Measurement comparison and Monte Carlo analysis for volumetric‐modulated arc therapy (VMAT) delivery verification using the ArcCHECK dosimetry system

Abstract

The objective of this study is to validate the capabilities of a cylindrical diode array system for volumetric‐modulated arc therapy (VMAT) treatment quality assurance (QA). The VMAT plans were generated by the Eclipse treatment planning system (TPS) with the analytical anisotropic algorithm (AAA) for dose calculation. An in‐house Monte Carlo (MC) code was utilized as a validation tool for the TPS calculations and the ArcCHECK measurements. The megavoltage computed tomography (MVCT) of the ArcCHECK system was adopted for the geometry reconstruction in the TPS and for MC simulations. A 10×10 cm2 open field validation was performed for both the 6 and 10 MV photon beams to validate the absolute dose calibration of the ArcCHECK system and also the TPS dose calculations for this system. The impact of the angular dependency on noncoplanar deliveries was investigated with a series of 10×10 cm2 fields delivered with couch rotation 0° to 40°. The sensitivity of detecting the translational (1 to 10 mm) and the rotational (1° to 3°) misalignments was tested with a breast VMAT case. Ten VMAT plans (six prostate, H&N, pelvis, liver, and breast) were investigated to evaluate the agreement of the target dose and the peripheral dose among ArcCHECK measurements, and TPS and MC dose calculations. A customized acrylic plug holding an ion chamber was used to measure the dose at the center of the ArcCHECK phantom. Both the entrance and the exit doses measured by the ArcCHECK system with and without the plug agreed with the MC simulation to 1.0%. The TPS dose calculation with a 2.5 mm grid overestimated the exit dose by up to 7.2% when the plug was removed. The agreement between the MC and TPS calculations for the ArcCHECK without the plug improved significantly when a 1 mm dose calculation grid was used in the TPS. The noncoplanar delivery test demonstrated that the angular dependency has limited impact on the gamma passing rate (<1.2% drop) for the 2%–3% dose and 2 mm–3 mm DTA criteria. A 1° rotational misalignment introduces 11.3% (3%/3 mm) to 21.3% (1%/1 mm) and 0.2% (3%/3 mm) to 0.8% (1%/1 mm) Gamma passing rate drop for ArcCHECK system and MatriXX system, respectively. Both systems have comparable sensitivity to the AP misalignments. However, a 2 mm RL misalignment introduces gamma passing rate drop ranging from 0.9% (3%/3 mm) to 4.0% (1%/1 mm) and 5.0% (3%/3 mm) to 12.0% (1%/1 mm) for ArcCHECK and MatriXX measurements, respectively. For VMAT plan QA, the gamma analysis passing rates ranged from 96.1% (H&N case) to 99.9% (prostate case), when using the 3%/3 mm DTA criteria for the peripheral dose validation between the TPS and ArcCHCEK measurements. The peripheral dose validation between the MC simulation and ArcCHECK measurements showed at least 97.9% gamma passing rates. The central dose validation also showed an agreement within 2.2% between TPS/MC calculations and ArcCHECK measurements. The worst discrepancy was found in the H&N case, which is the most complex VMAT case. The ArcCHECK system is suitable for VMAT QA evaluation...

Comparing measurement‐derived (3DVH) and machine log file‐derived dose reconstruction methods for VMAT QA in patient geometries

“…A previously developed dose reconstruction tool was used in this study. (5) The tool is written in C++ and converts the machine log file output from VMAT deliveries into Pinnacle 3 9.0 treatment plan format. The reconstructed plans were imported to Pinnacle and recalculated to obtain the actual delivered dose in patient anatomy.…”

“…More recently, ArcCHECK has been validated in heterogeneous phantom geometries. Lin et al (5) performed an ArcCHECK comparison in an inhomogeneous geometry consisting of ArcCHECK phantom without the cavity insert and compared the exit dose measurements with Monte Carlo simulation, AAA based TPS dose calculation, and MatriXX ion chamber dosimeter. The authors found more than 98% gamma agreement between MC and ArcCHECK, and 97% between TPS and ArcCHECK when 1 mm dose grid size was used for dose calculation.…”

3D VMAT Verification Based on Monte Carlo Log File Simulation with Experimental Feedback from Film Dosimetry

“…On the theoretical definition of VMAT presented by Otto [2], Monte Carlo (MC) was already proposed as an effective and necessary tool [6, 7]. MC particle transport simulation is recognized for its higher accuracy to model linac heads, especially in non-standard dosimetric conditions, like the ones involved in VMAT treatments.…”

Validation of an improved helical diode array and dose reconstruction software using TG‐244 datasets and stringent dose comparison criteria

“…It can be employed by itself to compare measured and reference dose at the detectors’ locations 1 , 2 , 3 , 4 , 5 , 6 or in conjunction with 3DVH software (Sun Nuclear) that uses a AC‐based planned dose perturbation (ACPDP) algorithm to perform volumetric measurement‐guided dose reconstruction on a phantom or patient dataset 7 , 8 , 9 , 10 , 11 . While the dosimeter and associated software have been characterized and employed rather extensively, (1–3,5,7,9,10,12–18) two recent developments necessitated revisiting the subject.…”