(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose: To determine the dose calculation accuracy of the Eclipse Treatment Planning System (TPS) for small photon beams via diode measurements and following the recommendations on IAEA TRS-483.
Methods: The equipment used was the PTW 60018 silicon diode, the Leksell Gamma Knife spherical solid-water dosimetry phantom, a Siemens CT simulator, and a high definition Varian TrueBeam. A CT scan of the phantom+diode system was taken with an SRS protocol (1mm slice thickness). This scan was then imported into the TPS and five static radiotherapy plans were generated. Each plan contained nine beams of the same field size (1x1, 1.5x1.5, 2x2, 2.5x2.5, 3x3cm²) separated by 40° gantry angles and defined by the MLC; the jaws were set always 0.2cm wider in every direction. The prescription dose per plan was 1000cGy to the isocenter, equally weighted among all nine beams. The phantom+diode system was secured to the treatment couch and the diode’s sensitive volume was positioned at the isocenter via kV-kV image registration. All measurements made with the diode were cross-calibrated with a calibrated farmer ionization chamber under reference conditions and corrected with the field output correction factors outlined in TRS-483.
Results: The average deviation (µ ± s) between the planned and measured doses were (0.5±0.7)% for the 3x3cm² plan, (0.4±0.8)% for the 2.5x2.5cm², (0.8±0.7)% for the 2x2cm², (1.2±0.7)% for the 1.5x1.5cm² and (4.0±1.0)% for the 1.0x1.0cm².
Conclusion: Eclipse dose calculation agrees well (<1%) with the measurements for field sizes larger than 2x2cm², but the difference increases gradually (up to 4%) as the field size decreases. This deviation is acceptable given the high dose non-uniformity (~40%) within the PTV in SRS and SBRT applications. We plan to evaluate the angular dependence of the diode and output variation of the machine to further correct for the inherent angular dependence variations in these measurements.
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