Dosimetric impact of endoscopic titanium clips in photon and proton radiotherapy for esophageal cancer

Background To quantitatively assess the dosimetric influence of endoscopic titanium clips in photon and proton radiotherapy for esophageal cancer and determine whether clip-induced heterogeneities affect target coverage, esophageal wall dose, or plan robustness. Methods This retrospective study included 26 patients with esophageal squamous cell carcinoma who underwent metallic clip placement for radiotherapy localization. Photon IMRT/VMAT and proton pencil beam scanning plans were generated with clip densities represented either by native CT values or overridden to 0 HU. Multiple regions of interest, including the clip, 3-mm and 5-mm expansions and their intersections with the esophageal wall, GTV, and PTV, were evaluated. Dose–volume parameters were compared using paired tests and linear mixed-effects models. Proton plans incorporated robust optimization accounting for ±5 mm setup and ±3.5% range uncertainties. Results Clip-induced perturbations were small and localized in both modalities. Photon dose differences were ≤0.3 Gy across evaluated structures, while proton variations were ≤0.1 Gy, confined to the immediate clip vicinity. Target coverage remained stable, with all changes ≤1% of the prescription dose. Proton robustness was preserved under uncertainty scenarios, maintaining ≥95% CTV coverage. Differences between modalities were minimal, and although a few metrics reached statistical significance, the absolute dose changes remained small. Esophageal wall dose increases did not exceed 0.2 Gy (0.5% of prescription), well below established toxicity thresholds. Conclusion Endoscopic titanium clips introduce only minor, clinically insignificant dose perturbations in photon and proton esophageal radiotherapy. Proton sensitivity to local heterogeneities is effectively mitigated by robust optimization. These findings support the dosimetric safety and reliability of titanium clips as localization markers in contemporary treatment planning.