Impact of Variable RBE Modeling in Head and Neck IMPT and Dynamic ARC Proton Therapy

Document Type

Conference Proceeding

Publication Date

9-2026

Publication Title

International Journal of Particle Therapy

Abstract

Background and Aims: In proton therapy for head and neck cancer, using a constant relative biological effectiveness (cRBE) of 1.1 may underestimate the biological dose, especially in high linear energy transfer (LET) regions. This study evaluated the impact of two variable RBE models—the McNamara model (MM) and the Wedenberg model (WM)—on target coverage and organ-at-risk (OAR) dose metrics between intensity-modulated proton therapy (IMPT) and DynamicARC Proton (DARC) techniques. Methods: Ten bilateral head and neck cancer (diagnosis oropharyngeal cancer) patients were retrospectively planned in RayStation (Research version 2024A) using IMPT (three-field) and DARC: (two-partial-arcs) techniques. Prescription was 70 Gy(RBE) in 35 fractions with three dose levels (CTV_7000, CTV_5950, CTV_5600). All plans were robustly optimized (setup:±3mm; range uncertainty:±3%) and calculated using cRBE = 1.1 and the two variable RBE models (MM and WM), applying α/β ratios of 10 for targets and 3 for OARs. Target endpoints included CTV_7000 D95 and D0.03cc; OAR endpoints included Dmean to the oral cavity, ipsilateral-parotid, and contralateral-parotid. Results: For CTV_7000 D95, MM increased dose by 5.62 ±0.16 Gy(RBE) with IMPT and 5.30±0.35 Gy(RBE) with DARC; WM increases were 5.04±0.25 Gy(RBE) and 4.55± 0.54 Gy(RBE), respectively. For D0.03cc, MM increased dose by 6.73±0.78 Gy (RBE) (IMPT) and 6.86±0.59 Gy(RBE) (DARC); WM by 6.51 ±1.08 Gy(RBE) (IMPT) and 6.69±0.81 Gy(RBE) (DARC). For OARs, MM increased Dmean to the oral cavity by 5.81±1.44 Gy(RBE) (IMPT) and 5.57±1.57 Gy(RBE) (DARC), to the ipsilateral-parotid by 6.13±1.03 Gy(RBE) (IMPT) and 4.82±1.51 Gy(RBE) (DARC), and to the contralateral-parotid by 4.64±0.78 Gy(RBE) (IMPT) and 3.37±1.33 Gy(RBE) (DARC). WM yielded slightly smaller but comparable increases relative to cRBE. When comparing IMPT and DARC under variable RBE models, DARC consistently yielded lower Dmean to OARs. Under the MM, DARC reduced Dmean to the oral cavity by 2.72 ± 3.35 Gy(RBE), the ipsilateral-parotid by 4.69 ± 2.76 Gy (RBE), and the contralateral-parotid by 5.99 ± 1.37 Gy(RBE). Comparable reductions were observed with the WM: 2.64 ± 3.38 Gy (RBE), 4.88 ±2.92 Gy(RBE), and 6.07 ±1.60 Gy(RBE), respectively. Conclusions: For both IMPT and DARC, variable RBE substantially alters reported target dose metrics and increases OAR dose relative to cRBE. Under both variable RBE models, DARC demonstrated lower Dmean to the parotids and oral cavity compared with IMPT. These findings support incorporating LET considerations into plan evaluation and motivate further investigation of LET-based optimization and planning strategies for head and neck cancer.

Volume

21

Issue

Suppl

First Page

101636

Last Page

101636

Comments

PTCOG 64 Particle Therapy Co-Operative Group 64th Annual Conference, June 8-13, 2026, Deauville, France

DOI

10.1016/j.ijpt.2026.101636

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