Proposed linear energy transfer areal detector for protons using radiochromic film

Abstract

Radiation therapy depends on predictably and reliably delivering dose to tumors and sparing normal tissues. Protons with kinetic energy of a few hundred MeV can selectively deposit dose to deep seated tumors without an exit dose, unlike x-rays. The better dose distribution is attributed to a phenomenon known as the Bragg peak. The Bragg peak is due to relatively high energy deposition within a given distance or high Linear Energy Transfer (LET). In addition, biological response to radiation depends on the dose, dose rate, and localized energy deposition patterns or LET. At present, the LET can only be measured at a given fixed point and the LET spatial distribution can only be inferred from calculations. The goal of this study is to develop and test a method to measure LET over extended areas. Traditionally, radiochromic films are used to measure dose distribution but not for LET distribution. We report the first use of these films for measuring the spatial distribution of the LET deposited by protons. The radiochromic film sensitivity diminishes for large LET. A mathematical model correlating the film sensitivity and LET is presented to justify relating LET and radiochromic film relative sensitivity. Protons were directed parallel to radiochromic film sandwiched between solid water slabs. This study proposes the scaled-normalized difference (SND) between the Treatment Planning system (TPS) and measured dose as the metric describing the LET. The SND is correlated with a Monte Carlo (MC) calculation of the LET spatial distribution for a large range of SNDs. A polynomial fit between the SND and MC LET is generated for protons having a single range of 20 cm with narrow Bragg peak. Coefficients from these fitted polynomial fits were applied to measured proton dose distributions with a variety of ranges. An identical procedure was applied to the protons deposited from Spread Out Bragg Peak and modulated by 5 cm. Gamma analysis is a method for comparing the calculated LET with the LET measured using radiochromic film at the pixel level over extended areas. Failure rates using gamma analysis are calculated for areas in the dose distribution using parameters of 25% of MC LET and 3 mm. The processed dose distributions find 5%-10% failure rates for the narrow 12.5 and 15 cm proton ranges and 10%-15% for proton ranges of 15, 17.5, and 20 cm and modulated by 5 cm. It is found through gamma analysis that the measured proton energy deposition in radiochromic film and TPS can be used to determine LET. This modified film dosimetry provides an experimental areal LET measurement that can verify MC calculations, support LET point measurements, possibly enhance biologically based proton treatment planning, and determine the polymerization process within the radiochromic film.

Patient-Specific QA of Spot-Scanning Proton Beams Using Radiochromic Film

“…Radiochromic film can be used as a reference dosimeter for biomedical experiments with low-energy proton beams if appropriate LET corrections are applied [36]. There are numerous researchers re-ported that under-response of radiochromic film in the presence of proton SOBPs which could be defined as “quenching effect” of the dosimeter in the region [26–33,35–37]. “Quenching” is said to occur when, for a given physical dose, a dosimeter not limited to radiochromic film gives a lower output reading for radiation with high LET than for the same dose of low LET radiation.…”

“…Calibration films were irradiated at 18, 20, and 23 cm while the chamber array was irradiated at nearly identical depths of 18, 20 and 24 cm. The LET dependence of radiochromic films has been thoroughly investigated [6,18,22,26–33] and it has been established that the effect is highest for lower (< 11 MeV) proton energies [28]. However, for spot scanning beams, there may be a larger distribution of energies used to irradiate the PTV which could make accounting for LET corrections quite variable.…”

Comparison of linear energy transfer measurement for therapeutic carbon beam using CR-39 and TLD

“…Currently, many methods for LET measurements had been developed using different detectors, including dual ionization chambers (Tegami et al 2017), thermoluminescence detector (TLD) (Parisi et al 2019, Shimomura et al 2020, CR-39 (Hirano et al 2018), radiographic films (Mayer et al 2015), gel dosimeters (Maeyama et al 2015), optically stimulated luminescence dosimeters (OSLD) (Granville et al 2014a, 2014b, Christensen et al 2022, tissue equivalent proportional counter (Bianchi et al 2020), silicon on insulator dosimeter (Chacon et al 2020, Samnøy et al 2020, James et al 2021, Lee et al 2021 and so on. As most of these methods were still in the experimental exploration, few comparative studies between methods had been reported.…”

Technical note: Determination of proton linear energy transfer from the integral depth dose

“…Their method does not measure LET directly using its physics definition instead measures the ratio of ultraviolet to blue emission intensities as a surrogate of LET, and uses MC simulations to establish a relationship between the surrogate parameter and LET. A few other methods are available for proton LET measurement, [10][11][12][13][14][15] but all similarly employ surrogates and require MC simulations to calibrate the surrogates and LETs.Lineal energy,the energy imparted at each event in a volume of interest divided by the mean chord length in that volume, 16 is another surrogate of LET and can be measured with microdosimeters. 17,18 MC has been used to simulate the energy-dependent mean chord length in the detector sensitive volume.…”