than right one. Significant differences weren’t observed for organ peripheral dose with increase helmet size. The peripheral organ doses were increased with decrease of isocenter point as result of decrease depth of tumor due to decrease scattered radiation. Conclusion: Treatment planning should be considered gamma knife peripheral peripheral doses for organs near to target volume specially in surface tumor.