S1553
Physics - Autosegmentation
ESTRO 2026
of 0.67 ± 0.11, recall of 0.57 ± 0.14, precision of 0.86 ± 0.09, and HD95 of 5.33 ± 2.76 mm. Paired t-tests demonstrated that the integration of MRI significantly improved the Dice score, recall, and HD95 (p < 0.05), whereas the difference in precision was not statistically significant.
orientation were optimized by minimizing the voxel- wise Hounsfield unit differences between the image and its mirror reflection, using a robust Huber-loss– based similarity metric applied exclusively to bony structures to avoid that large tumor masses bias the MSP location (Figure 1). Tumor extension beyond the MSP was quantified by the maximum distance of any contralateral GTV-T voxel. Contralateral lymphatic spread patterns were then evaluated as a function of MSP extension. Patients were grouped into distance cohorts: <0 mm (DC<0, lateralized tumors not extending over MSP), 0–10 mm (DC<10), 10–20 mm (DC<20), and >20 mm (DC>20).
Conclusion: The integration of MRI with CT resulted in statistically significant improvements in Dice score, recall, and HD95 for automatic prostate CTV segmentation, indicating enhanced boundary delineation and spatial accuracy. Although precision remained comparable between the two models, the overall findings suggest that incorporating MRI can meaningfully improve segmentation performance and may support more reliable radiotherapy planning. Keywords: prostate cancer, magnetic resonance imaging, CT Automated Mid-Sagittal Plane Detection to Analyze Contralateral Lymphatic Spread in Oropharyngeal Cancer Yoel Pérez Haas, Loris Keller, Roman Ludwig, Noemi Bührer, Esmée L Looman, Panagiotis Balermpas, Jan Unkelbach Radiation Oncology, University Hospital Zurich, Zurich, Switzerland Purpose/Objective: Oropharyngeal squamous cell carcinoma (OPSCC) extending beyond the mid-sagittal plane (MSP) are known to exhibit higher contralateral lymph node involvement compared to lateralized primary tumors1. This study aimed to (1) develop a robust, automated algorithm to detect the MSP from planning CT images, (2) quantify the primary tumor’s contralateral extension as a continuous geometric metric, and (3) analyze how this distance correlates with contralateral Digital Poster 1128 lymphatic spread. This work aims to provide a quantitative basis for estimating contralateral involvement risk and guiding elective irradiation
Results: Across all patients, 25 % exhibited cLNL involvement (LNL II 22 %, LNL III 9 %, LNL IV 3 %). Contralateral prevalence increased with primary-tumor extension beyond the MSP: 13 % for lateralized tumors (DC<0), 23 % for small extension (DC<10), 35 % for moderate extension (DC<20), and 43 % for extensive extension (DC>20) (Figure 2).Level-specific rates also rose with increasing MSP extension: in LNL II, prevalence increased from 11 % (DC<0) to 20 % (DC<10), 31 % (DC<20), and 43 % (DC>20); and in LNL III, from 2 % (DC<0) to 7.5 % (DC<10), 10.4 % (DC<20), and 22.7 % (DC>20).
decisions in OPSCC. Material/Methods:
A fully automated MSP detection algorithm was developed for head and neck CT imaging and evaluated on 198 OPSCC patients treated at the University Hospital Zurich. The MSP position and
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