S1801
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
patient-specific anatomy. In our clinic, conventional treatment has been performed using two tangential photon beams with wedges, combining different energies and complementary fields. Recently, an intensity-modulated radiotherapy (IMRT) technique has been introduced to optimize dose conformity. This study compares both approaches in a cohort of 26 patients in order to evaluate their dosimetric differences and to assess the consistency of the treatment plan under simulated respiratory motion conditions. Material/Methods: Twenty-six patients prescribed 26 Gy in five fractions were planned using IMRT with four tangential fields of 6 MV and 10 MV. Plans were optimized with a maximum of 20 segments per field using the Elekta Monaco TPS with the XVMC algorithm. Subsequently, each treatment was replanned with a three- dimensional conformal radiotherapy (3DCRT) technique using the same planning system and a Collapsed Cone algorithm. All dose calculations were performed with a 2 mm grid. Relevant dosimetric parameters were evaluated for both the Planning Target Volume (PTV) and the Organs at Risk (OARs). Both IMRT and 3DCRT plans were verified using the Delta ⁴ (ScandiDos), and respiratory motion was simulated with HexaMotion, employing a two- dimensional breathing pattern acquired from a real patient. The analysis was based on the gamma index, considering the gamma passing rate, average gamma value, and dose deviation to assess plan accuracy and consistency. Results: For the PTV, dose coverage (D98%), high-dose regions (D2%), the homogeneity coefficient [1], and the Paddick conformity index were assessed. For the OARs, the analyzed parameters included V780 cGy for the ipsilateral lung, V650 cGy and V130 cGy for the heart, and D2% and mean dose for the contralateral breast. IMRT demonstrated superior conformity and homogeneity, consistent with the measurement results. When evaluated using the clinical gamma index criteria (3%, 2 mm, 10% threshold), IMRT showed stable performance under both static and respiratory motion conditions. Furthermore, IMRT maintained high accuracy even when stricter gamma index criteria (2%, 2 mm, 10% threshold) were applied. In contrast, 3DCRT performance declined under both the more stringent criteria and simulated respiratory motion.
Conclusion: Overall, the dosimetric comparison suggests that both techniques provide clinically acceptable treatment plans, although IMRT may offer advantages in terms of dose distribution uniformity. Measurements highlighted that IMRT demonstrated greater consistency than 3DCRT when evaluated under simulated respiratory motion. Future research will assess breathing patterns and impact on treatment stability and dosimetry. References: [1] ICRU Report 83. Journal of the ICRU, Volume10, No.1, 2010. Oxford University Press, 2010. DOI: 10.1093/jicru/ndq002[2] Choi, Y. E., Sung, K., Dong, K. S., Kim, H. J., & Lim, Y.-K. (2021). Dosimetric Impact of Respiratory Motion During Breast Intensity-modulated Radiation Therapy Using Four-dimensional Dose Calculations. Anticancer Research, 41(1), 417-427. https://doi.org/10.21873/anticanres.14791 Keywords: IMRT, Breast, Respiration Online Adaptive Radiotherapy (ART) Capabilities on ETHOS for Complex Pelvic Treatment Plans Jarno Bouveret, Vincent Fave, Amadine Halley radiooncology, Hôpital de la Tour, Meyrin / Genève, Switzerland Purpose/Objective: To describe and validate a structured optimization methodology enabling safe and effective online adaptive radiotherapy (ART) implementation on the ETHOS system for complex pelvic treatments involving multiple target volumes and dose levels. Material/Methods: A standardized optimization workflow was developed to minimize variability and ensure dosimetric stability. Optimization models were refined through retrospective analysis of a targeted subset of adaptive sessions, drawn from over 2,000 clinical treatments. Each iteration was validated and tested on new clinical cases. Results: Digital Poster 382 Successive optimizations enabled reliable treatment delivery for complex pelvic cases with multiple dose prescriptions. The refined models improved consistency in target coverage and OAR sparing.
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