Transmission beam planning for improved robustness and efficiency in proton therapy for head and neck cancer

Background and purpose

Optimizing head and neck cancer (HNC) plans with single-energy proton beams that fully traverse the patient (transmission beams) can improve robustness and delivery efficiency, complementing conventional approaches.

Materials and Methods

Experimental measurements, validated with Monte Carlo (MC) simulations, were carried out on a uniform water-equivalent plastic phantom (RW3) containing a metal component (2-Euro coins) irradiated with a single high energy proton field (228.6 MeV) to verify the transmission beam concept. 28 nasopharyngeal cancer (NPC) intensity modulated proton therapy (IMPT) were then optimized with nine coplanar single-energy fields (228.6 MeV), positioning the Bragg peaks well beyond the patient body, so called transmission beam mode. These plans (IMPT-TB) were compared to conventional IMPT and volumetric modulated arc therapy (VMAT) photon plans in terms of dose distributions quality, expected organ at risk (OAR) toxicity, robustness and delivery time.

Results

Transmission beams minimized dose perturbation by metal objects (∼7% max relative variation at 18 cm depth). IMPT-TB plans achieved comparable dose distribution and expected toxicities to IMPT, increasing the dose bath (+96 % vs. IMPT) but remaining significantly lower than VMAT (−31.4 %). For 94 % of patients (N = 26), IMPT-TB met at least one additional dose constraint that the corresponding IMPT plan failed to satisfy. Moreover, in the analyzed subgroup (N = 5), IMPT-TB plans delivered with our synchrotron exhibit a 67 % reduction in beam time compared to IMPT plans.

Conclusions

IMPT-TB plans demonstrated enhanced robustness and significantly faster delivery compared to IMPT. Transmission beams could be clinically implemented, also in conjunction with standard IMPT, for proton radiation treatment of NPC.