Optimization in IMRT: EUD concept revisited for voxel based evaluation.

Purpose/Objectif: Treatment optimization comes to find the treatment parameters, such as map fluence for IMRT, which minimize a score function.The score function is a mathematical tool checking the relevance of a treatment plan according to the dose constraints for the target volume and normal tissues involved in the irradiation. Dose constraints are currently introduced inside commercial TPS by DVHs points. DVHs are 2D representation of 3D physical dose distribution and do not take into account the spatial information or the non-linear dose response of the volumes of interest.To overcome this lack of information, we are developing a biology based score function which takes into account the spatial information at the voxel level. Materials/Methods: The Equivalent Uniform Dose (EUD} concept is based on radiobiological principles and is a measurement of dose weighted for biological effects. Nevertheless, EUD is defined at the structure level and the spatial information is loss.We have developed a dedicated application to IMRT treatment plan evaluation and optimization within the MathLab framework.Spatial information and fractionation effect are taken into account by applying on dose matrices a filter based on local linear quadratic EUD (LQ EUD) values for each voxel. Compensation for hot and cold spots is a weakness for EUD models and it was compared by calculating the resultant LQ EUD.This limitation is overcome using this methodology. Results: The filter was applied on a dose distribution corresponding to a target volume of 93.2cm ~, prescription dose 44Gy.Hot spots and cold spots were defined according to ICRU report S0.The target includes 7 isolated cold spots (12-41 Gy): 6 smaller than lcm 3 (2.3% of the whole volume) and another one representing 3.7% of the volume. 69% of the volume is overdosed (47-60 Gy) leading to EUD and LQ EUD values of 48Gy.Filtered LQ EUD is less sensitive to the compensation for hot and cold spots. We found LQ EUD value of 45 Gy when the volume element analysis is 2cm 3. Conclusions: The calculation of local EUD is a way to summarize the strength of heterogeneous spots. Our analysis at the voxel level is a way to keep LQ EUD concept and to overcome DVHs restrictions.The introduction of a biological model could lead to better plans and the development of such treatment plan evaluation is the beginning of a biological based treatment optimization project