Maintaining dosimetric quality when switching to a Monte Carlo dose engine for head and neck volumetric‐modulated arc therapy planning

Abstract

Head and neck cancers present challenges in radiation treatment planning due to the large number of critical structures near the target(s) and highly heterogeneous tissue composition. While Monte Carlo (MC) dose calculations currently offer the most accurate approximation of dose deposition in tissue, the switch to MC presents challenges in preserving the parameters of care. The differences in dose‐to‐tissue were widely discussed in the literature, but mostly in the context of recalculating the existing plans rather than reoptimizing with the MC dose engine. Also, the target dose homogeneity received less attention. We adhere to strict dose homogeneity objectives in clinical practice. In this study, we started with 21 clinical volumetric‐modulated arc therapy (VMAT) plans previously developed in Pinnacle treatment planning system. Those plans were recalculated “as is” with RayStation (RS) MC algorithm and then reoptimized in RS with both collapsed cone (CC) and MC algorithms. MC statistical uncertainty (0.3%) was selected carefully to balance the dose computation time (1–2 min) with the planning target volume (PTV) dose‐volume histogram (DVH) shape approaching that of a “noise‐free” calculation. When the hot spot in head and neck MC‐based treatment planning is defined as dose to 0.03 cc, it is exceedingly difficult to limit it to 105% of the prescription dose, as we were used to with the CC algorithm. The average hot spot after optimization and calculation with RS MC was statistically significantly higher compared to Pinnacle and RS CC algorithms by 1.2 and 1.0 %, respectively. The 95% confidence interval (CI) observed in this study suggests that in most cases a hot spot of ≤107% is achievable. Compared to the 95% CI for the previous clinical plans recalculated with RS MC “as is” (upper limit 108%), in real terms this result is at least as good or better than the historic plans.

A head and neck treatment planning strategy for a CBCT‐guided ring‐gantry online adaptive radiotherapy system

“…All boxplots in this study were based on the difference between the achieved goal in Ethos/clinical plans and institutional clinical goals with one acceptable variation: D 0.03cc ≤107% instead of ≤105%. This allowed variation was associated with plan quality changes observed when moving from collapsed cone calculation approaches to more rigorous algorithms 25 . For most goals, the PTV and OAR dose differences from institutional guidelines in both Ethos‐ and clinical‐initial SIB plans were comparable.…”

“…This allowed variation was associated with plan quality changes observed when moving from collapsed cone calculation approaches to more rigorous algorithms. 25 For most goals, the PTV and OAR dose differences from institutional guidelines in both Ethos-and clinical-initial SIB plans were comparable. However, PTV High achieved goals in Ethos were clinically unacceptable in some cases.…”

“…In this study, clinical plans were calculated based on collapsed cone convolution superposition, whereas Ethos used the more precise Acuros XB. Collapsed cone versus Monte Carlo (comparable to the Acuros XB) calculations of identical plans showed increased hot spots for Monte Carlo 25,27,28 . For this reason, D 0.03cc ≤ 107% (vs. 105%) was used in this work.…”

“…To minimize clinical goals, only the most important goal for each OAR was used, and goals for OARs intersecting or very close to PTVs that may cause conflict and prevent the IOE from generating a clinically acceptable plan were excluded altogether (these were decided on a case‐by‐case basis by identifying regions within PTVs that had unacceptably high hot spots or appreciable under‐coverage and were in close proximity to OARs with conflicting objectives). For hotspot control, a goal of D 0.03cc ≤ 105% was used when planning, with 107% acceptable variation, which was based on what was found to be achievable for H&N plans using a similar dose calculation algorithm 25 …”

“…For hotspot control, a goal of D 0.03cc ≤ 105% was used when planning, with 107% acceptable variation, which was based on what was found to be achievable for H&N plans using a similar dose calculation algorithm. 25 Additional structures and goals were introduced to control undesired hotspots in PTV high, medium and low, such as V 105% ≤ 5% with intermediate priority, as well as additional goals (D 0.03cc ≤ 105% and V 105% ≤ 5%) to cropped structures of PTV Med, Low defined as follow: PTV Med and/or PTV Low were cropped a certain distance from PTV High . This distance was determined based on the prescription dose to PTV Med and/or PTV Low relative to the prescribed dose to PTV High .…”