Radiation Protection in the Commissioning and the use of a iort-dedicated mobile linac

Iort (Intra-Operative Radiation Therapy) is a radiotherapy treatment technique consisting in the administration during a surgical intervention, of a single and high radiation dose (up to 30 Gy) to the tumour bed/environment, after the surgical removal. the main objective of IORT is to increase both the probability of local control of the tumour and the therapeutic ratio between local control of the tumour and tolerance of the adjacent healthy tissues and organs: this goal can be achieved through a better definition of the target volume and the displacement and/or screening of the organs and tissues at risk during the surgical intervention. IOT is not a new treatment technique: it had been proposed in 1909 at the beginning of radiotherapy, when only X-ray orthovoltage therapy equipment (120-250 kV) were available, and the results obtained did not justify the development and a widespread use of this technique. The main reason for this situation lied in the unfavourable depth dose distribution of the X-ray beams. When, in the years after 1970, the new linear accelerators (linacs), able to produce electron beams with energies >4 MeV were introduced in the clinical use, IORT was against taken into serious considerations by radio therapists and surgeons. Abe et al, demonstrated that IORT could offer great advantages over conventional radiotherapy with external beams, not only for tumours located nearby critical organs and tissues, but also for locally advanced tumours, difficult to be controlled by surgery or external radiotherapy alone, thanks to its possibility of delivering high radiation doses to the regions of possible macroscopic local diffusion of tumour cells. This possibility is offered by an appropriate choice of the energy, and consequently of the range in the tissues, of the electron beams. As a simple mnemonic rule, we remember that the maximum range in water (and therefore in the soft tissues) of the electron beams, if expressed in cm, is approximately equal to half of the electron beam energy, expressed in MeV, and that the useful range, or therapeutical range (corresponding to a DPP of 80%), always expressed in cm, is approximately equal to 1/3 of the electron beam energy, always expressed in MeV. From this considerations, it is clear that the availability of electron beams with initial energy in the range between 4 and 15 MeV allows to treat with IORT any target, up to a thickness of approximately 5 cm. the main reason why IORT with conventional linacs did not have a great diffusion, despite of its theoretical advantages, was the need of one of these two radical choices: the installation of an operating room inside the bunker of a linac or the transport of the patient, under anaesthesia, from the operating room to the bunker of the linac and back to the operating room after the irradiation: it is clear that both these two choice create a lot of problems and difficulties. These difficulties have been solved a few years ago with the development and the introduction in the clinical use of mobile linacs producing only electron beams, that can easily be transported and employed directly in the operating room. It is clear that the use of such linacs in a space not specially designed sets important problems of radiation protection, that must be analysed and solved specifically for each situation. At the beginning of 1999 a mobile Linac (Novac 7, manufactured by Hitesys, Aprilia, Italy) able to produce electron beans in the energy range 4.5-9 MeV became available at the European Institute of Oncology, Milano, for being employed in traditional operating rooms, especially for the IORT of early breast cancer patients. (Author