Follicular lymphoma with marked monocytoid or plasmacytoid differentiation and tiny or indistinct follicles: a case study of four patients.

We report four cases of exceptional follicular lymphoma (FL) showing marked monocytoid or plasmacytoid differentiation and tiny or indistinct follicles, mimicking marginal zone lymphoma or plasma cell neoplasm. The lymphoma of patient 2 had lymphoepithelial lesions. The lymphoma of patient 4, who had a history of typical FL, was composed of only monocytoid cells, suggesting secondary involvement of the monocytoid component of FL. The lymphomas of patients 1 and 2 had tiny follicles immunoreactive with CD10 and BCL-2, whereas samples from patients 3 and 4 were negative for CD10. Fluorescence in situ hybridization (FISH) analysis showed IGH/BCL2 fusion in three of the four lymphomas (patients 1, 3, and 4). Histological analysis showed time- and site-dependent differences in FL characteristics in patients 2 and 4. Identifying tiny CD10+/BCL-2+ neoplastic follicles, detecting IGH/BCL2 fusion by FISH, and considering a history of FL are crucial for diagnosing such exceptional cases of FL.