S1308
Clinical - Urology
ESTRO 2026
Digital Poster 4868
PTV-based plans (p=0.06). The robust optimization approach led to a substantial reduction in the mean V40Gy, V60Gy, and V70Gy values for the rectum when compared to PTV-based planning, with a decrease of 7.34%, 4.87%, and 0.58%, respectively. A similar trend was seen for the bladder, with reductions of 9.61%, 6.37%, and 2% in V50Gy, V60Gy, and V70Gy, respectively (p<0.05). Robust optimization also resulted in improved sparing of the small bowel, with significant reductions in V45Gy and V50Gy values. The V45Gy was reduced by 29.42cm3, and the V50Gy by 15.27cm3.Table 1: Patient-specific dose metrics for nominal robust and PTV-based plans
Modelling the dosimetric impact of adaptive bladder radiotherapy on incidental pelvic nodal irradiation Vincent Vakaet 1,2 , Karole Warren-Oseni 3,2 , Maneesh Singh 1,2 , Jun Hao Lim 4 , Fiona McDonald 5,2 , Vibeke N Hansen 3,2 , Victoria Harris 6 , Aslam Sohaib 7 , Robert Huddart 1,2 , Shaista Hafeez 1,2 1 Urology Unit, The Royal Marsden NHS Foundation Trust, London, United Kingdom. 2 Radiotherapy and Imaging, The Institute of Cancer Research, London, United Kingdom. 3 Joint Department of Physics, The Royal Marsden NHS Foundation Trust, London, United Kingdom. 4 Clinical Oncology, Nottingham University Hospital NHS Trust, Nottingham, United Kingdom. 5 Radiotherapy, The Royal Marsden NHS Foundation Trust, London, United Kingdom. 6 Clinical Oncology, Guy’s and St. Thomas’ NHS Foundation Trust, London, United Kingdom. 7 Radiology, The Royal Marsden NHS Foundation Trust, London, United Kingdom Purpose/Objective: Global practice differs in how elective pelvic nodal irradiation is approached when using radiotherapy for the radical treatment of localised muscle-invasive bladder cancer (MIBC). In contrast, pelvic lymph node dissection is an essential part of a radical cystectomy [1]. Up to 30% of patients may have occult pathological lymph node involvement at surgery [2]. Despite this, bladder-only radiotherapy has traditionally been linked with low rates of pelvic relapse [3]. We aim to model the changing patterns of incidental dose to the pelvic lymph nodes with the development of image- guided adaptive bladder radiotherapy strategies to assess their potential future impact on disease control. Material/Methods: Ten patients with T2-T4aN0M0 unifocal MIBC treated within an institutional clinical research and ethics committee-approved adaptive bladder radiotherapy protocol (NCT01124682) were selected for further analysis [4]. Pelvic lymph node regions (common, external, internal iliac, obturator, and presacral) were contoured according to consensus guidelines and combined into a composite nodal clinical target volume (CTVn). Three planning strategies were compared: (1) 3DCRT with a 1.5 cm uniform margin applied to the whole bladder (PTVbladder), (2) plan-of- the-day library of 3 VMAT plans (PTVbladder small, medium, large) and (3) VMAT delivery of a tumour- focused simultaneous integrated boost (SIB; PTVboost) with a lower dose delivered to the uninvolved bladder (PTVuninvolved bladder). PTVbladder/ PTVboost were planned to 64 Gy in 32 fractions (f), with PTVuninvolved bladder planned to 52 Gy in 32f. Dose–volume parameters, including the Dmean and
Figure 1: DVH metrics for CTV68 (blue), CTV50 (red), rectum (brown), bladder (yellow), and small bowel (purple) in nominal and perturbed scenarios for robust plans Conclusion: Robust optimization in hypo fractionated VMAT with SIB for prostate cancer provides benefits over PTV- based planning, including improved OAR sparing and enhanced robustness against setup uncertainties, while maintaining equivalent target coverage. Keywords: Plan robustness, OAR sparing, setup uncertainties
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