3190 - Treatment Planning Strategy Selection Modulates Linear Energy Transfer (LET) Distributions in Nasopharyngeal Carcinoma Proton Therapy: Implications for the Mitigation of RBE Uncertainties

12:30pm - 01:45pm ET

Poster Hall - Exhibit Hall A

Screen: 14

Presenter(s)

Jingfen Yang, - Department of Radiation Physics, Shandong Cancer Hospital and Institute, Shandong First Medical University and Shandong Academy of Medical Sciences, jinan,

J. Yang¹, and T. Dai²; ¹Department of Radiation Physics, Shandong Cancer Hospital and Institute, Shandong First Medical University and Shandong Academy of Medical Sciences, jinan, China, ²Department of Radiation Oncology Physics and Technology, Shandong Cancer Hospital and Institute, Shandong First Medical University and Shandong Academy of Medical Sciences, Jinan, China

Purpose/Objective(s): Radiotherapy is the primary treatment approach for nasopharyngeal carcinoma (NPCa), however, the proximity of target to critical organs necessitates advanced techniques to minimize toxicity.

Materials/Methods: This study evaluated four intensity-modulated proton therapy (IMPT) treatment planning strategies for NPCa, focusing on both physical dose and linear energy transfer (LET) distributions—a key determinant of variable proton relative biological effectiveness (RBE). Twelve NPCa patients underwent robustly optimized IMPT planning (setup uncertainty: 0.3 cm; range uncertainty: 3.5%) with identical constraints. Strategies differed in beam arrangements: Strategy 1 utilized two bilateral fields; Strategies 2 and 3 employed one anterior and two bilateral fields (the difference is that Strategy 2 incorporated a 1.5cm robust target volume (RTV) margin near the brainstem, while Strategy 3 did not); Strategy 4 utilized one non-coplanar vertex and two bilateral fields.

Results: Dosimetric analysis revealed comparable target coverage across all strategies (CTV D₉₈, D₉₅, D₂; CI, HI; p>0.05). However, significant LET variations emerged in organs-at-risk (OARs). Strategy 1 and Strategy 4 demonstrated significantly lower LET in deep structures (brainstem L₁, inner ears L_mean) but higher superficial LET (facial regions) compared to Strategies 2 and 3. Specifically, Strategy 4 yielded the lowest brainstem L₁, while Strategy 1 and Strategy 4 increased oral cavity L_mean. Strategies 2 and 3 produced higher maximum LET in optic structures (chiasm, nerves) and higher mean dose/LET to eyeballs/lenses. LET-volume histograms and difference maps confirmed negligible CTV LET differences but distinct spatial patterns: Strategies 1 and 4 elevated superficial LET but spared deeper OARs, whereas Strategies 2 and 3 increased LET posterior to targets. No significant differences existed between Strategy 2 and Strategy 3.

Conclusion: These findings demonstrate that beam angle selection profoundly influences OAR LET distributions—critical for mitigating RBE uncertainties—without compromising target dosimetry. Non-coplanar (Strategy 4) and bilateral-only (Strategy 1) strategies offer advantages in reducing LET to serial OARs like the brainstem and optic apparatus.