Background: Adenoid cystic carcinomas (ACCs) are extremely rare, but they are a treatment challenge because they are highly radioresistant and are surrounded by several 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 ACC.
Patients and methods: Ten patients with nonoperative, bulky, and radioresistant ACCs were enrolled. Spherical vertices with a 1-cm diameter and average centre-to-centre distance of 3.51 cm were delineated within the gross tumour volumes. The prescription was 15 Gy (relative biological effectiveness [RBE]) to vertices and 3–3.5 Gy (RBE) to the periphery as clinical boost target volumes (CTVboosts) in one fraction. Photon, proton, and carbon-ion LRT plans were generated based on this. Peak-to-valley ratios (PVDRs) and dose delivered to the vertices, CTVboost, and OARs were compared among the three plans. The OAR doses were also compared between LRT plans and one fraction of clinical boost plans.
Results: The mean for photon, proton, and carbon-ion LRT plans were 4.78 (range: 4.34–5.36), 4.82 (range: 4.15–5.37), and 4.69 (range: 4.31–5.28), respectively. The mean for the same plans were 3.42 (range: 3.15–3.79), 2.93 (range: 2.19–3.74), and 3.58 (range: 3.09–4.68), respectively, with no significant differences in both. The brain stem, chiasm, optic nerve, parotids, spinal cord, and brain were better protected in proton and carbon-ion LRT plans than in photon LRT plans. Further, these plans did not introduce more doses to the OARs compared to the one-fraction clinical boost plan, with a significant difference.
Conclusion: Despite the minimal difference in PVDR between proton and carbon-ion LRT plans and photon LRT plans, the former can better protect OARs than photon LRT plans. Therefore, PBS proton and carbon-ion LRT can be used for sinonasal ACC.