ESTRO 2025 - Abstract Book

S3514

Physics - Optimisation, algorithms and applications for ion beam treatment planning

ESTRO 2025

Conclusion: Daily robust optimization with enlarged RRS can effectively mitigate dose degradation resulting from CT number errors in CBCT-based online-adaptive IMPT planning. Even for relatively large CT number errors, the online CBCT based plans outperformed our current clinical trigger-based offline adaptive approach in terms of mitigation of extreme coverage losses and toxicity risks. With the expected introduction of CBCT-based online-adaptive IMPT for head-and-neck, this clinically available method can ensure robustness against (residual) CT-number errors.

Keywords: CBCT; in-room CT; online adaptive proton therapy

2874

Digital Poster An Effective LET Approach for Assessing Sequential Boost Proton Therapy Plan in Head and Neck Cancer Hong Qi Tan 1,2,3 , Calvin Wei Yang Koh 1 , Kah Seng Lew 1 , Clifford Ghee Ann Chua 1 , Andrew Wibawa 1 , Zubin Master 1 , James Cheow Lei Lee 1 , Sung Yong Park 1 1 Radiation Oncology, NCCS, Singapore, Singapore. 2 School of physical and mathematical science, NTU, Singapore, Singapore. 3 Duke-NUS Medical School, Duke-NUS, Singapore, Singapore Purpose/Objective: Proton therapy plays an increasingly important role in the treatment of head-and-neck cancer due to the reported decrease in the incidence of radiation toxicity compared to intensity-modulated radiotherapy. Sequential boost is commonly used in the treatment due to multiple target volumes and different radiosensitivity within the target. When assessing the composite biological dose from different phases using variable relative biological effectiveness (RBE) model, it can be challenging to determine the contribution of high linear energy transfer (LET) radiation from each phase, especially when the dose tolerance of organs-at-risk is exceeded. In this work, we proposed a framework to calculate the effective LET in the composite plan which can be easily correlated to the LET from each phase to facilitate a more comprehensive assessment of the treatment plan and its potential radiobiological impact. Material/Methods: The voxel-wise effective RBE in the composite plan is defined as RBE E = (RBE 1 D 1 +RBE 2 D 2 ) / (D 1 + D 2 ) for a generic two phases plan. The LET and voxel dose (D 1 and D 2 ) from each phase can be extracted from RayStation 12B. In this work, we use the Wedenberg’s RBE variable RBE model to calculate LET 1 and LET 2 . Finally, using the Wedenberg’s RBE model, we can invert the linear-quadratic (LQ)-based RBE e equation to solve for the effective LET. We showed the result of our effective LET framework on a sequential boost nasopharyngeal cancer plan treated with two phases of 44Gy/22fx and 29.9Gy/13fx.