Deformable abdominal phantom for the validation of real-time image guidance and deformable dose accumulation

Abstract

Purpose: End-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.

Methods: The 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.

Results: The 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.

Conclusions: A 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.

Keywords: Deformable dose accumulation; motion management; phantoms; three-dimensional dosimetry.

Figure 1

a) Rendering and b) axial cross section of the deformable abdominal phantom that was developed. The phantom platform features a programmable motion stage and plunger apparatus to drive motion, a deformable phantom section with low‐density organ sections, and a cavity for containing a removable deformable 3D dosimeter. 3D, three‐dimensional.

Figure 2

Axial slice of a CT image of the PVCP abdominal phantom. PVCP, polyvinyl chloride plastisol.

Figure 3

a) Model of the molding method used to fabricate the PVCP shells which encased the nPAG. b) Model of the 3D‐printed insert used to create the asymmetric inner cavities in the PVCP shells. c) Image of a completed deformable dosimeter. 3D, three‐dimensional; nPAG, normoxic polyacrylamide gel; PVCP, polyvinyl chloride plastisol.

Figure 4

The irradiation setup of the abdominal phantom apparatus.

Figure 5

A central axial slice of the (a) nPAG measured dose distribution and (b) the TPS planned dose distribution for one of the three dosimeters irradiated with the liver SBRT treatment. The two distributions appear qualitatively similar in the high‐dose regions, but the nPAG distribution shows a distinct lack of response near the PVCP shell wall, which is shown in deep red. Each grid mark is spaced by 2.5 cm. nPAG, normoxic polyacrylamide gel; PVCP, polyvinyl chloride plastisol; SBRT, stereotatic body radiotherapy; TPS, treatment planning system.

Figure 6

Central z‐profile of the dose distributions displayed in Fig. 5. The nPAG measured profile matched the TPS calculated profile in the high‐dose regions above 11 Gy with a 2.4% average absolute deviation. The nPAG showed a distinct falloff in dose due to oxygen inhibition near the edges of the gel due to oxygen inhibition. nPAG, normoxic polyacrylamide gel; TPS, treatment planning system.

Figure 7

Central coronal slices of the isodose maps of the undeformed dosimeter (a), the dosimeter with 1 cm of deformation (b), the dosimeter with 2 cm of deformation (c), and the dosimeter that was irradiated with three fractions, each with a different deformation state (d). Red arrows approximate the location of the deformation site and the direction of deformation. Each grid mark is spaced by 2.5 cm.

Figure 8

Central y‐profiles through each dose distribution measured by the dosimeters during the static deformation experiment.

Figure 9

Central slices of γ maps utilizing 3%/5 mm criteria. The dosimeter that underwent 1 cm of deformation (a), 2 cm of deformation (b), and three fractions with the three deformation states (c) were compared to the baseline undeformed dosimeter for this analysis. Red arrows approximate the location of the deformation site and the direction of deformation. Each grid mark is spaced by 2.5 cm.

Figure 10

Central coronal slices of the isodose plots of the static undeformed dosimeter (a), the ungated dynamically deformed dosimeter (b), and the beam‐gated dynamically deformed dosimeter (c). Red arrows approximate the location of the deformation site and the direction of deformation. Each grid mark is spaced by 2.5 cm.

Figure 11

Y‐profiles through each dose distribution measured by the dosimeters during the dynamic deformation experiment.

Figure 12

Central slices of γ maps utilizing 3%/5 mm criteria. The dynamically deformed dosimeter without beam gating (a) and the dynamically deformed dosimeter that utilized beam gating (b) were compared to the static undeformed dosimeter. Red arrows approximate the location of the deformation site and the direction of deformation. Each grid mark is spaced by 2.5 cm.