High spatial resolution dosimetry with uncertainty analysis using Raman micro-spectroscopy readout of radiochromic films.

Abstract

The purpose of this work is to develop a new approach for high spatial resolution dosimetry based on Raman micro-spectroscopy scanning of radiochromic film (RCF). The goal is to generate dose calibration curves over an extended dose range from 0 to 50Gy and with improved sensitivity to low (<2Gy) doses, in addition to evaluating the uncertainties in dose estimation associated with the calibration curves. Samples of RCF (EBT3) were irradiated at a broad dose range of 0.03-50Gy using an Elekta Synergy clinical linear accelerator. Raman spectra were acquired with a custom-built Raman micro-spectroscopy setup involving a 500 mW, multimode 785 nm laser focused to a lateral spot diameter of 30µm on the RCF. The depth of focus of 34µm enabled the concurrent collection of Raman spectra from the RCF active layer and the polyester laminate. The preprocessed Raman spectra were normalized to the intensity of the 1614cm-1 Raman peak from the polyester laminate that was unaltered by radiation. The mean intensities and the corresponding standard deviation of the active layer Raman peaks at 696, 1445, and 2060cm-1 were determined for the 150×100µm2 scan area per dose value. This was used to generate three calibration curves that enabled the conversion of the measured Raman intensity to dose values. The experimental, fitting, and total dose uncertainty was determined across the entire dose range for the dosimetry system of Raman micro-spectroscopy and RCF. In contrast to previous work that investigated the Raman response of RCFs using different methods, high resolution in the dose response of the RCF, even down to 0.03Gy, was obtained in this study. The dynamic range of the calibration curves based on all three Raman peaks in the RCF extended up to 50Gy with no saturation. At a spatial resolution of 30×30µm2 , the total uncertainty in estimating dose in the 0.5-50Gy dose range was [6-9]% for all three Raman calibration curves. This consisted of the experimental uncertainty of [5-8]%, and the fitting uncertainty of [2.5-4.5]%. The main contribution to the experimental uncertainty was determined to be from the scan area inhomogeneity which can be readily reduced in future experiments. The fitting uncertainty could be reduced by performing Raman measurements on RCF samples at further intermediate dose values in the high and low dose range. The high spatial resolution experimental dosimetry technique based on Raman micro-spectroscopy and RCF presented here, could become potentially useful for applications in microdosimetry to produce meaningful dose estimates in cellular targets, as well as for applications based on small field dosimetry that involve high dose gradients.