Comparative Analysis of Proton and Carbon-Ion Therapy for Head and Neck Cancer: LET-Dependent α/β Ratios in EQD2 and NTCP.

Proton therapy (PT) and carbon-ion therapy (CIT) differ not only in their physical dose characteristics but also in their biological effects, attributable to variations in fractionation protocols and linear energy transfer (LET). These differences make direct comparisons challenging. This study aims to comprehensively compare the treatment planning of protons and carbon ions for head and neck cancer by incorporating LET-dependent variable α/β ratios into the calculations of equivalent dose in 2 Gy fractions (EQD2) and normal tissue complication probability (NTCP). We retrospectively created treatment plans for nine head and neck cancer patients using proton therapy and carbon-ion therapy. Variable α/β ratios, calculated via the microdosimetric kinetic model, were integrated into EQD2 calculations. NTCP assessments for both late and acute toxicities utilized the Lyman-Kutcher-Burman model. In addition to comparing EQD2 and NTCP outcomes between protons and carbon ions, we conducted robustness evaluations that considered a 2 mm positioning error and a 3.5% range uncertainty. The α/β ratio for protons remained consistent at approximately 3 Gy across all tissues. In contrast, the α/β ratio for carbon ions varied significantly, reaching up to 20 Gy along the beam path. This variability in α/β ratios showed that carbon ions achieved lower EQD2 values for organs at risk (OARs) such as the brain, skin, and oral mucosa compared to protons. Furthermore, NTCP analysis revealed a significant reduction in acute toxicities, particularly dermatitis, with carbon-ion therapy. Both treatment modalities effectively minimized the probability of late toxicities. No significant differences in robustness were observed between proton therapy and carbon-ion therapy. EQD2 calculations incorporating variable α/β ratios revealed that carbon ions offer a dosimetric advantage over protons for certain OARs. However, this difference was relatively minor and did not suggest a clinically significant impact beyond acute toxicities.