PhD abstract

Electronic Brachytherapy (eBT), also called contact radiotherapy, is a cancer treatment technique using low energy X-Rays (≤ 50 keV) generated by X-Ray tubes which are placed in close contact with the treated lesions. The latest evolutions of miniaturized X-Ray tubes led to the development of new treatment systems, such as the INTRABEAM® system of the ZEISS Company which is the most available eBT system and the only one currently used in France. Beside its medical benefit, the potential major advantages of treatment by eBT are the drastic decrease in patient discomfort and treatment cost. In the case of breast cancer treatment with such technique, the treatment is given in a single session that lasts 20 to 50 minutes where a high dose, in the order of 20 Gy, is delivered to the tumor bed surface in contact with spherical applicators associated to the X-Ray source. The delivered dose decreases rapidly with depth (< 1 Gy after a few centimeters) enabling to preserve neighboring healthy tissues. In France, the first IORT treatment performed was in Nantes in 2011. Today, ten medical centers offer IORT treatment using the INTRABEAM® system. Consequently, several medical physicists addressed to the French national metrology laboratory for ionizing radiation (LNHB) their need for a dosimetric traceability with a reference independent from the manufacturer. This need was reaffirmed by the French Authority for Health (HAS), in their report on the evaluation of the IORT for breast cancer treatment published in April 2016. This thesis work is a contribution to the metrological work initiated by LNHB for enhancing the safety of employing IORT by eBT systems. It was limited, within the thesis period, to the INTRABEAM® system associated with a 4 cm diameter spherical applicator. The thesis work was oriented towards three main objectives. The first one concerned the establishment and the transfer of a primary dosimetric standard, in terms of absorbed dose to water at 1 cm depth in water. The methodology was developed and applied on the INTRABEAM® system with 4 cm spherical applicator, for which, the dosimetric reference was established. The second objective was to use a dosimetric gel and the Monte Carlo method to assess the 3D spatial distribution of the relative absorbed dose delivered by such a system. The dosimetric gel system used was a Fricke-based hydrogel read by Magnetic Resonance Imaging at Service Hospitalier Frédéric Joliot in Orsay (SHFJ). The gel reading was calibrated, in terms of absorbed dose for low energy X-Rays (< 50 keV), and then used to define the relative dose distributions of the INTRABEAM® X-Ray source associated with the 4 cm spherical applicator in the axial and transverse planes of the X-Ray source probe tip. The last objective was to compare the dosimetric data delivered by Zeiss, for the INTRABEAM® system used at St. Louis hospital in Paris, by the ones obtained in the current study for the same system. Significant discrepancies were found from this comparison between the doses delivered by Zeiss and those obtained in the current study. Discrepancies were also observed in a separate work conducted by the PTB under a different INTRABEAM® configuration. Some reasons of these discrepancies are outlined and discussed in this study.

Key words

intrabeam, contact X-Ray therapy (CXRT), axxent, electronic brachytherapy, low-energy X-Rays, primary standards

PhD thesis