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.

Inclusion criteria:

• Patients with abdominal and/or pelvic lymph node recurrences of solid tumors.
• No more than 5 metastatic lesions in no more than 2 organs and a controlled primary tumor site.
• Diagnostic imaging includes at least a PET scan or CT thorax/abdomen, of which one is not older than 4 weeks at the time of referral for SBRT.
• Primary tumor must be treated at least 4 months before the diagnosis of metastasis.
• Patients must be 18 years or older.
• Written informed consent.

Exclusion criteria:

• Prior radiotherapy in the same field.
• Second primary malignancy except in situ carcinoma of the cervix, adequately treated non-melanoma skin cancer, or other malignancy treated at least 3 years.
• Serious concomitant systemic disorders that would compromise the safety of the patient or his/her ability to complete the study, at the discretion of the investigator.

De-identified Individual Participant Data (IPD) underlying the results of this study will be shared with qualified researchers. All shared data will be coded and stripped of personal health information (PHI). Approval of the data request and execution of all required agreements (e.g., Data Sharing Agreement) will be required prior to release.

Data requests may be submitted starting 12 months after publication, and data will remain available for 24 months thereafter. Requests for extensions will be considered on a case-by-case basis.

Access will be granted to qualified researchers conducting ethically approved, scientifically sound studies. Requests must include:

• A brief research proposal describing objectives and methods
• (If applicable) A Statistical Analysis Plan
• Documentation of ethical approval
• Agreement to sign a Data Sharing Agreement