Abstract
Purpose:To evaluate the variation of dose calculations using a treatment planning system (TPS) over a two year period and assessment of the need for TPS QA on regular intervals.Methods:Two phantoms containing solid water and lung‐ and bone‐equivalent heterogeneities were constructed in two different institutions for the same brand treatment planning system. Multiple plans, consisting of photons and electron beams, including IMRT and VMAT ones, were created and calculated on the phantoms. The accuracy of dose computation in the phantoms was evaluated at the onset by dose measurements within the phantoms. The dose values at up to 24 points of interest (POI) within the solid water, lung, and bone slabs, as well as mean doses to several regions of interest (ROI), were re‐calculated over a two‐year period which included two software upgrades. The variations in POI and ROI dose values were analyzed and evaluated.Results:The computed doses vary slightly month‐over‐month. There are noticeable variations at the times of software upgrade, if the upgrade involves remodeling and/or re‐commissioning of the beams. The variations are larger in certain points within the phantom, usually in the buildup region or near interfaces, and are almost non‐existent for electron beams.Conclusion:Routine TPS QA is recommended by AAPM and other professional societies, and is often required by accreditation organizations. The frequency and type of QA, though, is subject to debate. The results presented here demonstrate that the frequency of these tests could be at longer intervals than monthly. However, it is essential to perform TPS QA at the time of commissioning and after each software upgrade.
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