Does intensity modulation increase target dose calculation errors of conventional algorithms for lung SBRT?

Abstract

Purpose: Conventional dose algorithms (Type A and Type B) for lung SBRT can display considerable target dose errors compared to Type-C algorithms. Intensity-modulated techniques (IMRT/VMAT) are increasingly being utilized for lung SBRT. Therefore, our study aimed to assess whether intensity modulation increased target dose calculation errors by conventional algorithms over conformal techniques. Methods: Twenty lung SBRT patients were parallely planned with both IMRT and dynamic conformal arc (DCA) techniques using a Type-A algorithm, and another 20 patients were parallely planned with IMRT, VMAT, and DCA using a Type-B algorithm. All 100 plans were recalculated with Type-C algorithms using identical beam and monitor unit settings, with the Type-A/Type-B algorithm dose errors defined using Type-C recalculation as the ground truth. Target dose errors for PTV and GTV were calculated for a variety of dosimetric end points. Using Wilcoxon signed-rank tests (p < 0.05 for statistical significance), target dose errors were compared between corresponding IMRT/VMAT and DCA plans for the two conventional algorithms. The levels of intensity modulation were also evaluated using the ratios of MUs in the IMRT/VMAT plans to those in the corresponding DCA plans. Linear regression was used to study the correlation between intensity modulation and relative dose error magnitudes. Results: Overall, larger errors were found for the Type-A algorithm than for the Type-B algorithm. However, the IMRT/VMAT plans were not found to have statistically larger dose errors from their corresponding DCA plans. Linear regression did not identify a significant correlation between the intensity modulation level and the relative dose error. Conclusion: Intensity modulation did not appear to increase target dose calculation errors for lung SBRT plans calculated with conventional algorithms.