ESTRO 2025 - Abstract Book

S2765

Physics - Dose prediction, optimisation and applications of photon and electron planning

ESTRO 2025

Conclusion: Implementation of RapidPlan resulted in significant reduction in OAR doses while maintaining adequate target coverage. These reductions, particularly in rectum and bladder doses, could decrease treatment-related toxicities and improve patient quality of life. Additionally, the improved plan consistency suggests that KBP can help standardize treatment quality across different planners. These results support the broader adoption of KBP in clinical practice for improving treatment plan quality.

Keywords: Knowledge-Based Planning, Dose planning, RapidPlan

References: [1] Kurativ strålebehandling for prostatacancer, version 3.0, 2023. Available from: https://ducg.dk/daproca prostatacancer/kliniske-retningslinjer.

1423

Mini-Oral Quantification of plan aperture modulation in radiotherapy treatment plans Victor Hernandez 1 , Iñigo Lara-Aristimuño 1 , Ruben Abella 1 , Jordi Saez 2 1 Medical Physics, Hospital Universitari Sant Joan de Reus, Reus, Spain. 2 Radiation Oncology, Hospital Clinic, Barcelona, Spain Purpose/Objective: International guidelines emphasize the need to quantify plan modulation as part of PSQA for IMRT/VMAT, recommending documenting the modulation of treatment plans and replanning in cases of excessive modulation to prevent failures in treatment verifications[1-3]. However, no consensus exists on how to quantify plan modulation and existing metrics suffer from important limitations[4-6]. This study proposes a novel metric, Plan Aperture Modulation (PAM), to quantify the modulation of radiotherapy treatment plans. PAM aims to provide a clear geometric interpretation, addressing limitations of previous complexity metrics and facilitating integration into TPSs and clinical workflows. Material/Methods: PAM is defined as the average fraction of the target area located outside the beam aperture, weighted across all control points in a treatment plan. It was evaluated on IMRT/VMAT plans for different anatomical sites—prostate with lymph nodes and lung stereotactic body radiation therapy. Plans with varying complexities were generated using the Eclipse TPS, and PAM was compared to established metrics including Plan Modulation (PM), Modulation Complexity Score (MCS), and monitor units per Gray (MU/Gy). The relationship between PAM and Modulation Factor (MF), defined as the relative increase in MUs due to aperture modulation was also analysed using analytical expressions[7]. Results: PAM provided a clear assessment of plan modulation and was validated across different delivery platforms, such as C-arm linacs and Halcyon systems and treatment techniques. The metric outperformed the previous metrics and indicated a zero modulation for Dynamic Conformal Arc plans (Figure 1), offering an intuitive and robust interpretation. Additionally, PAM was independent of confounding factors, such as treatment technique, beam energy, delivery system, dose calibration conditions, and patient anatomy. Furthermore, the derived equations allowed the direct determination of MF based on PAM regardless of other factors (Figure 2), offering major advantages in treatment plan evaluation.