SU‐F‐T‐280: An Assessment of Deviations in Gamma Index Between PTW Octavius4D Phantom Measurements and Various Treatment Planning Systems

Purpose: There is a variety of treatment planning systems (TPS) available for clinical use all with highly variable settings to calculate patient dose. Patient specific IMRT QA is often a daily procedure at most clinics and each clinic devises their own policy on the TPS settings for testing the quality of the patient plan against a measured dose on IMRT QA phantoms. With this in mind, it is the aim of this study to evaluate the effect of dose calculation parameters on IMRT QA. Methods: Six patient plans were optimized and calculated using Pinnacle3 (v9.10). Patient QAs were calculated using the PTW Octavius4D phantom with a dose grid resolution of 3mm and a phantom density of 1.045g/cc. Using PTW's VeriSoft, the calculated dose distribution of Pinnacle3, Monaco, and Eclipse were compared to the delivered plan (the control) on the Octavius4D. For each TPS, the dose grid was varied from 2–4mm, and the phantom density varied ±10% in increments of 2%. Additionally, statistical uncertainties ranging from 0.7%–3% were calculated for Monaco. Using the evaluation metric of 3% and 3mm with a threshold of 10% of the max dose, the average and standard deviation of the 3D γ-index were calculated for each plan. The γ-index from axial, sagittal, and coronal slices at isocenter and the overall 3D γ-index were used for the comparisons. Results: The disparity in γ-index was small between most plan modifications. An extreme example is one case in which a 3D slice yielded a γ-index that differed up to 12.9% whereas the maximum variation in total volume for any case was merely 2.3%. The average standard deviation of all slices was ±1.49% and the average standard deviation for total volume was only ±0.55%. Conclusion: Dose calculation resolution and algorithm does not significantly affect the patient QA measurements.