Accuracy and reliability of a commercial treatment planning system in nontarget regions in modern prostate radiotherapy

BACKGROUND: The currently available treatment planning systems (TPSs) are neither designed nor intended for accurate dose calculations in nontarget regions. The aim of this work is to quantify the accuracy and reliability of nontarget doses calculated by a commercially available TPS. METHODS: Nontarget doses calculated by the collapsed cone (CC) (v5.2) algorithm implemented in the RayStation (v6) TPS were compared to measured values. Different scenarios were investigated, from simple static fields to intensity modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) treatment plans. Deviations and confidence limits (CLs) were calculated between results of calculations and measurements—applying both local (δ) and global (Δ) normalization—for various points of interest (POIs). Results were based on a single‐institution experience for one clinical test case (prostate) and evaluated against internationally accepted criteria. RESULTS: Overall, the TPS underestimated the nontarget dose by an average of −17.7% ± 25.3% for IMRT. Quantitatively similar results were obtained for VMAT (−17.6% ± 21.2%). POIs receiving < 5% of the prescription dose were significantly underestimated by the TPS (p‐value < 0.05 for both IMRT and VMAT). Dose calculation accuracy was also determined by the contribution of secondary radiation, with measured doses for out‐of‐field POIs being significantly different from calculated values (p‐value < 0.01 for both IMRT and VMAT). Although the CL(δ) in nontarget regions failed the proposed tolerance criteria (40%) for both IMRT (68.8%) and VMAT (52.6%), the CL(Δ) was within the tolerance limit (4%) for both treatment techniques (1.9% for IMRT and 1.3% for VMAT). No action levels (7%) were exceeded. CONCLUSIONS: Based on the currently available benchmarks our TPS is considered acceptable for clinical use, although the dose in some POIs was poorly predicted by the CC algorithm. Some areas were pointed out where TPSs and linear accelerator control systems can be improved.