Grade (G) 3B follicular lymphoma (FL) is a rare FL subtype which exists on a histological continuum between ‘low-grade’ (Grade 1, 2 and 3A FL) and diffuse large B-cell lymphoma (DLBCL) appearing to share features with each. Clinical characteristics and outcomes are poorly understood due to lack of adequate representation in prospective trials and large-scale analyses. We analyzed 157 G3BFL cases from 18 international centers, and two comparator groups; G3AFL (n=302) and DLBCL (n=548). Composite histology with DLBCL or low-grade FL occurred in approximately half of G3BFL cases. With median 5 years follow-up, G3BFL overall (OS; P<0.001) and progression-free survival (PFS; P<0.001) were superior to DLBCL however G3BFL patients were younger (P<0.001) with better performance status (P<0.001), less extranodal disease (P<0.001) and more frequently normal LDH (P<0.001) at baseline. G3BFL and G3AFL OS and PFS were similar (OS: P=0.83, PFS: P=0.80). After frontline immunochemotherapy, 24% of G3BFL relapsed; DLBCL in 63% and low-grade FL in 19%. 8% of relapses occurred beyond 5 years. In this G3BFL cohort, the R-IPI successfully delineated risk groups, but FLIPI did not. We conclude that immunochemotherapy-treated G3BFL has similar survival outcomes to G3AFL, yet a favourable baseline profile and distinctly superior prognosis compared to DLBCL.
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