"Dose distribution close to metal implants in gamma knife radiosurgery: a Monte Carlo study" [Med. Phys. 30, 1812-1815 (2003)].

We have calculated the dose enhancements close to metal implants in Gamma Knife radiosurgery using different beam sizes, i.e., 4, 8 and 18 mm collimator helmets sin addition to the previous calculations using the 14 mm collimator helmetd. For the 8 and 18 mm helmets, the obtained results were similar to those obtained using the 14 mm helmet, which can be explained by the similar beam profiles with and without the platinum implant in the region from the maximum down to the platinum–phantom interface. On the other hand, the dose enhancement for the smallest 4 mm collimator helmet is higher than those using the 8, 14 and 18 mm collimator helmets, which can be explained by the rapid falloff of the steep gradient of the dose profile without the platinum implant. In our recent paper entitled “Dose Enhancement Close to Platinum Implants for the 4 MV, 6 MV and 10 MV Stereotactic Radiosurgery,” it was found that the dose enhancement due to the foreign platinum implant increased with the beam energy but decreased as the beam size increased. Comparatively higher dose enhancements were observed when using smaller collimators. As there are similarities in the physics for the Gamma Knife and Linac based radiosurgery, these results have prompted us to extend our previous studies on the dose distribution close to metal implants in Gamma Knife radiosurgery to further studies using other collimator helmets, viz., 4, 8 and 18 mm. From our previous results on Gamma Knife radiosurgery, there were dose enhancements close to some foreign metal implants when a 14 mm collimator shot was delivered at the unit center point s100, 100, 100d of a water phantom with a diameter of 160 mm sFig. 1d. The dose enhancement is defined as the percentage difference between the dose profiles with and without the platinum implant at the platinum– phantom interface. The dose enhancement is due to the generation of secondary electrons from the photoelectric effect, which is favored for higher atomic numbers. Dose enhancements as high as 10% were observed close to a platinum implant along the x and y axes, while dose enhancements as high as 20% were observed close to the platinum implant along the z axis at the superior position of the metal– phantom interface. In that study, only the 14 mm collimator was studied because the order of magnitude for both the implants and the treatment area susing multiple shotsd were close to the real clinical situation. Moreover, the objective of that paper was to study dose enhancements for different implant materials. In the present Letter, we focus on platinum implants with different beam sizes and we have repeated the calculations