Session Item

Silicone-based radiochromic dosimeters allow for dose read-out in 3D with high spatial resolution. Additionally, they can be deformed, thus motivating our development of this dosimeter into a protocol-specific verification tool for several tumour sites influenced by motion and deformation that will be treated with protons within controlled clinical trials, including breast and liver cancer. However, the increase in linear-energy-transfer (LET) with depth can lead to under response near the Bragg peak. Since the dosimetric properties of this system are highly dependent on material composition and curing conditions, we investigated the impact of these factors on the LET dependency.

Cuvette-sized dosimeters were fabricated from silicone elastomer, curing agent (CA), chloroform and leucomalachite green for two compositions, containing 5% or 9% CA. The dosimeters were cured in a light-sealed box at 20^oC for 1, 3 or 5 days. Proton irradiation was conducted with monoenergetic beamlets (80 MeV), and an entrance dose of 5, 10 and 15 Gy. Solid water (SW) slabs were placed around the dosimeters, including a 2 cm SW slab as a build-up. Dosimeters were scanned pre- and post-irradiation with a 1D optical scanner (wavelength: 635nm, 0.2 mm steps), with the change in optical density defined as OD=log10(I_pre/I_post), where I_pre/post is the laser intensity measured after the cuvette. Monte Carlo (MC) simulations in TOPAS were used to estimate dose (D_MC) and dose averaged LET  (LET_d) distributions, using an experimentally validated beam model. Finally, dose response (OD/D_MC) was estimated with linear regression for the three dose levels measured at 2 cm depth in the dosimeter and used to find the measured dose (D_meas). The quantity (1 - D_meas/ D_MC)*100 was chosen to quantify the LET dependency.

Most of the change happened already after 3 days of curing. For 5% CA dosimeters, LET dependency decreased from 42% for 1 day of curing (averaged in the LET interval 8.5-10 keV/μm) to 39% for 5 days, whereas for 9% CA it decreased from 41% for 1 day of curing to 26% for 5 days. The dose response (OD/D_MC) decreased by 13% from 1 to 5 days for 5% CA, and by 34% for 9% CA dosimeters.

Longer curing led to a reduction in LET dependency for both compositions. For the 5% CA, the reduction was minimal, and coupled with the decrease in dose response, there was no advantage in extending the curing process. For the 9% CA we also observed a decrease in dose response, but the significant reduction in LET dependency (more than 20%) could still be an advantage for some applications. Regardless, it will always be necessary to correct for the LET dependency of both dosimeter compositions.