P264] Time resolved dose rate distributions in radiosurgery and their impact on biological effectiveness of treatment

Abstract

Purpose To obtain time-resolved dose rate distributions (TR-DRD) in radiosurgery applications, and evaluate the impact of TR-DRD on the biological effectiveness of treatment. Methods Treatment plans of 74 acoustic schwannoma patients treated with Leksell Gamma Knife® C or PERFEXION™ were meta-analyzed. The 3D dose distribution within the Planning Target Volume (PTV) for each patient was obtained from the superposition of relevant data for each delivered GK-shot. Single shot dosimetry calculations were based on the nominal dose rate at the center of each shot and corresponding relative dose profile data. TR-DRD were estimated neglecting 60Co source decay during the irradiation of each shot. A formalism including frequently used radiobiological tools that accounts for the effect of dose rate and incorporates fast and slow components of repair, was developed on the basis of the Biologically Effective Dose (BED). Patient and plan characteristics as well as dose and dose rate indexes incorporating the TR-DRD were introduced and their correlation with the corresponding radiobiological parameters was examined using the Spearman rank test. Results The nominal mean dose rate at focus (NMDR-F) presented a high correlation with the percentage of the total dose delivered with dose rates lower than 1 Gy/min, TD 1 Gy / min , ( r = - 0.837 and p = 5.32 E - 20 ) and the percentage of the PTV volume receiving dose rates lower than 1 Gy/min from all shots, V 1 Gy / min ( TR-DRD ) ( r = - 0.830 and p = 6.43 E - 20 ). A statistically significant correlation was also found between the number of iso-centers used per PTV volume (Unit Isocenter, UI) and the V 1 Gy / min ( TR-DRD ) ( r = 0.268 and p = 0.021 ) as well as the percentage of the PTV volume receiving dose values lower than 15 Gy, V 15 Gy ( r = 0.543 and p = 5.90 E - 07 ). The Equivalent Uniform BED (EUBED) was significantly correlated with the UI ( r = - 0.4318 and p = 0.0001 ), the UI/NMDR-F ( r = - 0.3955 and p = 0.0005 ), the V 1 Gy / min ( TR-DRD ) ( r = - 0.3155 and p = 0.0062 ) and the V 15 Gy ( r = - 0.569 and p = 1.25 E - 07 ). Conclusion The observed correlation between the TR-DRD based parameters and the EUBED suggests that the former should be taken into account at the planning stage of radiosurgery, either directly or indirectly via plan characteristics readily evaluated in every day clinical routine (NMDR-F, UI) that are correlated with TR-DRD based dose and dose rate parameters.