RADIATION PROTECTION OF PATIENT DURING STEREOTACTIC RADIOSURGERY. Part I: Single isocenter phantom measurements

Radiation dose to organs outside the radiotherapy treatment field can be significant and clinical interest. Therefore, in order to estimate peripheral doses (PD) during the stereotactic radiosurgery treatment with Leksell Gamma Knife for single isocenter treatment technique in particular as a part of the method can be described the all plan treatment in general. Knowledge of PD can be used effectively during the optimization of treatment planning in order to deliver a very accurate dose of radiation to well-defined target with minimal damage to surrounding healthy tissue and other organs. Moreover, PD can be limiting factor during the treatment of some non-malignant lesion or pregnant woman. Alderson-Rando phantom was used for PD measurements. The Dose of 40 Gy was delivered in a single fraction radiosurgery to midline hypothetical target volume situated close to the center of calva. Treatment planning for single target covered by 50% isodose of each collimator was performed with Leksell Gamma Plan (ver. 5,32) treatment planning system. The treatment plans for a single, 5, 10, 15, 20 and 25 shots, keeping the same position and the same doses were produced. Automatic positioning system (APS) was used for positioning of phantom during the treatment. The doses to different organs were measured during treatment by thermoluminescence dosimeter (TL). A simple exponential function can describe dependence of measured PD on distance from target for all collimator size were utilizing. Close to the target a scattering radiation within the patient was dominant, at distances beyond 40 cm from the target this component becomes less predominated and less dependence of the collimator size. While that the PD distances beyond 40 cm was mainly due to scattered radiation in the treatment room and leakage radiation. Contributions (DT) to the PD due the transportation of patient during the positioning were linearly increasing with the number of shots. DT does not practically contribute to the prescribed dose in target and to the laterally scattered radiation within patient. PD depends on collimator size and this dependence is more significant close to the target where it is proportional with the C3 and less significant far from the target where it is proportional with C only but more dependence on the number of shots. The number of shots as well as collimator size has influence on the magnitude of PD. Reduction of number of shots can decrease the PD for sites far from the target, on other hand the influence of collimator size is more significant for organs close to the target. Weighting the effect of the collimator size and number of shots can be used effectively during the optimization procedure to choose the most suitable treatment plan. 1 Faculty of Nuclear Sciences and Physical Engineering, Brehova 7, 115 19, Prague 1, Prague, Czech Republic.