Effect of the normalized prescription isodose line on the magnitude of Monte Carlo vs. pencil beam target dose differences for lung stereotactic body radiotherapy

Abstract

In lung stereotactic body radiotherapy (SBRT) cases, the pencil beam (PB) dose calculation algorithm is known to overestimate target dose as compared to the more accurate Monte Carlo (MC) algorithm. We investigated whether changing the normalized prescription isodose line affected the magnitude of MC vs. PB target dose differences. Forty‐eight patient plans and twenty virtual‐tumor phantom plans were studied. For patient plans, four alternative plans prescribed to 60%, 70%, 80%, and 90% isodose lines were each created for 12 patients who previously received lung SBRT treatments. Using 6 MV dynamic conformal arcs, the plans were individually optimized to achieve similar dose coverage and conformity for all plans of the same patient, albeit at the different prescription levels. These plans, having used a PB algorithm, were all recalculated with MC to compare the target dose differences. The relative MC vs. PB target dose variations were investigated by comparing PTV D95, Dmean, and D5 loss at the four prescription levels. The MC‐to‐PB ratio of the plan heterogeneity index (HI) was also evaluated and compared among different isodose levels. To definitively demonstrate the cause of the isodose line dependence, a simulated phantom study was conducted using simple, spherical virtual tumors planned with uniform block margins. The tumor size and beam energy were also altered in the phantom study to investigate the interplay between these confounding factors and the isodose line effect. The magnitude of the target dose overestimation by PB was greater for higher prescription isodose levels. The MC vs. PB reduction in the target dose coverage indices, D95 and V100 of PTV, were found to monotonically increase with increasing isodose lines from 60% to 90%, resulting in more pronounced target dose coverage deficiency at higher isodose prescription levels. No isodose level‐dependent trend was observed for the dose errors in the target mean or high dose indices, Dmean or D5. The phantom study demonstrated that the observed isodose level dependence was caused by different beam margins used for the different isodose levels: a higher prescription line required a larger beam margin, leading to more low‐density lung tissues in the field and, therefore, larger dose errors at the target periphery (when calculated with PB). The phantom study also found that the observed isodose level dependence was greater for smaller targets and for higher beam energies. We hereby characterized the effect of normalized prescription isodose line on magnitude of PTV dose coverage as calculated by MC vs. PB. When comparing reported MC dose deficiency values for different patients, the selection of prescription isodose line should be considered in addition to other factors known to affect differences in calculated doses between various algorithms.PACS number(s): 87.55.kh, 87.55.dk, 87.55.de

Verification of Acuros XB dose algorithm using 3D printed low‐density phantoms for clinical photon beams

“…In this work, we have investigated only the low‐density range for clinical energies of interest in the small field sizes, due to inherent limitations of the 3'D printing fabrication process employed. That said, the investigation of small field size and low‐density tissue range for AXB validation is of clinical relevance as these conditions are akin to those of lung SBRT treatments, which has been a topic of interest with respect to the application of AXB 21, 22, 23, 24, 25…”

Still equivalent for dose calculation in the Monte Carlo era? A comparison of free breathing and average intensity projection CT datasets for lung SBRT using three generations of dose calculation algorithms

“…[16][17][18][19][20][21][22][23] Moreover, the magnitude of the dose error varies widely from case to case, and can depend on a multitude of factors. [23][24][25][26][27] Interestingly, in a phantom study, 25 Aarup et al found that the difference in calculated target dose between MC and PBC algorithms depends systematically on the density of the surrounding lung tissue. Specifically, they compared dose calculation using different algorithms on a spherical "target" surrounded by simulated "lung" tissue of different densities.…”

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

“…Finally, our study used DCA as the conformal technique for comparison. While DCA is a popular conformal technique used for lung SBRT, 9,10,[14][15][16] other techniques are also used such as multiple noncoplanar conformal beams.…”

“…[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.…”

“…9 Dose calculation has been widely compared among the above algorithms for lung SBRT, with interalgorithm differences mostly observed for the target dose. 5,7,9,10,14 It is known that as compared with Type-C algorithms, Type-A and Type-B algorithms tend to overestimate the target dose for lung SBRT, and the magnitude of the dose error varies widely from case to case, up to over 30% for Type A and over 15% for Type B. 7,9,15 These results include both forward plans using conventional conformal techniques as well as inverse plans using modern intensity-modulated techniques.…”