Simulated versus realistic intra operative radiation therapy (I.O.R.T.) treatment in operating room: from knowledge of stray radiation to action

Intra-Operative Radiation Therapy (I.O.R.T.) is carried out with electron beams produced by a Linac (Linear Accelerator) generally used for conventional radiotherapy with external beam, or by dedicated accelerators that can be employed directly into an operating room. I.O.R.T. refers to the application of radiation during a surgical intervention, after the removal of a neoplastic mass. I.O.R.T. uses on the tumour area a direct irradiation, for the possible localisation of sub-clinic illness or macroscopic residue in the case of non-radical resection. Intra-Operative Radiotherapy foresees a single session only, generally preceded or followed by radiotherapy with external beam. It allows the achievement of a selective radiation boost on the tumour volume. In some cases, it can also be used as a one-time/stand alone treatment in initial cancer of small volume, or in unresectable malignancies for palliative purpose. The technical advantages of I.O.R.T. consist in the direct visual control of the target volume, and in the possibility to protect the healthy tissues by moving them away from the path of the radiation beam. The use of electron beams allows the administration of a homogeneous dose to a selected layer of tissues surrounding the tumour. The following professional staff forms the Operative Group: radiation oncologist, surgeon, anaesthetist, medical physicist, radiation technologist, nurse.The choice of a simulation geometry very similar to the clinical situation allows to evaluate radioprotection data very close to the real situation. For a fixed layout, an anthropomorphic phantom was positioned on the operating bed and a breast I.O.R.T. treatment was simulated positioning all the accessories of the operating room in their typical positions. A detailed dose mapping was performed with a Victoreen 450P ionisation chamber and with environment film-dosimeter on the walls of the operating room during the simulation of the clinical treatment. The simulation appears to be accurate and the simulation layout presents no significant differences compared to that of the clinical practice. Therefore, from a dosimetric point of view, there should not be any significant difference between the radiological risk evaluation performed during the acceptance of the accelerator and the effective risk for the operators and the public. At this moment sufficient dosimetric data obtained with the film dosimeters during the clinical practice are not available to perform a detailed comparison between the simulation and the realistic treatment; the clinical practice has just began and the first environment dosimeters will be analysed within two months. During the clinical practice measurements with the ionisation chamber Victoreen 450P were performed and these preliminary measurements confirmed that the simulations approximates very well the clinical situation. The investigation of the dosimetry during clinical practice is in progress and the comparison with the simulated data will also give an indication on the reproducibility of the barrier positioning during treatments. Furthermore, the knowledge of the detailed ambient dose distribution gives the information necessary to position the barrier efficiently, on the basis of the occupation factors of the surrounding of the operating room. Setting correctly the vertical barriers as by radioprotection prescription, the areas nearby the operating room (considering appropriately the occupation factor) can be considered as 'free zone' (which there is no risk for the operators to exceed 1 mSv of annual effective dose)