Dose perturbations in proton pencil beam delivery investigated by dynamically deforming silicone-based radiochromic dosimeters.

Objective.Proton therapy with pencil beam delivery enables dose distributions that conform tightly to the shape of a target. However, proton therapy dose delivery is sensitive to motion and deformation, which especially occur in the abdominal and thoracic regions. In this study, the dose perturbation caused by dynamic motion with and without gating during proton pencil beam deliveries were investigated using deformable three-dimensional (3D) silicone-based radiochromic dosimeters.Approach.A spread-out Bragg peak formed by four proton spots with different energies was delivered to two dosimeter batches. All dosimeters were cylindrical with a 50 mm diameter and length. The dosimeters were irradiated stationary while uncompressed and during dynamic compression by sinusoidal motion with peak-to-peak amplitudes of 20 mm in one end of the dosimeter and 10 mm in the other end. Motion experiments were made without gating and with gating near the uncompressed position. The entire experiment was video recorded and simulated in a Monte Carlo (MC) program.Main results.The 2%/2 mm gamma index analysis between the dose measurements and the MC dose simulations had pass rates of 86%-94% (first batch) and 98%-99% (second batch). Compared to the static delivery, the dose delivered during motion had gamma pass rates of 99%-100% when employing gating and 68%-87% without gating in the experiments whereas for the MC simulations it was 100% with gating and 66%-82% without gating.Significance.This study demonstrated the ability of using deformable 3D dosimeters to measure dose perturbations in proton pencil beam deliveries caused by dynamic motion and deformation.

Dose-response of deformable radiochromic dosimeters for spot scanning proton therapy.

Intrafractional motion and deformation influence proton therapy delivery for tumours in the thorax, abdomen and pelvis. This study aimed to test the dose-response of a compressively strained three-dimensional silicone-based radiochromic dosimeter during proton beam delivery. The dosimeter was read-out in its relaxed state using optical computed tomography and calibrated for the linear energy transfer, based on Monte Carlo simulations. A three-dimensional gamma analysis showed a 99.3% pass rate for 3%/3 mm and 93.9% for 2%/2 mm, for five superimposed measurements using deformation-including Monte Carlo dose calculations as reference. We conclude that the dosimeter's dose-response is unaffected by deformations.