Persistent small-cell lymphocytosis in dogs with a concurrent mediastinal mass has been associated with both thymoma and small-cell lymphoma. In thymomas, neoplastic thymic epithelial cells induce overproduction and release of polyclonal lymphocytes, whereas thymic lymphoma results in thymic effacement by a clonal expansion of neoplastic lymphocytes and subsequent leukemic phase of lymphoma. Flow cytometry has been used to differentiate these 2 entities by immunophenotyping mediastinal mass aspirates. It has been reported that cases with mediastinal masses in which ≥ 10% of the associated small-cell lymphocytes were double positive for CD4 and CD8 were thymomas, whereas masses associated with < 10% were suggestive of lymphoma. We report a unique case of thymoma-associated lymphocytosis lacking the classic CD4+CD8+ immunophenotype. Our findings suggest that there may be more diversity in the thymoma-associated lymphocyte immunophenotype than has been identified previously; immunophenotyping alone might not be sufficient to differentiate thymic small-cell lymphoma from thymoma-associated lymphocytosis. In dogs with mediastinal masses and peripheral lymphocytosis, employing a variety of testing modalities to avoid misdiagnosis is prudent. These modalities include cytologic and/or histologic evaluation, immunophenotyping, and clonality assessment.

伴有纵隔肿块的犬持续性小细胞淋巴细胞增多症与胸腺瘤和小细胞淋巴瘤有关。在胸腺瘤中，肿瘤性胸腺上皮细胞诱导多克隆淋巴细胞的过度产生和释放，而胸腺淋巴瘤通过肿瘤性淋巴细胞的克隆扩增和随后的淋巴瘤白血病期导致胸腺消失。流式细胞术已被用于通过对纵隔肿块抽吸物进行免疫表型分析来区分这两种实体。据报道，纵隔肿块中 ≥ 10% 相关小细胞淋巴细胞 CD4 和 CD8 双阳性的病例是胸腺瘤，而小于 10% 的肿块提示淋巴瘤。我们报告了一个独特的缺乏经典 CD4+CD8+ 免疫表型的胸腺瘤相关淋巴细胞增多症病例。我们的研究结果表明，胸腺瘤相关淋巴细胞免疫表型的多样性可能比以前确定的要多。单独的免疫表型可能不足以区分胸腺小细胞淋巴瘤和胸腺瘤相关淋巴细胞增多症。对于有纵隔肿块和外周淋巴细胞增多症的犬，谨慎使用多种检测方法以避免误诊。这些方法包括细胞学和/或组织学评估、免疫表型和克隆性评估。谨慎使用多种检测方式以避免误诊。这些方法包括细胞学和/或组织学评估、免疫表型和克隆性评估。谨慎使用多种检测方式以避免误诊。这些方法包括细胞学和/或组织学评估、免疫表型和克隆性评估。