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Upright positioning enhances beam angle optimization and organ sparing in head and neck carbon-ion radiotherapy with
Shiya Huang1,2,3,4, Liwen Zhang5,2,3, Rong Zhou1
1Department of Nuclear Medicine, Shanghai Proton and Heavy Ion Center, Fudan University Cancer Hospital, Shanghai, China.
Background:
Carbon-ion radiotherapy (CIRT) for head and neck tumors is typically delivered in the supine posture using fixed beam lines, which limits beam angle selection. Combining upright posture with fixed beam lines offers expanded angular access and potential dosimetric advantages, yet optimal angle configurations remain unclear. This study identifies optimal beam angles in head and neck CIRT by comparing dosimetry and robustness of upright and supine plans for fixed-beam systems, thereby supporting beam angle optimization and clinical implementation of upright treatment in fixed-beam systems.
Methods:
Twenty patients with head and neck cancer were retrospectively robustly optimized using four beam configurations: horizontal beams at 0° (S0) and with a 15° superior-oblique tilt (S15) in the supine posture, anterior beams at 15° (U15) and 45° (U45) in the upright posture. Plans were generated in RayStation (v10B) accounting for ± 3 mm setup and ± 3.5% range uncertainties. Target coverage (D95%, D2%, V95%, conformity index [CI], homogeneity index [HI]), plan robustness (DVH bands, worst-case scenario), and organ-at-risk (OAR) dosimetry (mean dose to cochleae and parotid glands, and brainstem D1cc) were compared. Statistical analyses used paired t-tests or Wilcoxon signed-rank tests.
Results:
All plans achieved comparable nominal target coverage and similar CI values. S15 showed significantly improved robustness (DVH band ΔD95% = 0.5 Gy(RBE), ΔV95% = 1.4%; worst-case ΔD95% = 0.3 Gy(RBE), ΔHI = 0.01, ΔCI = 0.02, all p < 0.05) and lower OAR doses versus S0 (cochlea: 28.4 vs. 30.6 Gy(RBE), parotid: 13.5 vs. 18.5 Gy(RBE), brainstem D1cc: 40.1 vs. 41.7 Gy(RBE), all p < 0.001). U15 exhibited comparable robustness to S15 with further reductions in cochlea (18.5 vs. 28.4 Gy(RBE), p < 0.001) and parotid sparing (11.9 vs. 13.5 Gy(RBE), p < 0.05). U45 showed the highest robustness and OAR sparing, except for the brainstem, where D1cc was significantly increased (50.9 Gy(RBE), p < 0.05).
Conclusions:
The anterior beams at 15°in the upright setup (U15) showed the best balance of robustness and OAR sparing, making it the preferred option. The 15°-angled supine setup (S15) is a practical alternative. S0 and U45 are not recommended due to inferior robustness and higher brainstem dose, respectively.
