Abstract
PurposeIn this work we have developed a novel method of dose distribution comparison, the inverse gamma (IG) evaluation, by modifying the commonly used gamma evaluation method.MethodsThe IG evaluation calculates the gamma criteria (dose difference criterion, ΔD, or distance‐to‐agreement criterion, Δd) that are needed to achieve a predefined pass rate or gamma agreement index (GAI). In‐house code for evaluating IG with a fixed ΔD of 3% was developed using Python (v3.5.2) and investigated using treatment plans and measurement data from 25 retrospective patient specific quality assurance tests (53 individual arcs).ResultsIt was found that when the desired GAI was set to 95%, approximately three quarters of the arcs tested were able to achieve Δd within 1 mm (mean Δd: 0.7 ± 0.5 mm). The mean Δd required in order for all points to pass the gamma evaluation (i.e., GAI = 100%) was 4.5 ± 3.1 mm. The possibility of evaluating IG by fixing the Δd or ΔD/Δd, instead of fixing the ΔD at 3%, was also investigated.ConclusionThe IG method and its indices have the potential to be implemented clinically to quantify the minimum dose and distance criteria based on a specified GAI. This method provides additional information to augment standard gamma evaluation results during patient specific quality assurance testing of individual treatment plans. The IG method also has the potential to be used in retrospective audits to determine an appropriate set of local gamma criteria and action levels based on a cohort of patient specific quality assurance plans.
Highlights
Contemporary radiation therapy techniques can involve the use of complex radiation fields delivered with moving or static gantries to deliver modulated dose distributions
These techniques, which include intensity modulated radiation therapy (IMRT), volumetric modulated arc therapy (VMAT) and helical tomotherapy (HT), require careful quality assurance tests, to ensure that the planned dose distributions can be delivered by the treatment system[1,2,3,4,5]
Patient specific quality assurance (PSQA) is often performed prior to the radiation treatment, where the treatment beams are delivered to a phantom and the radiation doses are verified against those predicted in the treatment planning system (TPS)
Summary
Contemporary radiation therapy techniques can involve the use of complex radiation fields delivered with moving or static gantries to deliver modulated dose distributions. These techniques, which include intensity modulated radiation therapy (IMRT), volumetric modulated arc therapy (VMAT) and helical tomotherapy (HT), require careful quality assurance tests, to ensure that the planned dose distributions can be delivered by the treatment system[1,2,3,4,5]. The gamma index method[6] is the predominant dose
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