Clinical implementation of intensity modulated proton therapy in breath-hold for mediastinal lymphoma patients using surface monitoring and nasal high-flow therapy

Background and purpose: We report our first clinical experience treating mediastinal lymphoma patients in breathhold (BH) with intensity modulated proton therapy (IMPT), using optical surface scanning (OSS) for beam gating and nasal high-flow therapy (NHFT). Moreover, we evaluated BH stability throughout treatment and assessed adequacy of applied treatment margins. Materials and methods: Thirteen patients received IMPT-BH with anterior beams supported by NHFT and OSS. OSS enabled gated treatment delivery with a 3 mm breathing window and a maximum isocenter shift of 5 mm. OSS respiratory traces were synchronized with machine log-file beam delivery data to identify beam-on periods. BH stability, BH duration, and OSS-derived isocenter shifts were assessed during cone beam CT (CBCT) and treatment. Margin adequacy was evaluated using post-treatment CBCTs acquired for 34 fractions (2-3 CBCTs per patient), while plan robustness over time was assessed with weekly repeat-CTs. Results: Analyzing 187 fractions, the median number of BHs was 1 [1-6] per CBCT and 5 [3-20] per treatment delivery. Intra- and inter-BH surface stability were within 1 mm, and the median isocenter deviation during beam delivery was less than 3 mm. Average fraction time was 44 +/- 13 min, decreasing as patients became more experienced. One plan adaptation was required in the entire cohort. Residual target shifts during BH treatments were < 8 mm, comparable to motion in free-breathing. Conclusions: IMPT-BH with OSS and NHFT is feasible for treating mediastinal lymphoma, providing stable BHs, and acceptable treatment times. Interfraction motion was slightly higher than intrafraction motion, suggesting that combining bone alignment with soft tissue adjustment could improve setup accuracy.