24. Reproduction of some INTRABEAM® spectra using a standard X-ray generator

Abstract

Introduction Contact Radiotherapy by X-Ray Generator (CR-XRG) is a cancer treatment technique using low energy photons (⩽50 keV). The irradiation is done in contact with the tumor so it receives a high dose while preserving neighboring healthy tissues. The eventual goal of this work is to establish appropriate primary standards, in terms of air kerma, K air , and absorbed dose to water, D w , in order to calibrate the relative spatial distribution of absorbed dose. As a first step, the present study focusses on the characterization and reproduction of two beams employed in CR using a standard XRG available at LNHB. Since the INTRABEAM®, with spherical applicators, is widely used for breast cancer treatments, two of its emitted beams are considered here. Methods INTRABEAM® Spectra were measured using a CdTe detector highly collimated to reduce the photon flux. Standard XRG and chosen filters were used to reproduce the two INTRABEAM® spectra that comprise a continuous part and fluorescence peaks. The reproduced beams were then measured under the same conditions as the original ones. All spectra were corrected for measurement artefacts [1] . To quantify the reproduction accuracy, the following quantity that calculates the K air for a given normalized photonic spectrum is introduced: K air , norm = ∑ o ( E ) μ tr ( E ) ρ air . E . Δ E ∑ [ o ( E ) . Δ E ] . The K air , norm ratio between the original and reproduced spectra was compared to unity. To check the consistency of the approach, K air , MC values were also calculated using the MCNPX code [2] for the original and reproduced spectra. Results K air , norm and K air , MC ratios are less than 1.005 for the spherical applicator of 3 cm in diameter. However, these ratios are around 1.05 for the spherical applicator of 4 cm due to the presence of higher intense gold L-shell fluorescence peaks (induced by the INTRABEAM® anode made in gold). An attempt failed to reproduce these peaks, by introducing gold foils to the filter, because of their low fluorescence yield. Conclusions Whilst this study did not confirm the possibility to reproduce all kind of spectra, it did partially substantiate the method pursued. Further attempts to better reproduce these spectra will exploit the fluorescence tungsten peaks that are close to the gold ones. This research will serve as a base for future studies involving other INTRABEAM®’s applicators or CR systems.