This is a prospective, single-arm, pilot feasibility clinical trial designed to evaluate the feasibility and safety of a simulation-free adaptive radiotherapy workflow to enable single-session celiac plexus SBRT planning and delivery. In this trial, the treatment itself is non-investigational (standard-of-care celiac plexus SBRT) but the treatment workflow (simulation-free, using adaptive radiotherapy to compress treatment planning and delivery into a single session) is novel. Investigators hypothesize the successful completion of the simulation-free ART workflow through treatment delivery in the first on-table treatment attempt for at least 80% of patients.

A standard treatment for pain caused by cancer affecting the celiac plexus (a group of nerves in the upper abdomen behind the stomach and pancreas) is a type of radiation therapy called stereotactic body radiotherapy (SBRT). In the usual process for this type of radiation treatment, the patient would first have a special radiation planning scan called a computer tomography simulation (CTsim) to create a personalized treatment plan. The planning process usually takes 5 to 10 days, which means treatment may begin up to 2 weeks after the initial CTsim scan. Recent technology has allowed for treatment to be planned without a separate CTsim scan, called a CTsim-free workflow. In this study, investigators will use pre-existing scans (such as diagnostic imaging) to create the patient's treatment plan. On the day of treatment, the care team will use a technique called Adaptive Radiotherapy (ART) to adjust the plan based on the patient body's position and anatomy that day, allowing planning and treatment to happen in a single visit.

Inclusion Criteria:

• Subjects must have histologically or cytologically confirmed cancer that is metastatic or unresectable, and considered appropriate per the treating physician to receive celiac plexus SBRT.
• Age >18 years. Because no data are currently available on the use of celiac axis SBRT in subjects ≤18 years of age, children are excluded from this study.
• Performance status: ECOG Performance status ≤ 3
• Severe retroperitoneal pain syndrome (radiates from the lower back to the upper abdomen, belt- like distribution), intensity of at least 5 on 11-point Brief Pain Inventory (BPI, average pain) scale.
• Subjects must have anatomical involvement of the celiac plexus on the diagnostic CT, PET/CT, or MRI. This includes: Any pancreatic cancer, any other cancer that on imaging demonstrates either gross involvement of the celiac blood vessels or celiac plexus on imaging OR haziness around the celiac blood vessels that typically implies tumor engulfment.
• Prior chemotherapy or biological treatment is allowed, but any active oncological treatment should be stopped at least 1 week prior to radiation therapy and renewed at least 1 week following radiation therapy.
• Subjects must have the ability to understand and the willingness to sign a written informed consent document.
• Subjects must have a diagnostic CT of the abdomen and pelvis, with or without contrast, acquired < 28 days prior to the study consent.

Exclusion Criteria:

• In ability to tolerate lying supine and still for at least 45 minutes.
• Performance status: ECOG Performance status of 4.
• Previous radiotherapy to the upper abdomen overlapping with the projected site of treatment.
• Pregnant or breastfeeding women are excluded from this study.
• Subjects with conditions associated with increased risk of side effects from radiation such as inflammatory bowel disease and scleroderma.
• Women of childbearing potential must have a negative pregnancy test within 14 days of study entry. If pregnancy test is not clinically indicated as determined by treating physician or protocol principal investigator, documentation of this exception is sufficient in lieu of a pregnancy test.