A comparative assessment of histology, immunohistochemistry and clonality in the differential diagnosis of splenic lymphoid nodules in dogs

Canine splenic lymphoid nodules are histologically classified as indolent lymphomas (marginal zone lymphoma – MZL; mantle cell lymphoma - MCL) or lymphoid nodular hyperplasia, simple (SNH) or complex (CNH) type. Nevertheless, their differentiation may be difficult on a plain morphological basis, because of similar histologic appearance and poorly defined diagnostic criteria. In order to evaluate the possible contribution of image analysis, immunohistochemistry (IHC) and clonality, we reviewed 30 surgical samples of splenic lymphoid nodules histologically diagnosed as 10 MZL, 3 MCL, 11 SNH and 6 CNH. Proliferative activity was evaluated together with immunophenotype with a double staining technique for Ki67-index and CD3 or CD79a. Image analysis was then performed to assess in each nodule the percentage of T/B-cell areas and the corresponding proliferative activity. Three cases formerly diagnosed as MZL were reclassified as lymphoid hyperplasia upon evaluation of CD3 and CD79a patterns. The percentage of CD79a-positive areas was significantly higher in lymphomas (mean, 3.5 ± 11.3%) compared with nodular hyperplasia (mean, 24.2 ± 10.9%; P = 0.001). The proliferative activity of B-cells was lower in hyperplastic lesions than in lymphomas (median Ki67-index, 2.2% and 5.5%, respectively; P=0,014). Regardless of the histological diagnosis, Ki67-index was higher in non-B cells than in B cells in all cases (median, 18.7%; P < 0.001). The best cut off value discriminating between lesions diagnosed as hyperplasia or lymphoma was a B cell area of at least 27% with a Ki67 index above 3%. None of the dogs except one received adjuvant treatment besides splenectomy. Dogs were monitored for a median follow-up time of 947 days (range, 133-2261) and in no case a relapse was documented. Overall median survival time was 1237 days, with no significant difference between lymphomas and hyperplasia. Surprisingly, clonality results showed a monoclonal or biclonal rearrangement also in the vast majority of cases diagnosed as hyperplasia, suggesting a pathogenetic continuum with lymphoma. In conclusion, the combination of histology and IHC may help to improve the diagnostic accuracy of canine splenic lymphoid nodules, even if the long-term behavior of these lesions appears similar