Montecarlo simulation code in optimisation of the IntraOperative Radiation Therapy treatment with mobile dedicated accelerator

The principle of optimisation of the EURATOM 97/43 directive foresees that for all medical exposure of individuals for radiotherapeutic purposes, exposures of target volumes shall be individually planned, taking into account that doses of non-target volumes and tissues shall be as low as reasonably achievable and consistent with the intended radiotherapeutic purpose of the exposure. Treatment optimisation has to be carried out especially in non conventional radiotherapic procedures, as Intra Operative Radiation Therapy (IORT) with mobile dedicated LINear ACcelerator (LINAC), which does not make use of a Treatment Planning System. IORT is carried out with electron beams and refers to the application of radiation during a surgical intervention, after the removal of a neoplastic mass and it can also be used as a one-time/stand alone treatment in initial cancer of small volume. IORT foresees a single session and a single beam only; therefore it is necessary to use protection systems (disks) temporary positioned between the target volume and the underlying tissues, along the beam axis. A single high Z shielding disk is used to stop the electrons of the beam at a certain depth and protect the tissues located below. Electron back scatter produces an enhancement in the dose above the disk, and this can be reduced if a second low Z disk is placed above the first. Therefore two protection disks are used in clinical application. On the other hand the dose enhancement at the interface of the high Z disk and the target, due to back scattering radiation, can be usefully used to improve the uniformity in treatment of thicker target volumes. Furthermore the dose above the disks of different Z material has to be evaluated in order to study the optimal combination of shielding disks that allow both to protect the underlying tissues and to obtain the most uniform dose distribution in target volumes of different thicknesses. The dose enhancement can be evaluated using the electron back scatter factor (BSF) and comparing percent depth dose curves in different target volume thicknesses for disks of different Z. Since measuring BSF can be quiet complicated a Monte Carlo study was performed. The main goal of the paper is to study the optimal combination of shielding disks to be used in intraoperative radiotherapy (IORT) for a dedicated LINAC with a beam mean energy of 7.2 MeV. Simulated depth-dose curves without shields were compared with measured data obtained (1) with motorised diode in water phantom and with (2) gaf-chromic film in RW3 slab phantom. The simulated depth dose curve in presence of the shields was compared with preliminary gaf-chromic HS film data, obtained for a target volume of 2,0 cm thickness; the material of the disk in contact with the target volume is aluminium (Al), copper (Cu) or lead (Pb), while the disk below is always lead or copper in order to protect tissues below. Work is in progress for the comparison of all the simulated data with measured data for all the disk combinations at different target volume thicknesses.