This research study will enroll people who have cancer in their abdomen or pelvis that was treated previously with radiation therapy. The purpose of this research study is to test the safety and possible harms of treating tumors in these regions with another round of radiation therapy, called reirradiation or "reRT." The researchers want to find out what effects (good and bad) reRT has in people with cancer in the abdomen and pelvis.

Inclusion Criteria:

• A primary or metastatic lesion in the abdomen or pelvis for which reirradiation will be delivered. There is no maximum lesion size.
• Single course of prior radiation therapy with prescribed equivalent dose in 2 Gy fractions (EDQ210) of at least 40 Gy.
• 50% isodose line of the prior radiation therapy course is expected to overlap at minimum the 50% isodose line of the radiation therapy delivered in this study.
• At least 6 months from completion of prior radiation therapy to initiation of study therapy.
• Eastern Cooperative Oncology Group (ECOG) performance status 0-2.
• Life expectancy at least 6 months.
• Women of childbearing potential and men must agree to use adequate contraception (hormonal or barrier method of birth control) prior to study entry and for the duration of study participation. Abstinence is acceptable if it is the participant's preferred method. Should a participant become pregnant or suspect pregnancy while participating in this study, the treating physician must be informed immediately.
• Concurrent hormonal therapy or immunotherapy such as immune checkpoint inhibitor is allowed

Exclusion Criteria:

• Contraindication to having an MRI scan.
• Lesion that would receive study therapy arises from or invades a gastrointestinal (GI) luminal organ (a lesion abutting but not invading a GI luminal organ is permitted)
• Lesion that would receive study therapy is not located in the abdomen or pelvis.
• > 1 course of overlapping radiation therapy delivered to the abdomen or pelvis.
• < 70% of the gross tumor volume (GTV) receives at least the 95% of the prescribed dose on the original plan using the simulation day anatomy.
• More than 1 lesion in the abdomen or pelvis that requires reirradiation.
• History of inflammatory bowel disease.
• Unable to acquire DICOM radiation therapy treatment plan from the prior radiation therapy.
• Unresolved grade 2+ toxicity (Common Terminology Criteria for Adverse Events version 5.0) from previous anti-cancer therapy.
• Any condition in the opinion of the investigator that would interfere with evaluation of study treatment or interpretation of participant safety or study results.
• Individuals who are pregnant.
• Chemotherapy given within 1 week prior to or following reirradiation.
• Vascular endothelial growth factor (VEGF) inhibitor given within 8 weeks prior to or following reirradiation