ESTRO 2025 - Abstract Book

S4339

RTT - Treatment planning, OAR and target definitions

ESTRO 2025

With daily online SC delineation and plan optimisation using SC ‘cord of the day’; daily change in contoured position of the spinal cord resulted in an improvement of PTV coverage in 4/5 cases and 2/5 cases on MRLT2 and MRL3D based contours, respectively. However these were only minor absolute values (-2.42 - +1.01%) in planning target volume (PTV) coverage. Conclusion: This work has highlighted that the SC PRV receives excess dose due to the inaccuracy of the SC position using standard workflows. An MRL workflow eliminates this excess dose with accurate online SC delineation. Elimination of fusion and registration error potentially enalbes reduced margins around this critical organ at risk. MRL imaging is suitable for SC delineation in an online spine SBRT adaptive workflow. Future directions of work include SC auto contouring to improve delineation variability. Alternatively, adapt-to-position workflows may be faster. References: 1. Chuang C, Sahgal A, Lee L, Larson D, Huang K, Petti P, et al. Effects of residual target motion for image tracked spine radiosurgery. Med Phys 2007;34. 2. Oztek MA, Mayr NA, Mossa-Basha M, Nyflot M, Sponseller P, et al. The Dancing Cord: Inherent Spinal Cord Motion and Its Effect on Cord Dose in Spine Stereotactic Body Radiation Therapy. Neurosurgery 2020 3. Fürweger C, Drexler C, Kufeld M, Muacevic A, Wowra B. Patient motion and targeting accuracy in robotic spinal radiosurgery: 260 single-fraction fiducial-free cases. Int J Radiat Oncol Biol Phys 2010;78 4. Warfield SK, Zou KH, Wells WM. Simultaneous truth and performance level estimation (STAPLE): An algorithm for the validation of image segmentation. Trans Med Imaging 2004;23 Keywords: SBRT , MR-linac, ART Digital Poster Analysing Replanning Contributing Factors to Standardise Adaptive Head and Neck Proton Therapy Sze Yarn Sin 1 , Semaya Natalia Chen 1 , Geoffvinc Ng 1,2 , Hong Qi Tan 1,3 1 Division of Radiation Oncology, National Cancer Centre Singapore, Singapore, Singapore. 2 Division of Physics and Applied Physics, Nanyang Technological University Singapore, Singapore, Singapore. 3 Oncology Academic Clinical Programme, Duke-NUS Medical School, Singapore, Singapore Purpose/Objective: Proton therapy is the current state-of-the-art radiotherapy and it has increasingly become a preferred treatment for locally advanced and inoperable head and neck (HN) cancer due to lower integral dose and dose sparing to the neighbouring OAR structures. Adaptive proton therapy is exceptionally important as the Bragg peak position is highly sensitive to changes in density and anatomy. At present clinical setting in National Cancer Centre Singapore, replan is decided after re-CT simulation and dose evaluation. The final verdict is dependent on RTTs, physicists’ recommendations and clinicians’ decision. Therefore, in this preliminary study, we investigate the contributing factors for re-CT simulation and the results of dose evaluation and its relation to the replanning decision. Material/Methods: HN proton patients treated with 60-74Gy/30-35 fractions from March to September 2024 were retrospectively selected. 42 patients underwent re-CT simulation with Siemens SOMATOM-X.cite simulator and the CT dose evaluation were generated with RayStation(v12B). The reasons for the eventual adaptive replanning were classified into tumour regression, internal anatomical changes, weight loss and setup issues. The dose coverage in the clinical target volume (CTV) with the highest prescribed dose were calculated using the D99%, D98% and D95% dose metrics. Mann-Whitney U tests were employed to test for significant difference between the dose coverage metrics and CTVs, with and without replans. 1605