Session Item

Radiochromic films, widely used in dosimetry, suffer from a decrease in relative efficiency (RE) when exposed to high-LET radiation. This effect is particularly relevant for beams of heavy ions and low-energy protons and must therefore be accounted and corrected for. RE calibrations published in the literature are not entirely consistent with each other, mainly due to the different methods used for estimating the LET of the beams. We propose a common framework for integrating available data and back it up with experimental measures using a 10-MeV proton beam. 

Literature data on relative efficiency for films of the EBT family was extracted from 10 different sources. Reported LET values were recalculated using Monte Carlo to extract dose-averaged, in-water average beam LET values in the active layer of the film (Figure 1, left panel). Additionally, 60 experimental data points for proton energies in the 1-10 MeV energy range were measured for each of the three film types under study: EBT2, EBT3 and unlaminated EBT3 (Figure 1, right panel). 

Measured values were consistent with recalculated literature values. Combined literature and measured data in the LET range of 5-80 keV/µm was fitted to a two-parameter model with expression RE(LET) = 1 – a · LET^b. The results of the fit support the hypothesis of a linear decrease of RE with LET, with no remarkable differences between the three types of films analyzed.

Fig 1. RE vs LET in active layer from different published datasets (left) and measured values (right). Marker color indicates type of film of the set: purple=EBT1, red=EBT2, blue=EBT3 and green=Unlaminated EBT3. Confidence bands represent 1-sigma confidence interval derived from the fit parameters.

An expression was derive to employ calibrated radiochromic films with low-energy protons taking LET quenching into account. The final relative error in RE using our models for a given beam energy and air gap ranges between 2% and 20%.