Dosimetry of high intensity electron beams produced with dedicated accelerators in Intra-Operative Radiation Therapy (IORT)

The technique of High Dose 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 volume target whose extension and depths will be determined directly during the operation. Since the possibility of devising a treatment plan using a TPS (Treatment Planning System) is not available, it is necessary to know the physical and geometric characteristics of the beam. Defining the physical characteristics of the beam entails both measuring the delivered dose and defining (monitoring) procedures. In any case a much higher dose will be released than occurs with conventional external accelerators. The ionization chamber recommended by the standard protocols for radiotherapy cannot be used because of the ion recombination inside the gas. In this work we propose the use of a calorimetric phantom, the Dosiort, to measure the beam properties. We describe the main characteristics and some preliminary 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 set-up is a solid phantom of density approaching 1 g/cc with sensitive layers of scintillating fibres at fixed a position 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 (quality, 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 for MU.