CT-guided online adaptive SBRT for abdominal and pelvic lymph node oligometastases:Daily anatomy based plan selection

Stereotactic body radiation therapy (SBRT) has emerged as an effective treatment option for patients with oligometastatic disease, improving progression-free and overall survival through the precise delivery of high radiation doses while sparing surrounding organs at risk (OARs). However, SBRT for abdominal and pelvic lymph node oligometastases remains challenging due to inter- and intrafraction organ motion caused by deformation, variable filling, respiration, and peristalsis. These anatomical variations may lead to target underdosage or excessive OAR exposure, potentially resulting in local tumor progression and treatment-related toxicity. The aim of this thesis was to optimize clinical and dosimetric outcomes for patients with abdominal and pelvic lymph node oligometastases using radiotherapy technician–led, CT-guided online adaptive SBRT with a plan-of-the-day approach. First, the safety of SBRT for abdominal and pelvic lymph node oligometastases was evaluated through a systematic review, demonstrating low rates of severe toxicity, with late grade 3 and 4 events occurring in 0.6% and 0.1% of patients, respectively, predominantly in high-dose and re-irradiation settings. Subsequently, the clinical feasibility, safety, and efficacy of online adaptive SBRT were prospectively investigated in the phase II STEAL study. Adaptive plans were selected in more than half of all fractions, resulting in excellent local control (96% at 1 year) and a median overall survival of 4.1 years, with minimal severe toxicity. Dosimetric analyses showed that online adaptation significantly increased the delivered dose to the target without increasing gastrointestinal organ doses compared to standard-of-care plans, supporting the clinical benefit of adaptive treatment strategies. In addition, this thesis explored the relationship between delivered dose and toxicity, demonstrating that higher gastrointestinal doses were associated with acute gastrointestinal toxicity and enabling the development of a normal tissue complication probability model suitable for online adaptive use. Late ureteral toxicity was found to be rare and not solely dose-dependent, indicating the relevance of patient- and treatment-related factors beyond dosimetry. Finally, simulations showed that personalized adaptive fractionation could substantially reduce the number of treatment fractions, improving patient comfort and treatment efficiency while maintaining dosimetric quality. Together, these findings support CT-guided online adaptive SBRT as a safe, effective, and forward-looking treatment strategy for abdominal and pelvic lymph node oligometastases and provide a foundation for further clinical optimization.