Motion-induced proton dose change measured by 3D deformable dosimeters in an anthropomorphic phantom
MO-0480
Abstract
Motion-induced proton dose change measured by 3D deformable dosimeters in an anthropomorphic phantom
Authors: Simon Jensen1,2, Stefanie Ehrbar3, Esben Worm4, Ludvig Muren1,2, Stephanie Tanadini-Lang3, Jørgen Petersen5,4, Peter Balling6,7, Per Poulsen1,2
1Aarhus University, Department of Clinical Medicine, Aarhus, Denmark; 2Aarhus University Hospital, Danish Centre for Particle Therapy, Aarhus, Denmark; 3University Hospital Zürich and University of Zürich, Department of Radiation Oncology, Zürich, Switzerland; 4Aarhus University Hospital, Department of Medical Physics, Aarhus, Denmark; 5Aarhus University, Department of Clinical Medicine, Aarhus, Switzerland; 6Aarhus University, Department of Physics and Astronomy, Aarhus, Denmark; 7Aarhus University, Interdisciplinary Nanoscience Center, Aarhus, Denmark
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Purpose or Objective
Intra-fractional motion has a detrimental effect on proton therapy delivery for tumours in the thorax and abdomen, an effect that may be mitigated by real-time motion-management methods such as respiratory gating. Dosimetric studies on complex intra-fractional motion, interplay effects and motion mitigation have been limited to simulations or 1D point, 2D planar, and 2.5D dose measurements. In preparation for a clinical trial, this study demonstrates how anthropomorphic deformable 3D dosimeters embedded in a deformable abdominal phantom can be applied to measure the effects of complex motion and gating on clinically relevant pencil beam scanning (PBS) proton treatments in the liver.
Material and Methods
A deformable abdominal phantom was modified to hold an anthropomorphic 3D dosimeter shaped as a human liver (Figure 1). A 3-field PBS proton plan was made on the exhale phase of a 4DCT scan using multi-field robust optimization (±5 mm shift along each axis including ±3.5 % range uncertainty). The plan was normalized to deliver a mean fraction dose of 12 Gy to a clinically realistic clinical target volume (CTV) in the liver. Two batches, each with two radiochromic silicone-based 3D dosimeters, were made. In both batches, one dosimeter was irradiated in a stationary position in the abdominal phantom. The second dosimeter was irradiated while the abdominal phantom moved sinusoidally with a 10 mm peak-to-peak amplitude without (Batch 1) and with (Batch 2) respiratory gating using a 50% duty cycle around the exhale phase monitored by an external marker block (Varian RPM system). The dosimeters were read out using an optical CT scanner which provided a 3D distribution of the radiation induced change in optical attenuation coefficients (Δα). In this study, Δα was used as a metric for the dose, which allowed direct comparison between the dynamic dose and the static dose in both batches. The motion-experiment datasets were image registered for alignment and evaluated using a global 3D gamma analysis with the stationary experiment as a reference for all voxels with signal above 8% of the maximum Δα. Additionally, a Δα-volume histogram of the CTV was constructed for all experiments normalized to the mean Δα in the CTV for the stationary scenario.
![](https://www.estro.org:443/ESTRO/media/Abstracts/976/fdc093f5-6641-4b6e-9bb2-b78988cb62e8.png)
Results
The gamma pass rates for the motion experiments were 68% (3%/3 mm) and 44% (2%/2 mm) without gating and 96% (3%/3 mm) and 85% (2%/2 mm) with gating (Figure 2A and 2B). Furthermore, Δα-volume histograms (Figure 2C) show better CTV coverage in the gated than non-gated scenario. Further experiments are needed to investigate the influence of interplay effects, CTV and motion magnitude.
![](https://www.estro.org:443/ESTRO/media/Abstracts/976/5b74a84a-6e13-4b75-add3-fdca05e3c2e4.png)
Conclusion
An anthropomorphic 3D dosimeter was for the first time embedded in a deformable abdominal phantom, and successfully measured the motion-induced dose perturbation of liver proton PBS treatments.