3D dosimetry of high intensity therapeutic electrons beams

The technique of High Dose Rate Intra-Operative Radiation Therapy (HDR-IORT) consists in the delivery of irradiation immediately following the removal of a cancerous mass, where the same incision is used to direct the radiation to the tumour bed. Given its particular characteristics, IORT requires dose measurements that are different from those requested in external radiotherapy treatments. The main reason lies in the fact that in this case a single high dose must be delivered to a target volume whose extension and depth will be determined directly during the operation. Because of this peculiar characteristics, until now there is not a dosimetric system able to detect the electron beam giving at once a realtime response and an extensive spatial measure of the absorbed dose. In this work we present the results obtained by using two orthogonal layers of a calorimetric phantom, the Dosiort, to measure the beam properties. We describe the main characteristics and some results of the Dosiort System, which is proposed within the framework of a research project of the INFN (Italian National Institute of Nuclear Physics). The final set-up is a solid phantom of density approaching 1 g/cm 3 with sensitive layers of scintillating fibres at fixed positions in a calorimetric configuration for the containment of electrons of energy 4-12 MeV. The prototype will be able to define the physical and geometrical characteristics of the electron beam (energy, isotropy, homogeneity, etc) and to measure the parameters needed to select the energy, the intensity and the Monitor Units (MU) for the exposition: Percentage Depth Dose; Beam profiles; Isodose curves; Values of dose per MU. In this paper we report in particular the measurement of the read-out dynamic range and the first qualitative study of the results which can be extracted from the measurements taken with the XY double layer in a test beam.