S2770
RTT - RTT contouring, target definition, and treatment planning
ESTRO 2026
25 fractions to the primary (CTV1) and elective nodes (CTV2, CTV3), and 55Gy simultaneous-integrated-boost to involved nodes (CTV55). Daily CBCTs (n=248) were reviewed; CTVs and OARs were recontoured and transferred to the planning CT for dose recalculation. Bladder/Rectum volumes (cc) were taken from Eclipse contours. Density override structures accounted for gaseous and contour changes. When bony anatomy showed significant deviations, deformable registration using Velocity was performed. Data normality was assessed. Associations between bladder/rectum volume changes and dosimetric parameters (CTV D98%, bladder/rectum V30Gy/V40Gy) were analysed using Spearman’s rank correlation ( ρ ) and descriptive statistics (median, Interquartile range). Results: Bladder volume varied considerably (median change from planned: –5.05%; range: -237.7 to 236.6 cc (– 76.9% to 182.7%)). Rectum volume showed greater variability (median change: –8.88%; range: –107.6% to 293.8%) (Figure 1). No significant correlations were found between bladder volume and bladder dose metrics (Table 1). Rectum volume positively correlated with higher rectum doses: V30Gy (rho=0.53, p=<0.01; median difference from planned=8.74%, IQR − 0.16– 14.24 %) and V40Gy (rho=0.61, p<0.01; median 12.11%, IQR − 0.16–19.64 %). Changes in CTV1 coverage were minimal (D98% median − 0.16Gy, IQR − 0.79–0.11Gy) and CTV55 similarly (median − 0.07Gy, IQR − 0.42– 0.01Gy) (Figure 1). Despite large anatomical variability, PBT maintained robust target coverage and bladder sparing.
Mini-Oral 1215
Impact of inter-fractional bladder and rectal volume changes on proton beam therapy dosimetry in cervical cancer Courtney Reynolds 1 , Vossco Nguyen 1 , Molly Munro 1 , Asma Sarwar 2 , Wiwatchai Sittiwong 3 , Deepali Purohit 1 1 Radiotherapy Physics, University College London Hospitals, London, United Kingdom. 2 Radiation Oncology Consultant, University College London Hospitals, London, United Kingdom. 3 Clinical Fellow - Radiotherapy, University College London Hospitals, London, United Kingdom Purpose/Objective: Extended-field radiotherapy is commonly used for locally advanced cervical cancer with multiple pelvic or para-aortic nodes, but it irradiates large volumes of OARs, causing toxicities.1 Proton beam therapy (PBT) could offer more conformal dose distribution and lower integral dose yet is more sensitive to anatomical changes compared to VMAT. This is particularly relevant given frequent organ filling and motion in cervical cancer treatment. A retrospective PBT planning study was done to assess the impact of inter- fractional bladder and rectal volume changes on CTV coverage (D98%>95%) and rectum/bladder dose metrics (rectum V30Gy<80%, V40Gy<70%, bladder V30Gy<85%, V40Gy<75%) in cervical cancer patients, using planning CTs and daily CBCTs from 10 VMAT- treated patients. Material/Methods: Ten patients previously treated with VMAT were retrospectively replanned for PBT (Multi-field Optimisation, Eclipse v16). Each plan received 45Gy in
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