S2762
RTT - RTT contouring, target definition, and treatment planning
ESTRO 2026
patient. Time taken to edit the AI- generated structures was recorded 3 months following initial data collection, and geometric analysis was completed in the form of Dice Similarity Coefficient (DSC) and 95% Hausdorff Distance (HD) measurements. Results: There was a statistically significant overall mean time reduction of 46% (p < 0.001) when editing AI- generated contours compared with manual delineation. Significant time savings were also achieved for the bladder (76%), rectum (54%), sigmoid (43%), and bowel bag (39%) (all p < 0.001). The mean DSC values were 0.93, 0.84, 0.56, and 0.81, and mean 95% HD values in mm were 3.98, 7.05, 33.71, and 32.19 for the bladder, rectum, sigmoid, and bowel bag Findings indicate that the AI contouring programme provides clear time-saving benefits, both overall and across each OAR structure. The geometric analysis results are promising, with three of the four structures achieving DSC values of over 0.8. 95% HD values showed a wider variety of results, highlighting the need to review all contours prior to approval. There were no large differences in time saving benefits between hysterectomy and non-hysterectomy patients, with geometric accuracy scores also showing relative similarity. The findings strongly support the continued local use of the AI contouring system to drive workflow efficiency, with clinician oversight remaining essential to uphold contour accuracy and clinical confidence. Recommendations include a revision of local protocol to perform AI delineation on every third CT slice for all OAR structures. Dosimetric analysis and a qualitative review could be carried out for future research. References: 1. Ferlay J, Ervik M, Lam F, Laversanne M, Colombet M, Mery L, Piñeros M, Znaor A, Soerjomataram I, Bray F (2024). Global Cancer Observatory: Cancer Today. Lyon, France: International Agency for Research on Cancer. https://gco.iarc.who.int/today Keywords: AI contouring, Gynaecological Cancer, Radiotherapy respectively. Conclusion: Pioneering routine use of rectal spacer in prostate VMAT: technical and dosimetric advancements Susana B Oliveira 1,2 , João Santos 3 , Guy Vieira 4 , Isabel Faria 5 , Norberto Pereira 6 1 Radiotherapy Service, Instituto Português de Oncologia do Porto, Porto, Portugal. 2 PhD Programme in Nanoscience and Biomedicine, University of Vigo, Vigo, Spain. 3 Medical Physics Service, Research Centre Digital Poster 262
(CI-IPOP), Instituto Português de Oncologia do Porto, Porto, Portugal. 4 Joaquim Chaves Saúde, Clínica de Radioncologia do Algarve, Faro, Portugal. 5 Escola Superior de Saúde, Instituto Politécnico do Porto, Porto, Portugal. 6 Psychology Service, Instituto Português de Oncologia do Porto, Porto, Portugal Purpose/Objective: To assess the technical feasibility and dosimetric impact of rectal spacer use in prostate cancer patients treated with Volumetric Modulated Arc Therapy (VMAT) in routine clinical practice. Material/Methods: A retrospective dosimetric analysis was performed on 80 patients with localized prostate cancer treated between 2020–2021. Patients were equally divided into two groups: VMAT alone (n=40) and VMAT with rectal spacer insertion (n=40). Treatment plans were generated with Eclipse TPS v15.6, following QUANTEC dose constraints. Dose–volume histograms (DVH) were analysed for the rectum, bladder, penile bulb, femoral heads, and planning target volume (PTV), considered organs at risk (OAR) in prostate radiotherapy. Key metrics included rectal V50–V75, bladder V65–V80, femoral head V50, penile bulb mean dose, and PTV conformity/homogeneity indices. Groups were compared using the Mann–Whitney U test (p<0.05). Results: Spacer use significantly reduced rectal doses across all thresholds (median V70: 7.05% vs 0.15%, p<0.001). Bladder doses were slightly lower in the spacer group but not statistically significant, remaining within tolerance in both cohorts. Femoral head doses were negligible (<0.3% V50). The penile bulb received a significantly lower mean dose with spacer use (18.3 Gy vs 11.5 Gy, p=0.003). PTV coverage and homogeneity were maintained, with conformity index slightly improved in the spacer group (0.996 vs 0.999, p=0.01). No technical issues or workflow delays were observed during treatment delivery. Conclusion: Our results confirm, in routine practice, the role of rectal spacer integration in prostate VMAT as a simple technical solution that achieves meaningful OAR sparing, preserves target coverage, and streamlines workflow. Rectal spacer use should be considered standard practice in prostate radiotherapy to optimise dosimetry, treatment quality and patient safety, supporting broader adoption in advanced and hypofractionated regimens. References: 1. Michalski, J.M.; Gay, H.; Jackson, A.; Tucker, S.L.; Deasy, J.O. Radiation Dose-Volume Effects in Radiation-Induced Rectal Injury.Int. J. Radiat. Oncol. Biol. Phys. 2010, 76 (Suppl. S3), S123–S1292. Pinkawa, M. Current role of spacers for prostate cancer radiotherapy. World J. Clin. Oncol. 2015, 6, 189–
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