NTCP models based on CT-guided online adaptive SBRT for abdominal lymph node oligometastases.

Background And Purpose: This study prospectively analyzed the correlation between dose and toxicity in CT-guided online adaptive stereotactic body radiotherapy for abdominal and pelvic lymph node (A-P LN) oligometastases. The aim was to design an NTCP model to predict acute gastrointestinal (GI) toxicity.

Materials And Methods: Patients with oligometastatic A-P LN received 45 Gy in five fractions. Three treatment plans were created using a pre-treatment diagnostic CT scan. After a pre-fraction in-room CT scan, the radiotherapy technologist used a decision tree to select the optimal plan. Dose volume histogram (DVH) parameters (Dmax, D0.2 cc, D0.5 cc, D1cc D2cc D5cc and D10cc) were extracted from the fraction CT with the selected library plan. The DVH-parameters were analyzed and used for NTCP modelling based on logistic regression analysis.

Results: In total, 55 treatments were performed in 52 patients. No acute or late grade ≥ 3 GI toxicity was observed, while 20 (36.4 %) patients experienced one or more acute grade ≤ 2 GI toxicities. The median dose to the gastrointestinal organs (GIO) was significantly higher across all DVH parameters in patients with toxicity. The NTCP model predicted a 50 % chance of acute grade ≤ 2 GI toxicity at a D0.5 cc of 57 EQD210.

Conclusion: Significant correlations between GI dose parameters D0.2 cc, D0.5 cc, D1cc D2cc, and D5cc and acute grade ≤ 2 GI toxicity were found. These models have the potential to demonstrate the extent to which toxicity rates can be reduced by lowering the dose.