9-year-old castrated male beagle dog, Canis familiars.The dog was referred to a veterinary surgeon for evaluation of a possible splenic mass. An abdominal ultrasound showed a 2 inch mass in the cranial aspect of the spleen, which was confirmed at surgery for a splenectomy. No other gross abnormalities were noted by the submitting veterinarian.
The surgeon reports a single splenic nodule and no other gross abnormalities.
The splenic nodule consists of coalescing nodules of lymphocytes.Â The intervening tissue is congested with a mixture of plasma cells, myeloid and erythroid precursors, megakaryocytes and hemosiderophages.Â The coalescing lymphoid tissue consists mainly of marginal zone cells which are intermediate in size (nuclei approximately 1.5 times the diameter of a red blood cell).Â The cells have a scant to moderate amount of eosinophilic cytoplasm and round to oval nuclei with a single prominent central nucleolus.Â Admixed throughout this proliferation are smaller numbers of larger cells with oval open nuclei (interpreted as dendritic cells), remaining germinal center cells, and scattered lymphocytes with small hyperchromatic nuclei (measuring 1 times the size of a red blood cell).Â Mitoses within the marginal zone cell population range from 0-2 in a single high power field.
Splenic Marginal zone lymphoma (probable).
Immunohistochemistry: The lymphocytes are strongly CD79a-positive and CD3-negative.Â PCR for antigen receptor rearrangement (performed by the Leukocyte Antigen Biology Laboratory at UC Davis): Molecular clonality analysis of IgH2, IgH3 and KDE (B cell) revealed polyclonal rearrangements.
Marginal zone lymphoma
Marginal zone lymphoma (MZL) is an indolent B-cell neoplasm derived from the cells of the marginal zone of lymphoid follicles.Â Three types of MZL with different clinical and molecular characteristics are recognized in humans: extranodal mucosa-associated lymphoid tissue (MALT) lymphoma, splenic MZL (SMZL) and nodal MZL (NMZL).Â In humans, SMZL is rare (1-3% of lymphomas) and usually involves the spleen, bone marrow, and blood.Â The disease can present as an incidental finding or with symptoms of splenomegaly.Â Anemia, lymphocytosis, or thrombocytopenia are reported in about 25% of cases.(7) Currently, there is no known genetic abnormality specific for SMZL, but deletions of chromosome 7q are found in 30-50% of cases.(5) Therapy for SMZL in humans remains controversial with options including splenectomy, various chemotherapeutic agents, or rituximab alone.(1,5,7) Most disease-related deaths in SMZL are associated with transformation to diffuse large B cell lymphoma.(5)
Indolent lymphomas in dogs include follicular, mantle cell, marginal zone, and T-zone lymphomas.Â Valli et al examined 66 dogs with indolent lymphoma which included 33 dogs with nodal MZL and 13 cases of splenic MZL; however, only 3 of these cases had outcome data available.(9) Two recent studies describe SMZL in 5 and 34 dogs, respectively, and provide better insight into clinical characteristics and outcome of this disease.(4,6) In the larger study by OBrien et al, the overall median survival time (MST) after splenectomy was 383 days and dogs that had MZL as an incidental finding had a longer MST (1,153 days) compared to dogs with clinical signs associated with MZL (309 days).Â Other factors such as lymph node involvement, hemoabdomen, adjuvant chemotherapy, and concurrent malignancies did not influence survival.(4)
Assessment of tissue architecture is needed for a diagnosis of MZL, and therefore, histopathology is required.Â MZL has a distinct nodular pattern in which the lighter-staining neoplastic marginal zone cells form a dense cuff around small foci of darkly stained mantle cells (fading follicles).Â The neoplastic marginal zone lymphocytes are intermediate in size, with nuclei measuring approximately 1.5 times the diameter of a red blood cell, and have a single prominent central nucleolus.Â Benign marginal zone hyperplasia (MZH) has a similar architectural appearance, although the expanded marginal zone is heterogeneous, containing a mixture of small and intermediate sized lymphocytes.(10) Differentiating between MZH and MZL is challenging because MZL arises on the background of MZH.Â Therefore, immunophenotyping and molecular clonality are ultimately required for a definitive diagnosis.
This case highlights the difficulty in differentiating between MZL and MZH and this distinction is especially difficult in the spleen of dogs.Â Lymphoid nodular hyperplasia and complex nodular hyperplasia (-ï¿½-ï¿½fibrohistiocytic nodules) are common in the canine spleen; it is possible that many cases of nodular hyperplasia contain areas of MZL.Â In this case, the architectural feature of homogenous coalescing marginal zone cells was more suggestive of MZL than MZH.Â The fields in which there were up to 2 mitotic figures further supported this diagnosis, and is likely indicative of later stages of disease development, as mitoses increase with disease progression.(9,10)
As expected, the cells were CD79a positive and CD3 negative, indicating a B cell phenotype.Â Tissues were sent to the Leukocyte Antigen Biology Laboratory at UC Davis.Â Molecular clonality analysis of IgH2, IgH3, and KDE (B cell) revealed polyclonal rearrangements, which suggested a reactive process rather than a neoplasm.Â However, upon review of the histopathological and immunohistochemical findings, the reviewing pathologists at UC Davis were also highly suspicious of MZL and that the PARR testing might be a false negative result.Â The OBrien paper also found a subset of cases (27%) that did not demonstrate a clonal population (pseudoclonal and polyclonal rearrangements)(3), which is consistent with the published sensitivity of this PCR-based test.(2,8) In that study, the polyclonal rearrangements were attributed to a mutation in V or J segments of Ig.Â Similarly, the most likely reasons for false negative results in this case are mutation of gene segments that are not covered by the primer sets or mutation of primer sites during somatic hypermutation.Â In support of this suspicion is the fact that the IgH3 locus did not show a robust polyclonal curve as would be expected in a hyperplastic lesion, but instead had non-reproducible peaks of variable height within a weak polyclonal background.
Spleen: Lymphoma, intermediate size, low grade, consistent with marginal zone lymphoma.
The contributor provides an exceptional overview to splenic marginal zone lymphoma and discusses its diagnostic challenges, specifically in distinguishing from hyperplastic nodules.Â Splenic nodular hyperplasia is a common finding in dogs and presents with variable histologic appearance depending on its cellular constituents.Â The simple or lymphoid form of hyperplasia is composed of discrete lymphocytes often forming follicles with germinal centers.Â The complex form of nodular hyperplasia additionally contains a proliferative stroma.Â Some variations of these lesions were previously diagnosed as fibrohistiocytic nodules, and recent advances in immunohistochemistry has led to their reclassification into a diverse group of diseases to include the above hyperplastic nodules, histiocytic sarcoma, and various subtypes of lymphoma.(3)
Using the grading criteria based on mitotic figures per single 400x field (indolent = 0-1, low = 2-5, intermediate = 5-10, high >10) and cell size determined by comparison to red blood cells (small = 1xRBC, intermediate = 1.5xRBC, large â¥ 2xRBC), we identified this neoplasm as intermediate size and low grade, with definitive classification of MZL requiring immunophenotyping and molecular clonality testing.(8)
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10. Valli VE, Veterinary Comparative Hematopathology.Â 1st ed.Â Ames, IA: Blackwell Pub; 2007.