Real world data on histological transformation in patients with follicular lymphoma: incidence, clinico-pathological risk factors and outcome in a nationwide Danish cohort.

We investigated incidence, risk factors and outcome for follicular lymphoma (FL) patients with histologic transformation (HT) found at primary diagnosis (discordant/composite, dc-tFL) or sequentially (s-tFL). Between 2000 and 2015, 2773 patients were identified. The majority of patients (2252, 81%) did not experience HT (nt-FL), while 224 (8%) had dc-tFL and 297 (11%) s-tFL. The risk of HT was 2.2% per year and 9.6% at 5 years. Age ≥60, a high FLIPI risk score and LDH-elevation were associated with increased risk of HT. Calculated from primary diagnosis and compared with nt-FL, 5-year overall survival (OS) was inferior in both s-tFL and dc-tFL (nt-FL: 82%, s-tFL: 68%, dc-tFL: 68%; p = .001), whereas 5-year progression-free survival (PFS) was worse only in s-tFL (s-tFL: 18%, dc-tFL: 58%, nt-FL: 60%). Calculated from time of HT, s-tFL had inferior outcome compared to dc-tFL for both OS (s-tFL: 47%, dc-tFL: 68%, p = .001) and PFS (s-tFL: 35%, dc-tFL: 58%, p = .001).