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

Dandan Zheng,Xiaoying Liang,Sumin Zhou,Shuo Wang,Vivek Verma

doi:10.1002/acm2.12266

Abstract

PurposeConventional 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.MethodsTwenty 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.ResultsOverall, 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.ConclusionIntensity modulation did not appear to increase target dose calculation errors for lung SBRT plans calculated with conventional algorithms.

Highlights

Stereotactic body radiotherapy (SBRT) is an increasingly common treatment for patients with inoperable early stage non-small cell lung cancer (NSCLC) and other lung tumors, with well-demonstrated efficacy and minimal side effects.[1,2,3] Accurate dose calculation during treatment planning is especially important in SBRT, owing to highfractional doses delivered over a small number of fractions
These latter algorithms are usually categorized from Type A to Type C, with increasing dose calculation accuracies:[8,9] (1) Type A: algorithms with a one-dimensional equivalent path length correction such as pencil beam (PB) convolution and ray tracing;[5,7,10] (2) Type B: algorithms applying two-dimensional corrections such as collapsed cone convolution (CCC)[11] and analytical anisotropic algorithm (AAA);[12] and (3) Type C: advanced algorithms such as fast Monte Carlo algorithms[4,13] and Boltzmann Solver-based algorithms such as acuros external beam (AXB).[9]
We investigated the impact of intensity modulation on target dose errors of conventional Type-A and Type-B dose algorithms relative to the more accurate Type-C algorithms on 20 randomly selected lung SBRT patients for each algorithm

Summary

Introduction

Stereotactic body radiotherapy (SBRT) is an increasingly common treatment for patients with inoperable early stage non-small cell lung cancer (NSCLC) and other lung tumors, with well-demonstrated efficacy and minimal side effects.[1,2,3] Accurate dose calculation during treatment planning is especially important in SBRT, owing to highfractional doses delivered over a small number of fractions. Two factors unique to lung SBRT increase the difficulty of achieving this necessary accuracy. Tissue heterogeneity between the high-density tumor and the surrounding low-density lung tissue complicates dose calculations owing to the loss of charged particle equilibrium.[4] Second, the small field sizes associated with the small target volume further exacerbate this problem.[5,6,7]. These latter algorithms are usually categorized from Type A to Type C, with increasing dose calculation accuracies:[8,9] (1) Type A: algorithms with a one-dimensional equivalent path length correction such as pencil beam (PB) convolution and ray tracing;[5,7,10] (2) Type B: algorithms applying two-dimensional corrections such as collapsed cone convolution (CCC)[11] and analytical anisotropic algorithm (AAA);[12] and (3) Type C: advanced algorithms such as fast Monte Carlo algorithms[4,13] and Boltzmann Solver-based algorithms such as acuros external beam (AXB).[9]

Methods

Results

Conclusion