[P172] Establishment and transfer of a primary dosimetry standard for breast cancer IORT using the INTRABEAM(R) system

Purpose Intraoperative radiation therapy (IORT) using low-energy X-rays (⩽50 keV) delivers a concentrated dose by placing, during surgery, a miniaturized X-ray tube in contact with the treated tumor bed. In France, the most used IORT treatment system is the INTRABEAM one, manufactured by the Zeiss Company, which is delivered with a database used to assess the absolute dose distribution. This work aims at establishing traceability to a dosimetry reference independent from the manufacturer, as recommended in the report published in 2016 by the French National Authority for Health on the evaluation of such treatment. Methods Several types of IORT low-energy photon sources are commercially available and any metrology laboratory cannot afford to be equipped with all of them. So a multi-step methodology, applicable to any of these sources, was developed both to establish and transfer a dosimetry reference. This methodology removes the requirement to bring together, within the same laboratory, the source and the dedicated, but non-transportable, primary standard Free-Air-Chamber (FAC). Firstly, the photon energy spectrum of the commercial source is characterized at the hospital, then the spectrum is reproduced at the primary laboratory using a conventional X-ray generator. The reproduced beam is then characterized, in terms of air kerma rate, using a primary FAC and also used to calibrate a transfer cavity ionization chamber, which is in turn used to calibrate the INTRABEAM® system. Finally, a Monte Carlo model of the source is developed to calculate a conversion factor, from air kerma to absorbed dose to water under reference conditions. Results This methodology was applied to the INTRABEAM® system of a hospital associated with a 4 cm spherical applicator. The absorbed dose to water was established at a depth of 1 cm in water. The Zeiss Company recently changed its dosimetry procedure, going from the TARGIT method to the non-TARGIT one. Our value is higher by 33% and 16% respectively than those given by the TARGIT and non-TARGIT methods. Conclusions The present study shows, for IORT X-ray sources, the importance of making available to end-users independent dosimetry measurements traceable to national standards.