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

* Prove the effectiveness of 20 Gy (and non-inferiority to actually recommended 30 Gy) with respect to response rate 6 months after radiotherapy. * Correlation of blood serum biomarker levels with lymphoma response to radiation treatment * Recording of survival rates, quality of life (QoL), radiogenic toxicities and inflammation relevant molecules in blood serum. Primary Objective: Response rate 6 months after end of treatment, 4 categories according to GELA-criteria: CR (complete remission) = CR or pMRD (probable minimal residual disease), PR (partial remission) = rRD (responding residual disease), NC (no change), PD (progressive disease) Secondary Objectives: QoL according to EORTC (QLQ C30 and STO22). EFS=Event-free survival (time to any failure or death from any cause, patients in CR or PR), LSS=lymphoma-specific survival (time to death related to lymphoma or associated with the treatment, all patients), PFS=Progression-free survival (time to progression of lymphoma or death from any cause, all patients), OS=overall survival (time to death from any cause, all patients). Level of cytokines IL-1β, IL-4, IL-8, TNFalpha and other inflammation relevant molecules Syndecan1, MMP-2 and S100 proteins. Acute toxicities during treatment according to NCI-CTC, chronic toxicities according to NCI-CTC/ LENT-SOMA. Monitoring of Adverse Events (AEs) and Serious Adverse Events (SAEs) -Assessment of safety: Monitoring of Adverse Events (AEs) and Serious Adverse Events (SAEs)