Feasibility of lattice radiotherapy using proton and carbon-ion pencil beam for sinonasal malignancy

Sinonasal malignancies are a treatment challenge because of their complex anatomy and close proximity to organs at risk (OARs). We aimed to investigate the feasibility of lattice radiotherapy (LRT) using pencil beam scanning (PBS) proton or carbon-ion beams in the treatment of sinonasal malignancies.A total of 10 patients with nonoperative and bulky sinonasal adenoid cystic carcinomas (ACC) were enrolled. Spherical vertices with a 1 cm diameter and average center-to-center (c-t-c) distance of 3.51 cm were delineated within the gross tumor volumes (GTVs). The prescription doses were 15 Gy[relative biologic effectiveness (RBE)] to the vertices and 3 to 3.5 Gy(RBE) to the periphery, delivered as clinical target volume boosts (CTVboosts) in 1 fraction. Photon, proton, and carbon-ion LRT plans were generated. Peak-to-valley dose ratios (PVDRs) and the doses delivered to the vertices, the CTVboost, and OARs were compared among the 3 plans.The mean PVDRmin values for the photon, proton, and carbon-ion LRT plans were 4.78 (range, 4.34 to 5.36), 4.82 (range, 4.15 to 5.37), and 4.69 (range, 4.31 to 5.28), respectively. The mean PVDRmean values for the same plans were 3.42 (range, 3.15 to 3.79), 2.93 (range, 2.19 to 3.74), and 3.58 (range, 3.09 to 4.68), respectively. There were no significant differences between the PVDRmin and PVDRmean values across the 3 LRT plans. Most critical organs were better protected in the proton and carbon-ion LRT plans than in the photon LRT plans. The photon LRT plans showed the highest maximum degree (Dmax) of vertices. Furthermore, these plans did not introduce more doses to the OARs compared to the 1-fraction clinical boost plan.Despite minimal differences in PVDR, proton and carbon-ion LRT plans can better protect OARs than photon LRT plans.