ESTRO 2025 - Abstract Book

S1528

Clinical – Mixed sites & palliation

ESTRO 2025

Purpose/Objective: This study prospectively analyzed the dosimetry of CT-guided online adaptive stereotactic body radiotherapy on abdominal and pelvic lymph node (A-P LN) oligometastases as in these patients, both mild and severe toxicity is of great importance. The aim was to correlate the delivered dose with acute gastro-intestinal (GI) toxicity and create a Normal Tissue Complication Probability (NTCP) model. Material/Methods: Patients with A-P LN oligometastases were treated with a total dose of 45 Gy in daily consecutive fractions of 9 Gy on the CyberKnife with an integrated CT scanner on rails [1]. For each patient, a library of three plans was created pre-treatment: Plan A, standard of care plan based on the planning CT and prescribed to the 90% iso-dose line; Plan B, adaptive plan with organ at risk (OAR) contours from a diagnostic CT; Plan C , adaptive plan based on the planning CT, but prescribed to the 80% isodose-line. After a pre-fraction in-room CT scan, the radiotherapy technologist used a decision tree to select the plan with the highest target coverage without exceeding OAR constraints. Dose volume histogram parameters of the target and OAR from the fraction CT with the given library plan were analyzed and the average was calculated over the five sessions for each patient. Toxicity was scored by a radiation oncologist using the CTCAE V4.0. Results: In total, 55 treatments were performed and 78% of the patients were treated with at least one adaptive plan. No grade ≥ 3 GI toxicity was reported. In 20 (36.4%) patients one or more acute grade ≤ 2 GI toxicities were observed. Significant correlations between the real given GI dose parameters (D 0.2cc , D 0.5cc , D 1cc D 2cc and D 5cc , Figure 1) and acute grade ≤2 GI toxicity were found. This correlation disappeared using the planned dose. NTCP models predicted a 50% chance of acute grade ≤ 2 GI toxicity at a D 0.5cc of 57 EQD 2 10 . A D 0.5cc < 35 Gy (49.6 EQD 2 10 ) resulted in a 44% risk of acute ≤ 2 toxicity.

Conclusion: With online adaptive SBRT, the real given dose to the OAR was obtained. It is in this study a prerequisite to develop NTCP models. Significant correlations between GI dose parameters D 0.2cc , D 0.5cc , D 1cc D 2cc and D 5cc and acute grade ≤2 GI toxicity were found. These models can assist in shared-decision making and support online adaptive treatment in the reduction of dose in order to minimize toxicity levels.

Keywords: Oligometastases, Online-adaptive, NTCP model