Clinicopathological features of in situ follicular neoplasm and relations with follicular lymphoma in Japan

Incidence, Clinicopathologic Features, and Genetics of in situ Follicular Neoplasia: A Comprehensive Screening Study in a Japanese Cohort

AIMS

In situ follicular neoplasia (ISFN) is a histologically recognisable neoplastic proliferation of follicular lymphoma (FL)-like B cells confined to the germinal centres. While some ISFNs are associated with overt FL, others are incidentally identified as isolated or pure forms in individuals without evidence of overt FL. The prevalence of incidentally found isolated ISFN is approximately 3% in Europe; however, no screening study has been conducted in Asia. To investigate the incidence and clinicopathologic characteristics of ISFNs in the Japanese population, we conducted histopathological screening of the lymph nodes (LNs) resected for solid tumours or inflammatory conditions.

METHODS AND RESULTS

We screened for ISFN in 5,700 LNs from 340 individuals using immunohistochemistry for BCL2 and identified seven ISFNs, with an incidence of 2.1%. The median age of the individuals with ISFN was 67 years, none of whom developed overt FL, with a median follow-up of 59 months. Next-generation sequencing was performed in five ISFNs, and ten variants in seven FL-associated genes were identified. The identified variants included HIST1H1E (2/5), ARID1A (2/5), KMT2D (1/5), CARD11 (1/5), BCL7A (1/5), CREBBP (1/5), and TNFRSF14 (1/5).

CONCLUSIONS

The incidence of isolated ISFN in the Japanese population is not significantly different from that in Europe, presumably reflecting the recent increase in FL in Japan. These incidentally found ISFNs have a low potential to transform into overt FL. Although mutations of FL-associated genes are already present in ISFNs, further molecular studies are needed to identify driver genes leading to the transformation of ISFN to overt FL.