S2797
RTT - RTT contouring, target definition, and treatment planning
ESTRO 2026
Míriam Mancera Soto 1 , Ainoa Vizuete Pérez 1 , Xavier Nolla Nieto 1 , Sonia Bermejo Martinez 2 , Jaume Puig Xampeny 1 , Guillermo Gómez de Segura Melcón 1 , Jéssica Jiménez Sánchez 2 , Núria Farré Bernadó 2 , Helena Vivancos Bargalló 1 1 Radiation Physics and Radiadion Protection, Hospital de la Santa Creu i Sant Pau, Barcelona, Spain. 2 Radiation Oncology, Hospital de la Santa Creu i Sant Pau, Barcelona, Spain Purpose/Objective: To evaluate the impact of the number of arcs in lung stereotactic body radiotherapy (SBRT) treatments using the VMAT technique, analysing the trade-off between dosimetric quality and treatment A retrospective planning study was conducted on 18 patients with early-stage non-small cell lung cancer (NSCLC) who were treated with SBRT. Three VMAT plans were generated in Eclipse v15.6 (Varian Medical Systems) for each patient including the following characteristics: 2 arcs (collimator angles 30° and 330°), 3 arcs (30°, 330° and 90°) and 4 arcs (30°, 330°, 85° and 95°), applying the same optimization objectives and constraints. AcurosXB (Dose medium) with a 0,125 cm grid was used for all plans. Dosimetric parameters were evaluated, including Paddick conformity index (CI), total monitor units (MU), irradiation time and dose administration time. Material/Methods: to organs at risk (OARs). Statistical analysis was conducted using Python-based tools for descriptive and correlation analysis. Results: Data normality was assessed with the Shapiro-Wilk test. Non-parametric tests were applied due to non- normal distribution. No statistically significant differences were observed among the treatment plans with different numbers of arcs for the analysed variables (p>0.00064 after Bonferroni correction). The Friedman test showed p = 0.58 for total MU and p = 0.80 for irradiation time. The mean CI was 0.78 ± 0.2 with 2 arcs, 0.84 ± 0.2 with 3 arcs, and 0.80 ± 0.3 with 4 arcs. Pairwise comparisons using the Wilcoxon signed- rank test yielded p-values of 0.001, 0.01, and 0.003; after Bonferroni correction, these were not significant.Doses to OARs were not significantly affected by arc number (Figure 1), depending mainly on PTV location (central vs. peripheral nodule).
Conclusion: This study suggests that UTE-derived dosimetry maps could have potential for dose calculation in head-and- neck cancers only using MRI images. Optimizations and verifications are still required to address the reduced dose calculations compared to the reference. An additional chemical shift encoded scheme in the spiral VIBE UTE could allow for better fat-water decomposition and avoid the need of CT images for tissue categorical mask computation. References: [1] Demol, B., Viard, R. and Reynaert, N. (2015), Monte Carlo calculation based on hydrogen composition of the tissue for MV photon radiotherapy. Journal of Applied Clinical Medical Physics, 16: 117-130.[2] Seco, J. and Evans, P.M. (2006), Assessing the effect of electron density in photon dose calculations. Med. Phys., 33: 540-552.[3] Biggs, S et al. (2022). PyMedPhys: A community effort to develop an open, Python-based standard library for medical physics applications. Journal of Open Source Software, 7(78), 4555,[4] – Sayaque et al. Magnetic resonance imaging with ultra- short echo time sequence for head and neck radiotherapy planning Physica Medica 2025; 133. Keywords: MRI-based, Head-and-neck
Digital Poster 4136 Impact of the number of arcs in lung SBRT: balancing dosimetric quality and treatment time.
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