Stereotactic Body Radiotherapy for Cancer Pain Relief

Inclusion Criteria: * Participants 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 participants ≤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 despite analgesic use. Note: previous use of celiac plexus block/neurolysis for pain control is allowed and will be recorded * Participants 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 must be stopped at least 1 week prior to radiation therapy and renewed at least 1 week following radiation therapy. * Participants must have the ability to understand and the willingness to sign a written informed consent document. * Participants must have a diagnostic CT, PET/CT, or MRI of the abdomen and pelvis, with or without contrast, acquired \< 28 days prior to the study consent. This diagnostic scan, to be used for pre-plan formation, must fully visualize the celiac plexus target intended for simulation-free treatment, with full axial field-of-view at the axial levels for treatment. The scan must be of sufficient clinical quality for the treating physician to delineate the target for treatment. Exclusion Criteria: * Prior radiotherapy to the upper abdomen. * Previous radiotherapy to the upper abdomen overlapping with the projected site of treatment. * Pregnant or breastfeeding women are excluded from this study. * Participants 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.