Study and development of an innovative electron beam imaging system for dosimetry in IORT (Intra Operative Radiation Therapy)

Intra Operative Radiation Therapy (IORT) is a technique based on delivery of a high dose of ionising radiation to the cancer tissue, after tumour ablation, during surgery, while reducing the exposure of normal surrounding tissue. The Novac7 is a new linear accelerator expressly conceived to perform in the operating theatre. This accelerator supplies electron beams with high dose rate (up to 20 cGy/min). Because of these peculiar characteristics, classical dosimetric techniques are not able to give at once a real-time response and an extensive measure of the absorbed dose. In the framework of a research contract between ENEA (Italian National Agency for Environment and Alternative Energy Research) and the Physics Department of the University of Bologna, Italy, a new system has been developed to study and visualise the Novac7 electron beam. The system is conceived as a grid that consists of two bundles of scintillating optical fibers (SOF) over-crossing each other, optically coupled with two arrays of photodiodes as read-out system. The instrument samples the delivered beam at 100 Hz. A dedicated software collects the signals from the photodiodes and reconstructs the bi-dimensional image. Moreover it provides a series of dosimetric parameters. We studied an appropriate reconstruction algorithm searching for the best compromise between computational complexity and high precision. An accelerator ranging from 3 to 12 MeV has been used for the experimental tests. The various phases of design, development and characterisation of the instrument will be illustrated, as well as some experimental tests performed with the prototype. The system is able to give a real time response (within 30 s), while the resolution is of 1 mm. The response is linear versus dose and the measures are not affected by the high dose rate. The conclusions confirm the capability of the instrument to overcome problems encountered with classic dosimetry, showing that the results obtained strongly encourage the continuation of this research.