(Sunday, 7/12/2020) 4:30 PM - 5:30 PM [Eastern Time (GMT-4)]
Room: Track 3
Purpose: The crystalline structure of the lithium pentacosa-10,12-dyinoiate (LiPCDA), used as the sensitive material in commercial Gafchromic films, can have two distinct forms: “hair-like” and “plate-like”. The objective is to investigate the relationship between the crystal structure of LiPCDA and dosimetric behaviour pertinent to its use in real-time optical dosimetry prototypes, namely sensitivity and polymerization kinetics.
Methods: We made both forms of LiPCDA monomer crystals in aqueous solutions and created the radiochromic films by coating the solutions onto Mylar film. The films were irradiated to 400 cGy with a 6 MV LINAC beam under standard conditions of 100 cm SAD, 10x10 cm2 field, and 1.5 cm depth. A custom Solid Water phantom with optical fibers and film holder, allowing for real-time measurements of light transmitted through the film during and after irradiation, was used. The transmitted photons as a function of time were collected at ~1 Hz during irradiation and for 1-hour thereafter. Radiation induced change in optical density (netOD) was calculated over the 10 nm band corresponding to the respective main absorbance peaks of each crystal structure with unirradiated film as the baseline. The post-exposure percent optical density change (%netOD) was calculated compared to netOD at the end of irradiation.
Results: The main absorbance peaks of the polymerized LiPCDA were 635 nm and 675 nm for “hair-like” and “plate-like” crystal structures, respectively. While the hair-like crystal of LiPCDA has absorbance spectra of commercial GafChromic EBT film, the plate-like crystal is similar to the older version, MD-55, which used pentacosa-10,12-dyinoic acid arranged in plates. Hair-like LiPCDA showed higher netOD per dose and lower %netOD compared to the plate-like crystal structure.
Conclusion: Location of main absorbance peaks, sensitivity to ionizing radiation dose and post-exposure increase in optical density depend on crystal structure of monomer rather than just on chemical formula alone.
Funding Support, Disclosures, and Conflict of Interest: Funding was provided by the Canadian Institutes of Health Research Project Grant PJT-162294
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