Study design and population

In this French multicentre case–control study, cases were adults (age ≥18 years) with RA fulfilling ACR-EULAR 2010 criteria in whom B-cell NHL or Hodgkin’s lymphoma developed after the diagnosis of RA. Exclusion criteria were T-cell lymphoma, a history of lymphoma before the RA diagnosis and a history of secondary Sjögren’s syndrome, given that this autoimmune condition is associated with increased risk of lymphoma.^{1 3} Cases were recruited following a call for observations mediated by the CRI-IMIDIATE network and among French Society of Rheumatology registries (AIR-PR, ORA, REGATE) and the Etude et Suivi des Polyarthrites Indifférenciées Récentes (ESPOIR) cohort. The AIR-PR, ORA and REGATE registries and the ESPOIR cohort have been described in previous studies.^17–20 Cases were excluded if the case report form was not sent to the investigating centre (Kremlin-Bicêtre Hospital).

Controls were drawn at random from the ESPOIR cohort among patients with RA fulfilling ACR-EULAR 2010 criteria who completed a 10-year follow-up. Cases and controls were matched on age (age at lymphoma diagnosis for cases and age at 10-year ESPOIR visit for controls). This variable was chosen because of its well-known association with risk of B-cell lymphoma in the overall population.²¹