SU‐C‐19A‐07: Influence of Immobilization On Plan Robustness in the Treatment of Head and Neck Cancer with IMPT

Abstract

Purpose:We evaluated the effect of interposing immobilization devices into the beam's path on the robustness of a head and neck plan.Methods:An anthropomorphic head phantom was placed into a preliminary prototype of a specialized head and neck immobilization device for proton beam therapy. The device consists of a hard low density shell, a custom mold insert, and thermoplastic mask to immobilize the patient's head in the shell. This device was provided by CIVCO Medical Solutions for the purpose of evaluation of suitability for proton beam therapy. See Figure 1. Two pairs of treatment plans were generated. The first plan in each pair was a reference plan including only the anthropomorphic phantom, and the second plan in each pair included the immobilization device. In all other respects the plans within the pair were identical.Results:In the case of the simple plan the degradation of plan robustness was found to be clinically insignificant. In this case, target coverage in the worst case scenario was reduced from 95% of the target volume receiving 96.5% of prescription dose to 95% of the target volume receiving 96.3% of prescription dose by introducing the immobilization device. In the case of the complex plan, target coverage of the boost volume in the worst case scenario was reduced from 95% of the boost target volume receiving 97% of prescription dose to 95% of the boost target volume receiving 83% of prescription dose by introducing the immobilization device. See Figure 2.Conclusion:Immobilization devices may have a deleterious effect on plan robustness. Evaluation of the preliminary prototype revealed a variable impact on the plan robustness depending of the complexity of the case.Brian Morse is an employee of CIVCO Medical Solutions.