A Prospective Phase III Study Comparing Standard Radiotherapy (24 Gy) With a PET REsponse-guided Very Low Dose (4 Gy) Staged Radiotherapy Strategy for Potentially Curable, Indolent B-cell Lymphomas

Inclusion Criteria: (Arms 1\&2) * Patients must be diagnosed with a follicular lymphoma or marginal zone lymphoma. Of note, there are now two widely-utilized pathology classification criteria used for mature B-cell lymphomas, the World Health Organization (WHO) 5th Edition Classification of Lymphoid Neoplasms and the International Consensus Classification (ICC) of Mature Lymphoid Neoplasms. Either criteria is acceptable and for the purposes of this protocol, the following diagnoses are included: * Follicular lymphoma * WHO 5th Edition * Classic follicular lymphoma (cFL) * Follicular lymphoma with uncommon features (uFL) * Pediatric type follicular lymphoma * Duodenal type follicular lymphoma * ICC * Follicular lymphoma, grades 1-2 or 3A * BCL2 rearrangement negative, CD23 positive follicle center lymphoma * Pediatric type follicular lymphoma * Duodenal type follicular lymphoma * Marginal zone lymphoma * WHO 5th Edition and ICC * Nodal marginal zone lymphoma * Pediatric nodal marginal zone lymphoma * Extranodal marginal zone lymphoma of mucosa associated lymphoid tissue (MALT) * Patients must have stage I or II disease (with stage I defined as involvement of one nodal region and stage II as two or more nodal regions involved on the same side of the diaphragm) * Patients should be newly diagnosed or previously observed with no prior lymphoma-directed therapy * If the patient is referred for localized gastric MALT lymphoma, there should be documented negative H. Pylori testing within 6 months prior to proposed radiotherapy * Age at the time of enrollment of ≥18 years * Patients must be able to start radiation within 2 months from time of randomization Exclusion Criteria: * Prior radiation to site(s) needing treatment * Follicular lymphoma, grade 3B (ICC) or Follicular large B cell lymphoma (FLBL by WHO 5th edition criteria) * Patients planned to receive concurrent systemic therapy (including oral steroids) for their lymphoma * Patient has cutaneous only iNHL defined as primary cutaneous B-cell lymphoma, primary cutaneous follicle center lymphoma, cutaneous marginal zone lymphoma or unspecified indolent lymphoma of the skin * Gross total resection of all disease * Tumor size measuring ≥5 cm in maximum diameter on any modality diagnostic imaging * Patients with any concurrent medical or psychiatric condition or disease which, in the investigator's judgment, would make them inappropriate candidates for entry into this study