ISRT 20 Gy for Indolent Localized Gastrointestinal (GI)-Lymphoma

Inclusion Criteria: * primary indolent gastric or duodenal lymphoma * pathology: marginal zone lymphoma (MZL) or follicular lymphoma (FL) * stage: clinical stage I or II (Ann Arbor classification) * H. pylori negative or antibiotic resistant lymphoma * IPI or FLIPI score low - high (0-4) * any size of tumor or affected lymph nodes * male or female with age ≥ 18 years * performance status ECOG 0 - 3 * written informed consent by the patient Exclusion Criteria: * prior radiation treatment of the gastrointestinal lymphoma * stage: clinical stage III or IV (Ann Arbor classification)-unability to understand the informed consent or unwillingness to participate in the study * severe comorbidity or organ dysfunction contraindicating the use of RT (liver cirrhosis Child-Pugh C, chronic obstructive pulmonary disease GOLD 4, heart insufficiency NYHA IV, dialysis dependent renal insufficiency, uncontrolled epilepsy) * known seropositivity for HIV * acute hepatitis B or C infection * chronic inflammatory bowel disease * prior malignant disease (exclusion: basalioma, non-metastasized solid tumor in constant remission diagnosed \>3 years ago) * pregnancy or breastfeeding * active substance abuse or severely compromised compliance