Extramedullary haematopoiesis (EMH) in the axillary lymph node of breast cancer patients is extremely rare but when encountered can represent a diagnostic challenge. We aim to highlight this incidental finding as a diagnostic pitfall which can be mistaken for metastatic carcinoma (particularly of the metaplastic type). We report the case of a 68-year-old Caucasian female with a family history of cancer. Core biopsy revealed that she had grade II oestrogen receptor-negative, Her2-positive invasive ductal carcinoma. She was offered neoadjuvant chemotherapy with Herceptin and subsequently underwent breast-conserving surgery. Microscopic examination of the post-treatment breast surgical specimen showed a partial pathological response with large areas of tumour regression. The sentinel lymph node showed frequent large single and multinucleate giant cells with hyperchromatic nuclei located predominantly within the subcapsular and medullary sinuses. The morphological differentials of metastatic carcinoma, sinus histiocytosis and extramedullary haematopoiesis were considered. A panel of immunohistochemistry showed these large cells to be negative for epithelial markers and CD68. They were strongly positive for CD42b (megakaryocyte marker). Smaller myeloperoxidase and factor VIII-positive cells were identified. The findings confirmed EMH. Sentinel nodes are often well scrutinised by pathologists for evidence of metastatic carcinoma as an important prognostic parameter both in the standard and neoadjuvant setting. Nodal megakaryocytes have been described in response to neoadjuvant chemotherapy particularly in association with Herceptin treatment. Pathologists’ awareness of this finding in the neoadjuvant setting is crucial to avoid a mistaken diagnosis of malignancy. A relevant immunohistochemical panel together with careful attention to morphology should help establish the correct diagnosis.

中文翻译：

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接受新辅助化疗的乳腺癌患者腋窝淋巴结髓外造血：一个潜在的诊断陷阱

乳腺癌患者腋窝淋巴结中的髓外造血 (EMH) 极为罕见，但在遇到时可能代表诊断挑战。我们旨在强调这一偶然发现是一个诊断陷阱，它可能被误认为是转移性癌（尤其是化生型）。我们报告了一名有癌症家族史的 68 岁白人女性的病例。核心活检显示她患有 II 级雌激素受体阴性、Her2 阳性浸润性导管癌。她接受了赫赛汀新辅助化疗，随后接受了保乳手术。治疗后乳房手术标本的显微镜检查显示部分病理反应与大面积肿瘤消退。前哨淋巴结显示频繁的大单核和多核巨细胞，染色质细胞核主要位于包膜下和髓质窦内。考虑了转移癌、窦组织细胞增生症和髓外造血的形态学差异。一组免疫组织化学显示这些大细胞上皮标志物和 CD68 呈阴性。他们对 CD42b（巨核细胞标记物）呈强阳性。鉴定出较小的髓过氧化物酶和因子 VIII 阳性细胞。调查结果证实了 EMH。在标准和新辅助治疗中，前哨淋巴结经常被病理学家仔细检查，以寻找转移癌的证据作为重要的预后参数。淋巴结巨核细胞已被描述为响应新辅助化疗，特别是与赫赛汀治疗相关。病理学家在新辅助治疗中意识到这一发现对于避免错误诊断恶性肿瘤至关重要。相关的免疫组化组以及对形态学的仔细关注应该有助于建立正确的诊断。