GammaPlan (R)-Leksell Gamma Knife (R) radiosurgery treatment planning verification method

This work provides a method for an independent check of Gamma Knife(R) Gamm aPlan(R) radiosurgery calculations, named the spherical approximation metho d or SAM. Based on skull dimension measurements, the treated volume of the head is modeled as a sphere of radius R. With this approximation, an analyt ical solution for fast ray tracing of the path length, for each of the 201 beamlets, of the Gamma Knife helmet collimator was possible. The dose rate at the focus of a single shot is the sum of the contributions of all active beamlets adjusted fur both the collimator factor and attenuation. For an a rbitrary point, the dose rare is derived at the beamlet level from the focu s values adjusted for the new path length attenuation and the appropriate c ollimators' off-axis profiles. The sum over all beamlets' contributions giv es the dose rate at that particular point. At the single shot level, SAM in dependent check results agree with the GammaPlan(R) for patient calculation s to better than +/-16% and, as expected, in spherical phantoms the agreeme nts improve to better than +/-11.0%. For an arbitrary point, multi-shot pro cedure, the agreement is better than +/-3% and +/-1.5, respectively. (C) 20 00 American Association of Physicists in Medicine. [S0094-2405(00)01708-9].