SU‐E‐T‐794: Effects of Dose Rate Variability for Volumetric Arc Treatments in Prostate Carcinoma

Purpose: To evaluate the effects of variable versus constant dose rate in volumetric modulated arc (VMAT) treatment planning for prostate cases. Method and Materials:A retrospective investigation on fifteen prostate cases was performed. Two dosimetrically identical linacs were commissioned in Pinnacle TPS. One machine was variable dose rate capable, while the other was constant dose rate capable for VMAT planning purposes. PTVs and OARs (rectum and bladder) were delineated for each case. Two VMAT plans were generated per case: each comprised of a single arc and comparing variable to constant dose rate. Both plans were normalized such that 95% of the PTV was covered by the same prescription dose. Plan quality was evaluated by several dose indices used as DVH objectives in the inverse optimization. In addition, dose inhomogeneity across the PTV was tallied together with MUs and estimated treatment times. Variable dose rare plans were used as reference. Results: The majority of rectum and bladder doses to 10%, 20%, and 40% of the OAR volumes differed by less than 3% between the two plans plans. Even for the cases with larger than 3% deviation the constant dose rate plans were clinically acceptable. Dose inhomogeneity across the PTV however was larger for constant dose rate plans, where for 80% of the cases it was over 3%, while it was within 3% for all variable dose rate cases. The MUs for the two delivering modes were comparable, while the treatment times differed three‐fold. Conclusions: The results indicate that variable and constant dose rate VMAT plans for prostate carcinoma result in comparable target coverage and OAR sparing. Nonetheless, the constant dose rate plans exhibit higher dose inhomogeneity across the target and the clinical applicability should be carefully evaluated for the potential hotspots in undesired location

SU‐GG‐T‐65: Dosimetric Verification of HDR Treatment Planning System Dose Calculations by Farmer Chamber Measurements in Water

Purpose: To evaluate the applicability of Farmer ion chambers for absolute dosimetry of high dose rate (HDR) brachytherapy sources in water and the potential to use this methodology for brachytherapy treatment planning system (TPS) dose calculation validation. Method and Materials: Water tank absolute dosimtric measurements of 192Ir doses were performed with a Farmer ionization chamber. Autoradiography was used for determination of absolute source position and subsequent alignment of the source with the chamber. A small solid water phantom was used for brachy catheter support. The phantom was immersed in a water tank equipped with a chamber holding mechanism capable of 1D motion. The chamber and the electrometer used for absolute dose measurements had ADCL traceable calibration factors for external beam reference dosimetry (calibrated in water with 60Co beam). Doses were measured from 2 cm to 11 cm away from a single dwell position. Measurements were compared to doses calculated by Oncentra HDR TPS as well as simple hand calculations, utilizing point source geometry, inverse square dose falloff and published data for radial dose falloff due to attenuation. Results: The measured and the TPS doses agreed within 3% up to 8 cm from the source, within 4% from 8 cm to 10 cm, and within 6% at 11 cm. The hand calculations, performed as a simple consistency check, agreed with the TPS within 1% across the entire tested range. Conclusions: The presented results indicate that Farmer chambers are suitable for absolute dosimetry of 192Ir sources over the useful HDR therapeutic range (2 cm to 10 cm). Absolute measurements in water performed with Farmer chambers can be easily employed for commercial TPS dosimetric commissioning as well as for routine dosimetric checks of the HDR delivery systems