Room: ePoster Forums
Purpose: To improve dosimetry and penumbra, we implemented a helmet shaped range shifter, or Range Shifter Helmet (RSH), for treating head and neck tumors with spot-scanning proton beam therapy. We describe the commissioning and dosimetric assessment of the RSH within our practice.
Methods: Three sizes of 3D-printed RSH were designed to treat patients of all sizes. To evaluate the helmetsâ€™ radiological properties, homogeneity was assessed by analyzing Hounsfield Unit variation. An in-house proton range measurement device was used to establish the relative stopping power (RSP). Spot lateral penumbrae were compared for three energy absorbers: RSH, Extended Range Shifter (ERS) placed in the treatment nozzle, and patient specific bolus (PSB) composed of Superflab. Additionally, Single Field Optimized (SFO) plans were calculated on Pixy Phantom to examine the influence of device-based setup uncertainties.
Results: The devices showed no radiological aberrations and had homogeneous material composition. The HU of the devices was assigned in the planning system based on the measured RSP of 1.053. Spot size (SS) comparison, defined as full width at half maximum (FWHM), between RSH, ERS, and PSB shows SS of RSH dosimetry to be 53.2% smaller than ERS and within 6.5% of PSB (Figs. 1&2). Relative shifts between the device and phantom of up to 3mm imposed DVH detriments of less than 1% at D95 for the target volume of an SFO plan. The worst case scenario (anterior vertex beam) caused perturbation of 3% in the D95 target coverage following a 6mm shift (Fig. 3).
Conclusion: Measurements and analyses showed RSH to be robust for the beam angles used to treat most targets in the head and neck regions. Additionally the dosimetry of the RSH-based solution is advantageous over other clinical solutions as it improves ease of use and dosimetry. Acknowledgments to Mayo Clinic Rochester for development of RSH.