S971
Clinical - Oligometastatic cancer
ESTRO 2026
but also offset the increased workload associated with OART, offering clinical and operational benefits. The trial is considered successful if a reduction of ≥ 30% in total number of fractions is achieved. Material/Methods: To date, 20 of 25 required patients with A-P LN oligometastases have been enrolled, with full inclusion expected by the end of 2025. This preliminary analysis includes 16 patients. The inclusion criteria were: ≤ 5 oligometastases in ≤ 2 organs, primary tumor controlled ≥ 4 months prior. Fourteen patients had prostate cancer as the primary tumor site and two had other primaries. Patients were treated with OART on the ETHOS platform (Varian Medical System Inc.). Three fractionation schedules were available: 5x9 Gy, 3x12.6 Gy, and 1x25 Gy. Dose guidance goals for organs of interest were radiobiologically equivalent across schedules. The optimal schedule was selected based on the pCT, prioritizing the lowest feasible number of fractions while respecting dose guidance goals of the organs of interest. If those goals could not be met, 5x9 Gy was selected and an underdosage to the PTV was allowed. Treatment time was measured from the first CBCT to post-treatment CBCT. Results: A 41.1% reduction in total fractions (53/90) was achieved. Eight (44%) 1-fraction treatments were delivered, one (6%) 2-fractions treatment and one (6%) 3-fractions treatment. In 8 cases (44%), the number of fractions could not be reduced. No grade ≥ 2 acute side effects (<3 months) was observed. Median (interquartile range) fraction treatment times [mm:ss] for 5-, 3-, 1-fractions and the whole cohort were: 27:05 (21:57-36:25), 32:10 (25:01-37:59), 32:50 (30:30-35:16), and 30:28 (22:29-36:25). In Table 1 the delivered dose to the targets and organs are given for various dosimetric parameters. No organ dose guidance goals were violated in any of the treatment fractions.
assuming that the α / β ratio is 10.
Conclusion: SBRT achieves excellent local control in oligometastatic rib metastases, with no local failures detected during follow-up. It also shows a favourable safety profile, with low rates of late toxicity and no rib fractures observed. To our knowledge, this represents one of the largest contemporary clinical cohorts focused on this anatomically challenging subsite, providing a strong foundation for future prospective study. Keywords: rib metastasis, stereotactic body radiotherapy, Reducing treatment fractions by personalized fractionation with online adaptive radiotherapy: a phase II study (STEAL-3) Erik van Lieshout, Mischa S. Hoogeman, Remi A. Nout, Maaike T.W. Milder, Joost J.M.E. Nuyttens Department of Radiotherapy, Erasmus MC Cancer Institute, Rotterdam, Netherlands Purpose/Objective: The aim of this prospective phase II study (STEAL-3, NL87228.078.24) is to evaluate whether personalized fractionation based on the planning CT (pCT) can reduce the total number of treatment fractions in online adaptive radiotherapy (OART) for abdominal- pelvic lymph node (A-P LN) oligometastases. This novel strategy could not only enhance patient convenience Poster Discussion 2057
Conclusion: Current results demonstrate that personalized fractionation in online adaptive radiotherapy reduces the total number of treatment fractions by 41.1%,
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