Extramedullary Haematopoiesis in Axillary Lymph Nodes of Breast Carcinoma Patients Receiving Neoadjuvant Chemotherapy: A Potential Diagnostic Pitfall.

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.