Polymer gel dosimetry - Using MRI-readout, simplified relatively non-toxic mixing procedure tested on FLASH

Abstract: Background: One of the polymer gel dosimeters available today is the N-isopropylamide (NIPAM) polymer gel dosimeter which has a less toxic monomer compared to other polymer gels and the advantage of being able to be manufactured under normal oxygen levels. Polymer gel dosimeters in general are advantageous independent dosimeters to use for verification of new radiation treatment techniques due to its favorable qualities such as very high resolution, 3D coverage, tissue equivalence and independence of energy and incident direction of the radiation beam. One of the new and upcoming treatment techniques today where there is a desire for an independent, high resolution 3D detector is FLASH, an irradiation technique where the radiation is delivered using ultra high dose rate of 40 Gy/s or more. The aim of this thesis was to start up a gel dosimetry laboratory and to investigate the feasibility of creating a NIPAM gel dosimeter with linear dose response using magnetic resonance imaging (MRI) readout, using relatively non-toxic ingredients and a simplified mixing procedure. An additional aim was to investigate the gel dose response when irradiated with FLASH. Method: NIPAM polymer gels were manufactured using tap water and concentrations of 5 % w/w (weight concentration) gelatin from porcine skin, 3 % w/w NIPAM, 3 % w/w N,N’-methylene-bis-acrylamide (BIS) and 21-26 mM tetrakis-hydroxymethyl-phosphonium-chloride (THPC). Vials containing the gel were irradiated with either 220 kV photons, 10 MeV electrons at conventional doe rates, or FLASH about 24 h after manufacturing. The vials were irradiated with doses up to around 40 Gy. The gel dose response was assessed through its R2 relaxation rate by acquiring T2 weighted MRI images of the vials approximately 24 h after irradiation. The doses delivered to the vials were calculated based on previous output measurements made on the specific machine or measured using film dosimetry. Results: The results show that intra-batch variations, with respect to the dose response reproducibility, are small with a standard deviation (SD) of the R2 relaxation rate between 0.010-0.021 s-1 for doses up to 10 Gy and with R2 values between 1.405 and 2.231 s-1. However, inter-batch variations are significantly larger with relative difference up to 19 %. The gel exhibits linearity (R2 0,98) of the dose response up to 28 Gy when irradiated with 220 kV photons, up to 23 Gy when irradiated with 10 MeV electrons at conventional dose rates and up to 27 Gy when irradiated with FLASH. Additionally, the results indicate a lower gel dose response when irradiated with FLASH compared to irradiation with 220 kV photons or 10 MeV electrons at conventional dose rates. Conclusions: It is feasible to use relative non-toxic ingredients (NIPAM), MRI-readout and a simplified mixing procedure with tap water and under normal levels of oxygen to obtain a gel dosimeter with linear dose response. The small intra-batch variations indicate very high dose response reproducibility while the larger inter-batch variations underline the need for calibration of each gel batch. The gel exhibited linearity of the dose response up to 23 Gy for all three radiation beam types used. A lower gel dose response was observed when irradiated with FLASH compared to irradiation with 220 kV photons or 10 MeV electrons at conventional dose rates.