Quiz. What is your diagnosis?

An 86 years old man underwent a bone marrow biopsy for thrombocytopenia and monoclonal gammopathy. Morphological examination showed a hypercellular marrow for the age of the patient (> 90%). A routinary first step immunohistochemical study better disclosed the presence of nodular aggregates of PAX5+ (Fig. 1A, 2,5×) B lymphoid cells in the context of bone marrow tissue. The aggregates were composed of small and mature lymphocytes (Fig. 1B, hematoxylin-eosin, 20×, arrows) with a prevalent B immunophenotype, as highlighted by CD20 immunostain (Fig. 1C, 20×). Nodules were surrounded by haematopoietic tissue represented predominantly (about 70% of marrow cellularity) by immature elements (Fig. 1B hematoxylin-eosin, 20×), with a positive immunostain for myeloperoxidase (MPO) (Fig. 1D, 20×) and CD34. Further immunophenotyping showed coexpression in B cells of CD5, CD23 and CD43 (cyclin D1, SOX11, CD10 and bcl6 were negative), in presence of admixed small T cells (CD3+, CD5+, CD43+). Immature haematopoietic cells, in addition to MPO and CD34, tested positive for CD117 (c-kit) (glicophorin-C, CD61, CD7, CD15 and Tdt were negative).