Intra-Follicular Proliferation Rate Is a Powerful Independent Prognosticator in Follicular Lymphoma.
Authors: Koster Ad , Tromp Hedwig A. , Raemaekers John M. , Borm George F. , MacKenzie Marius A. , Van Krieken Joannes H.J.M. 
Published: April 2020

Abstract
Several prognostic scores are used to predict the outcome of patients with follicular lymphoma (FL). Histologic grading is widely used, but poorly reproducible. Scoring systems such as the (FL)IPI consist of surrogate clinical markers that probably reflect underlying biological phenomena. We hypothesized that the proliferation rate of the intra-follicular neoplastic cells in FL may predict clinical outcome.
Proliferating cells were identified by using the Mib-1 antibody in pre-treatment lymph node biopsies of 51 patients. In each section 200 cells per follicle were assessed in five follicles. The proliferation ratio (PR) was defined as (no. of Mib-1 positive cells/ total no. of cells) x 100. The PR was assessed only in the follicles, since many proliferating, non-malignant cells are present in the inter-follicular area. The inter-observer variability was assessed by repeating this procedure for 25 cases by a second investigator blinded to the outcome of the first assessment, resulting in a correlation coefficient of 0.81 (p &lt; 0.001). All patients participated in a prospective multicenter trial on first-line treatment with the combination of 8 cycles of CVP (cyclophosphamide, vincristine and prednisone) chemotherapy plus interferon-alpha2b, followed by interferon-alpha maintenance in responding patients until relapse or progression. For statistical analysis the Kaplan-Meier product limit method, the log-rank test, the Cox proportional hazard model and the Mann-Whitney-U test were used.
The median age of the patients was 53.3 years; 37 patients had stage IV disease; in nine patients a wait and see interval preceded the treatment (median duration 13 months). A low IPI score was present in 27 patients, intermediate in 20 and high in one. (three patients unknown). After central revision of the original diagnosis six patients were classified as grade 3 FL, 45 as grade 1 or 2. The overall median PR was 16.9 (3.1–49.2). In grade 1 and 2 FL the median PR was 16.1, in grade 3 it was 24.2 (p = 0.02). After a median follow-up of 71 months the median progression-free survival (PFS) for all patients was 25 months. The median PFS was not reached in the patients with the PR below the median compared to only 15 months in the patients with the PR above the median (p = 0.00059). In patients with the PR below median, overall survival was not reached compared to only 42 months in patients with a high PR (p = 0.0019). The PR maintained its prognostic impact on PFS and OS when tested as a continuous variable (p = 0.016 and 0.053 resp.). When the six patients with grade 3 FL were excluded from the analysis, the results remained significant. Other parameters that were significantly associated with a worse OS in univariate analysis were male sex, the presence of bulky disease and intermediate or high IPI. In multivariate analysis the association of high PR with a worse OS remained significant.
We found that in FL intra-follicular proliferation rate is a strong independent prognostic factor of PFS and OS. Grade 3 FL appeared to have a higher PR than low grade FL, although numbers are small in this study. PR assessment is easy, relatively fast and reproducible. When confirmed in larger series, the intra-follicular PR could be used instead of histologic grading in identifying the aggressive types of follicular lymphoma, requiring other types of treatment.