Adaptive Fractionation in Online Adaptive Stereotactic Radiotherapy for Abdominopelvic Lymph Node Oligometastases

Oligometastases, a state of cancer with up to five metastases, was traditionally treated with systemic treatments like chemotherapy. Treatment with stereotactic body radiotherapy (SBRT) showed a high local control and improved disease-free survival. The use of SBRT also allows for the deferral of systemic treatment, thereby delaying its potential side effects. SBRT enables the delivery of a high dose to the tumor while minimizing the dose to organs at risk, reducing normal tissue damage, however, toxicity remains a potential issue in the abdominopelvic region, where lymph node oligometastases are often located near highly mobile, radiosensitive organs like the bowel. Online adaptive radiotherapy is used to address this issue, adapting the treatment plan to the anatomy of the day. Unfortunately, adaptive radiotherapy results in longer treatment delivery times than conventional radiotherapy. This can potentially be countered by increasing the fraction dose and reducing the number of fractions if the patient anatomy allows it. This is convenient for the patient as it reduces the number of hospital visits, and it could also reduce the total workload for the hospital. Therefore, there is not only a benefit of a reduction in toxicity by adaptive treatment, but also in reducing the total treatment time. This study aims to investigate if the number of adaptive fractions can be reduced by 30% for patients with abdominal or pelvic lymph node oligometastases.