Detector system dose verification comparisons for arc therapy: couch vs. gantry mount

Abstract

The aim of this study was to assess the performance of a gantry‐mounted detector system and a couch set detector system using a systematic multileaf collimator positional error manually introduced for volumetric‐modulated arc therapy. Four head and neck and esophagus VMAT plans were evaluated by measurement using an electronic portal imaging device and an ion chamber array. Each plan was copied and duplicated with a 1 mm systematic MLC positional error in the left leaf bank. Direct comparison of measurements for plans with and without the error permitted observational characteristics for quality assurance performance between detectors. A total of 48 different plans were evaluated for this testing. The mean percentage planar dose differences required to satisfy a 95% match between plans with and without the MLCPE were 5.2% ± 0.5% for the chamber array with gantry motion, 8.12% ± 1.04% for the chamber array with a static gantry at 0°, and 10.9% ± 1.4% for the EPID with gantry motion. It was observed that the EPID was less accurate due to overresponse of the MLCPE in the left leaf bank. The EPID always images bank‐A on the ipsilateral side of the detector, whereas for a chamber array or for a patient, that bank changes as it crosses the ‐90° or +90° position. A couch set detector system can reproduce the TPS calculated values most consistently. We recommend it as the most reliable patient specific QA system for MLC position error testing. This research is highlighted by the finding of up to 12.7% dose variation for H/N and esophagus cases for VMAT delivery, where the mere source of error was the stated clinically acceptability of 1 mm MLC position deviation of TG‐142.PACS numbers: 87.56.‐v, 87.55.‐x, 07.57.KP, 29.40.‐n, 85.25.Pb

Electronic portal imaging device-based three-dimensional volumetric dosimetry for intensity-modulated radiotherapy pretreatment quality assurance

“…Meanwhile, I'mRT MatriXX can be used as both gantry stationary and couch stationary detector system; if used as a gantry stationary detector system it does not comprehend the directional dependency. [9]…”

“…Originally described by Greer and Popescu, electronic PID (EPID) is a reliable dose verification system when corrected for systematic shifts and tilts. [789] The two available methods for EPID dosimetry are suitable for pretreatment verification as well as in vivo /transit dosimetry. The first method is the forward approach, where the measured dose in the portal imager plane is compared to the dose or photon fluence predicted by the treatment planning system (TPS).…”

A homogeneous water-equivalent anthropomorphic phantom for dosimetric verification of radiotherapy plans

“…These are carried out using portal imagers, film, ion chamber arrays, diode arrays, MU calculation software, and other methods. [ 2 8 9 10 11 12 ] All of these methodologies are based on phantom measurements; thus, it is necessary to establish the equivalence between heterogeneous patients and homogeneous water phantoms, and this equivalence has not previously been established in the literature. The present study establishes this equivalence.…”

Simple electronic portal imager-based pretreatment quality assurance using Acuros XB: A feasibility study

“…Pretreatment verification requires monitor unit (MU) verification and fluence measurements using either an ion chamber array, film, or portal imager (electronic portal imaging device [EPID]). [12]…”

“…EPID is a reliable dose verification system when considering the directional dependency of the beam-limiting devices and the imager panel systematic shifts and tilts. [25] There are two EPID dosimetry methods and both are suitable for in vivo dosimetry and pretreatment verification. The first method is based on the back-projection approach, where the dose fluence at EPID level is measured in the absence (free air or phantom measurement) or presence (transit dosimetry) of the patient.…”