S2806
RTT - RTT contouring, target definition, and treatment planning
ESTRO 2026
Purpose/Objective: MRI is optimal for soft-tissue visualisation in rectal cancer. Multimodality image registration of CT-MRI improves the accuracy of target delineation. However, CT–MRI rigid registration (RIR) remains challenging due to volumetric and positional changes between the elective volume, tumour, and organs at risk (OARs).Inaccurate registration can lead to over-or underestimation of the gross tumour volume (GTV). The region of interest (ROI) selected during registration plays a crucial role in its accuracy and clinical acceptability. Despite this, there is a lack of published guidance on the optimal ROI selection.Aim- to quantify the clinical acceptability of multiple ROI registrations and the differences between each.ObjectivesScore clinical acceptability of registrations, based on pre-defined scoring criteriaQuantify transformation matrix (TM) of 7 ROI definitions Material/Methods: Patients who had received radiotherapy for rectal cancer were included. All patients had a CT planning scan (2-2.5mm, L2/3 to mid-femur and intravenous- contrast). MRI was acquired on MR-Simulator Siemens Magnetom Sola-RT Pro-Edition 1.5T, sequences included 3D T2 SPACE (TR1200 TE120) isotropic whole- pelvis, 2D T2 TSE (TR 15950, TE 110) high resolution in axial plane, both acquired in supine position with bladder preparation.CT-MR were performed in Varian EclipseTM by one observer, first using extended range over the scan extent and then 7 user-defined ROI (a-g) selections (table 1).Registrations were assessed in transversal, sagittal and coronal planes. Window levels were set to pelvis and qualitative assessment included moving window\checker-board tools by the second observer1.Registrations were scored to determine CA. Anatomical boundaries were assessed using clinical target volume (CTV) and GTV. A 5-point Likert scale used was (1)very good-alignment – no adjustments required, (2)good , (3)acceptable, (4)poor and (5)very poor.ROI-g was considered gold standard, verified by observer 3. Varian VelocityTM was used to extract TM values for ROI a-g. Translational and rotational differences were derived by decomposing the rigid- body TM, calculated from ROI-g for ROI-a,b,c,d,e,f. Mean, SD and 95%CI were recorded.
year MDFS (80% vs 93%).Mean D2cc doses were: rectum 65.4 Gy, bladder 78 Gy, sigmoid 59.8 Gy, bowel 55.4 Gy. No grade ≥ 3 side effects were recorded. Rectal morbidity (G1–G2) occurred in five patients (23.8%). Two-year proctitis-free and rectal bleeding- free survival were 83.3% and 85.7%. Urinary morbidity (G1–G2) occurred in three patients; no grade ≥ 3 events. Vaginal atrophy, telangiectasia, and stenosis occurred in 52%, 33%, and 33%. No sacral fractures were observed. Conclusion: Rectal morbidity remains the main late adverse event in elderly patients receiving CRT and IGBT, mainly as proctitis or bleeding. The T-score may serve as a prognostic factor for MDFS and as a predictor for interstitial needle use. No significant association was found between T-score >4 and morbidity-free survival. Further studies are necessary to define predictors of radiotherapy-related morbidity in elderly LACC patients. References: Kobayashi D, Okonogi N, Wakatsuki M, et al. Impact of CT-based brachytherapy in elderly patients with cervical cancer. Brachytherapy. 2019;18(6):771 - 779.Chakraborty S, Geetha M, Dessai S, et al. How well do elderly patients with cervical cancer tolerate definitive radiochemotherapy using RapidArc. ecancermedicalscience. 2014;8:484.Wang W, Zhang J, Chen L, et al. Outcome and toxicity of radical radiotherapy or concurrent chemoradiotherapy for elderly cervical cancer women. BMC Cancer. 2017;17:350.Chuk E, et al. Clinical outcomes of 3 versus 4 fractions of magnetic resonance image- guided brachytherapy. Int J Radiat Oncol Biol Phys. 2024;120(1):95 - 107. Keywords: Elderly, Dosimetry, Cervical Cancer Can an individualised region of interest (ROI) in rigid registration for rectal cancer improve multi- modality image registration? Lynsey Devlin 1,2 , Stefanie Thomson 3 , Marimuthu Sankaralingam 3 , Timothy Mitchell 4 , Nicholas MacLeod 4 , Tareq Abdulla 4 , Olof Kjartansdottir 4 , Fatima Rushwan 4 , Aileen Duffton 1,2 Digital Poster Highlight 5123 1 Department of Radiotherapy, Beatson West of Scotland Cancer Centre, Glasgow, United Kingdom. 2 School of Cancer Sciences, University of Glasgow, Glasgow, United Kingdom. 3 Radiotherapy Physics, Beatson West of Scotland Cancer Centre, Glasgow, United Kingdom. 4 Department of Clinical Oncology, Beatson West of Scotland Cancer Centre, Glasgow, United Kingdom
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