The term splenic B-cell lymphoma/leukaemia, unclassifiable encompasses B-cell lymphoproliferative disorders that do not fulfil the criteria of other well-defined small B-cell lymphomas. The WHO classification includes within this category, two provisional disease entities: hairy cell leukaemia variant (HCL-v) and splenic diffuse red pulp small B-cell lymphoma (SDRPL) .
Hairy cell leukaemia variant
HCL-v is a very rare mature B-lineage lymphoproliferative disorder characterized by a clonal proliferation of B cells that morphologically resemble hairy cells but have a prominent nucleolus, resembling that of a prolymphocyte [1–3]; there is also splenic red pulp infiltration. This provisional disease entity occurs in the elderly without any male predominance. Splenomegaly is characteristic while palpable lymphadenopathy is usually minor or absent.
Haematological and pathological features
In contrast to hairy cell leukaemia (HCL), the white cell count is usually moderately elevated (5–300, median around 80 × 109/l) [4–7]. There may be mild anaemia and thrombocytopenia. The monocyte count is preserved. The neoplastic cells have moderately plentiful cytoplasm with irregular margins and a round nucleus with a large prominent nucleolus and some chromatin condensation (Figure 12.1). There may be some binucleated cells. Autoimmune manifestations are rare and, in contrast to splenic marginal zone lymphoma and SDRPL, the presence of a serum monoclonal paraprotein has not so far been reported. Bone marrow infiltration is usually interstitial and often intrasinusoidal  (Figure 12.2). Cells may be spaced, as in HCL, but this feature is not so consistently present. Reticulin deposition is increased but not to the extent that is usual in HCL so that it is usually possible to aspirate bone marrow. Splenic infiltration is preferentially in the red pulp and may be indistinguishable from that of HCL and SDRPL.
Peripheral blood film of a patient with hairy cell leukaemia variant (HCL-v) showing characteristic cells. Romanowsky stain, ×100 objective.
Section of a trephine biopsy specimen in HCL-v showing, in the centre of the photograph, intra-sinusoidal infiltration. Haematoxylin and eosin stain (H&E), ×100 objective.
Cytogenetic and molecular genetic abnormalities
Complex karyotypes and deletion of one TP53 allele in a proportion of cells are common, being seen in around 30% of patients [8, 9]. The presence of DNA copy number alterations in various chromosomes has also been documented. In addition to 17p deletion, gains of chromosome 5 (as in HCL) and deletion of 7q31-33 are most frequently found .
Somatic mutations of IGHV are present in around two thirds of cases with an increased usage of the IGHV4-34 family compared to normal B ...