LET_d Optimization Verification With an SOI Microdosimeter

Purpose: A first of its kind experimental verification of dose-averaged linear energy transfer (LET_d) optimized treatment plans for proton therapy has been carried out using a silicon-on-insulator microdosimeter at the Massachusetts General Hospital (MGH), Boston, USA. Methods and Materials: Three clinical treatment plans of a typical ependymoma structure set were designed using the standard clinical approach, the proposed protocol approach, and a one-field approach. The plans were then reoptimized to reduce the LET_d-weighted dose in the brain stem. All six plans were delivered in a solid water phantom and the experimental y_D‾ measured. Results: After LET_d optimization, a reduction in y_D‾ was found within the brain stem by an average of 12%, 19%, and 4% for the clinical, protocol, and one-field plans, respectively, while maintaining adequate coverage of the tumor structure. The experimental LET_d-weighted doses were in agreement with the treatment planning system calculations and Monte Carlo simulations and reinforced the improvement of the optimization. Conclusions: This work demonstrates the first experimental verification of the clinical implementation of LET_d optimization for patient treatment with proton therapy.