A comprehensive dosimetric study on switching from a Type-B to a Type-C dose algorithm for modern lung SBRT

Abstract

BackgroundType-C dose algorithms provide more accurate dosimetry for lung SBRT treatment planning. However, because current dosimetric protocols were developed based on conventional algorithms, its applicability for the new generation algorithms needs to be determined. Previous studies on this issue used small sample sizes and reached discordant conclusions. Our study assessed dose calculation of a Type-C algorithm with current dosimetric protocols in a large patient cohort, in order to demonstrate the dosimetric impacts and necessary treatment planning steps of switching from a Type-B to a Type-C dose algorithm for lung SBRT planning.MethodsFifty-two lung SBRT patients were included, each planned using coplanar VMAT arcs, normalized to D95% = prescription dose using a Type-B algorithm. These were compared against three Type-C plans: re-calculated plans (identical plan parameters), re-normalized plans (D95% = prescription dose), and re-optimized plans. Dosimetric endpoints were extracted and compared among the four plans, including RTOG dosimetric criteria: (R100%, R50%, D2cm, V105%, and lung V20), PTV Dmin, Dmax, Dmean, V% and D90%, PTV coverage (V100%), homogeneity index (HI), and Paddick conformity index (PCI).ResultsRe-calculated Type-C plans resulted in decreased PTV Dmin with a mean difference of 5.2% and increased Dmax with a mean difference of 3.1%, similar or improved RTOG dose compliance, but compromised PTV coverage (mean D95% and V100% reduction of 2.5 and 8.1%, respectively). Seven plans had >5% D95% reduction (maximum reduction = 16.7%), and 18 plans had >5% V100% reduction (maximum reduction = 60.0%). Re-normalized Type-C plans restored target coverage, but yielded degraded plan conformity (average PCI reduction 4.0%), and RTOG dosimetric criteria deviation worsened in 11 plans, in R50%, D2cm, and R100%. Except for one case, re-optimized Type-C plans restored RTOG compliance achieved by the original Type-B plans, resulting in similar dosimetric values but slightly higher target dose heterogeneity (mean HI increase = 13.2%).ConclusionsType-B SBRT lung plans considerably overestimate target coverage for some patients, necessitating Type-C re-normalization or re-optimization. Current RTOG dosimetric criteria appear to remain appropriate.

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

“…The dose errors observed in this study for Type‐A and Type‐B algorithms are in agreement with numerous previous studies 7, 9, 14, 15. Both algorithms overestimated the target peripheral or “cold spot” indices such as D95% and Dmin, and the magnitude of the overestimation varied widely from case to case.…”

“…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 algorithms4, 13 and Boltzmann Solver‐based algorithms such as acuros external beam (AXB) 9…”

“…Early lung SBRT primarily employed conformal techniques such as the 3D conformal beam technique and the dynamic conformal arc technique 1, 16. However, in recent years, intensity‐modulated techniques such as intensity‐modulated radiation therapy (IMRT) and volumetric‐modulated arc therapy (VMAT) have gained increasing popularity owing to superior organ‐at‐risk sparing, dose conformity, and fast delivery in the case of VMAT 9, 17…”

“…[5][6][7] The obstacle of dose calculation has been addressed by the development of more accurate dose calculation algorithms in commercial treatment planning systems, from earlier homogeneous dose calculations to newer dose algorithms incorporating heterogeneity correction. 4 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 Dose calculation has been widely compared among the above algorithms for lung SBRT, with interalgorithm differences mostly observed for the target dose.…”

Secondary monitor unit calculations for VMAT using parallelized Monte Carlo simulations

“…In general, these were the results of calculations performed for small fields or where inhomogeneities were present. 16 Through the application of parallel processing, calculation times Agreements between MUCheck and Eclipse for brain cases were among the worst presented in this study. This is potentially a result of the MC algorithm modeling the physics of radiation transport more accurately than Eclipse, which uses the Analytical Anisotropic Algorithm (AAA) to calculate dose.…”