Outline of FL cases included in the study. A total of 233 stage I cases were initially submitted from 2 clinical trials and 1 population-based cohort; 216 fulfilled all clinical inclusion criteria (EORTC study 20971, n = 143; GLSG, rituximab, and involved-field radiotherapy in early-stage FL [MIR (Mabthera and Involved Field Radiation)] study, n = 39; and the HMRN population-based registry, n = 34). In 82 of 216 cases with targeted NGS data, a complete data set on translocation, CNA, and mutations was successfully obtained, meeting all quality measures (ie, sufficient amount of DNA [>100 ng] from formalin-fixed paraffin-embedded [FFPE] material, and sequencing results with a minimum mean target coverage off >30 reads for paired-end sequencing and 300 000 reads for shallow sequencing). For 73 of 82 cases, complete IHC data of 7 markers (CD3, CD4, CD8, PD1, FOXp3, CD68, and CD163) of the microenvironment were available, meeting all quality measures, indicating sufficient amount of FFPE material to obtain two 1-mm cores, and the cores should contain >50% tumor tissue to score. As a reference cohort, 667 stage III/IV cases were selected from 2 clinical trials and 4 population-based cohorts, of which 391 fulfilled all clinical inclusion criteria (LYSA [Lymphoma Study Association] FL2000 study, n = 163; GLSG2000 study, n = 98; HMRN population-based registry, n = 100; Sweden population-based registry, n = 19; St. Bartholomew’s Hospital, London, n = 6; and Stanford University Hospital, Stanford, n = 5). In 139 of 391 cases, complete NGS data meeting all quality measures were obtained. For 120 of 139 cases, complete microenvironment information meeting all quality measures was also available. Depicted in the green box are the cases that are incorporated in the analysis.