ESTRO 2025 - Abstract Book

S1603

Clinical – Mixed sites & palliation

ESTRO 2025

This study aims to establish an optimized LSCLN- CTV delineation framework grounded in “block - reflux pattern”. By integrating robust anatomical landmarks and radiological imaging, we propose a novel paradigm for precise radiotherapy planning. Material/Methods: We retrospectively analysed cases from 2010 to 2024 for patients with gastrointestinal, gynaecological, and urogenital cancers, with LSCLNs confirmed positive via PET/CT or biopsy. Lung metastases or pleural effusion cases were excluded. For patients with oligometastatic LSCLNs, CTV delineation was based on the “block - reflux” lymphatic pattern and anatomical landmarks, particularly the scalene muscles and brachial plexus. Seventy percent of cases were used for optimization, and 30% for validation. Further we focused on newly diagnosed cohort of positive retroperitoneal lymph nodes (RPLNs), categorized into upper, middle, and lower levels, and evaluated their association with LSCLN metastasis via Hierarchical Logistic Regression model.

Results:

A total of 181 abdominopelvic cancer patients with LSCLN metastases were enrolled, with 90.6% concurrent positive RPLNs. For oligometastatic LSCLN patients (n = 76), CTV delineation based on the “block - reflux” pattern achieved 94.3% coverage in the optimization cohort (n =53) and 95.7% coverage in the validation cohort (n = 23). Additionally, among patients with concurrent V region involvement (n = 60), metastasis occurred exclusively in upper group (Va) in 20%, the lower group (Vb) in 26.7%, and both groups in 53%, revealing a nuanced interplay between Va and Vb that challenges the conventional notion of serial connection.