Med Phys. 2022 Mar 14. doi: 10.1002/mp.15597. Online ahead of print.
ABSTRACT
PURPOSE: Radiochromic material used in recent commercial films has been suggested as a candidate for in vivo dosimetry because of its dose sensitivity, real-time response, and atomic composition. It was observed that its sensitive material, Lithium Pentacosa-10,12-diynoate (LiPCDA), can have two distinct forms, with main absorbance peaks at ∼635 nm and ∼674 nm. The spectrum of the latter is similar to that of Pentacosa-10,12-diynoic acid (PCDA) used in the commercial predecessor, obtained through desiccation of the commercial film. Water was suggested to be a part of the crystal structure and thus its presence or absence would affect dosimetric parameters. The objective of this study was to: (a) investigate how desiccated commercial films compare to the native form in terms of macroscopic crystal structure, dose-response, signal linearity and post-exposure kinetics; (b) demonstrate proof of concept that the two versions can be combined into one optical dosimeter and measured simultaneously.
METHODS: Commercial radiochromic film, EBT-3, was desiccated for 10 days at 45°C. Using a 6 MV LINAC beam and standard set-up of 100 SAD, 10 cm x 10 cm field size, and 1.5 cm depth, commercial and desiccated films were irradiated to 50, 100, 200, 500, 1000, 2000, 3000 cGy and the latter to 4000, 5000 and 7000 cGy. A custom phantom equipped with optical fibres for real-time read-out was used for all measurements. Absorbance spectra were collected at ∼ 1 Hz before, during, and after irradiation. Data was collected for ∼1 hour after the end of irradiation for 200 cGy experiments. The radiation-induced change in optical density (∆OD) was calculated with a 10 nm band around the primary absorbance peak. The post-exposure percent optical density change was calculated and compared to ∆OD at the end of irradiation. Both commercial and desiccated films were also irradiated and measured simultaneously as proof of concept for using two materials within one optical path. For electron microscopy imaging, active material from commercial and desiccated films were imaged on a scanning electron microscope at an accelerating voltage of 10 kV.
RESULTS: Scanning electron microscope images showed that desiccated film was similar in topographical structure to the commercial EBT-3 form. It maintained a non-linear ∆OD with dose but resulted in ∼1/3 signal compared to the commercial film. Evaluation of post-exposure response showed significantly lower percent increase in ∆OD for desiccated film initially, with no statistically significant difference at 1 h after the end of irradiation. Combining both films and simultaneously measuring their absorbance illustrated that the two absorbance peaks were identifiable and resolvable to allow for an independent determination of dose from each.
CONCLUSIONS: Water is implicated in the crystal structure of the EBT-3 radiochromic film, with its removal through desiccation affecting both dosimetric and spectroscopic characteristics of the material. The two forms of radiochromic material (with and without water) are spectrally resolvable allowing for independent dose determination from each, opening up possibilities for dose measurements at different locations along a single fiber. This article is protected by copyright. All rights reserved.
PMID:35286716 | DOI:10.1002/mp.15597
