Abstract
PurposeEnd‐to‐end testing with quality assurance (QA) phantoms for deformable dose accumulation and real‐time image‐guided radiotherapy (IGRT) has recently been recommended by American Association of Physicists in Medicine (AAPM) Task Groups 132 and 76. The goal of this work was to develop a deformable abdominal phantom containing a deformable three‐dimensional dosimeter that could provide robust testing of these systems.MethodsThe deformable abdominal phantom was fabricated from polyvinyl chloride plastisol and phantom motion was simulated with a programmable motion stage and plunger. A deformable normoxic polyacrylamide gel (nPAG) dosimeter was incorporated into the phantom apparatus to represent a liver tumor. Dosimeter data were acquired using magnetic resonance imaging (MRI). Static measurements were compared to planned dose distributions. Static and dynamic deformations were used to simulate inter‐ and intrafractional motion in the phantom and measurements were compared to baseline measurements.ResultsThe statically irradiated dosimeters matched the planned dose distribution with an average γ pass rates of 97.0 ± 0.5% and 97.5 ± 0.2% for 3%/5 mm and 5%/5 mm criteria, respectively. Static deformations caused measured dose distribution shifts toward the phantom plunger. During the dynamic deformation experiment, the dosimeter that utilized beam gating showed an improvement in the γ pass rate compared to the dosimeter that did not.ConclusionsA deformable abdominal phantom apparatus which incorporates a deformable nPAG dosimeter was developed to test real‐time IGRT systems and deformable dose accumulation algorithms. This apparatus was used to benchmark simple static irradiations in which it was found that measurements match well to the planned distributions. Deformable dose accumulation could be tested by directly measuring the shifts and blurring of the target dose due to interfractional organ deformation and motion. Dosimetric improvements were achieved from the motion management during intrafractional motion.
Highlights
Patient motion can reduce the precision of external beam radiotherapy (EBRT), resulting in decreased target coverage and irradiation of nearby healthy structures
Profiles gathered through the middle of each dose distribution in the IEC defined z‐direction[37] of the axial slices are shown in Fig. 6, illustrating that the dose distributions are similar in the high‐dose regions, but the edge of the dosimeter has a distinct falloff
Measurements with the phantom and dosimeter where initially benchmarked with static irradiations measurements that matched well between the dose distributions planned with EclipseTM and the normoxic polyacrylamide gel (nPAG) dosimeter
Summary
Patient motion can reduce the precision of external beam radiotherapy (EBRT), resulting in decreased target coverage and irradiation of nearby healthy structures. This motion can be especially detrimental in the thoracic and abdominal regions of the body, where translational motion and deformation can be on the order of several centimeters.[1,2,3,4] Image‐guided radiation therapy (IGRT) has improved the precision of radiotherapy by using different imaging systems to reduce interfractional and intrafractional motion uncertainties. The treatment is adapted to account for the motion of the target by either gating or tracking the treatment beam.[8,9] Numerous imaging modalities have been utilized to monitor target motion including optical surface tracking,[10,11] magnetic resonance imaging (MRI) guidance,[12,13] ultrasound guidance,[14,15,16] and fluoroscopy.[17,18] Both interfractional and intrafractional motion management strategies have led to the reduction of treatment margins used for a variety of tumor types seen clinically.[19]
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