Benign inclusions in lymph nodes

Abstract

Benign inclusions are foci of non-neoplastic ectopic tissue in lymph nodes. They are classified into three types: epithelial, nevomelanocytic and decidual. It is important to identify them for the differential diagnosis with lymph node metastases, particularly among patients who present proliferative benign lesions. In general, epithelial inclusions are presented inside lymph nodes as epithelial cysts or as numerous structures resembling ducts. The cells of these structures may originate from the cells of paramesonephricus ducts, salivary glands, breast tissue, thyroid follicles, squamous epithelium or mesothelium. Paramesonephricus-type inclusions are almost exclusively found in pelvic lymph nodes and, in appearance, they resemble the epithelium of the uterine tube. Inclusions of breast tissue are composed predominantly of ectopic mammary glands and ducts that present diverse morphological characteristics that still have obscure etiology. Thyroid-type inclusions are frequently found in cervical and axillary lymph nodes, and it is believed that, embryologically, they arise from the mixing of tissues from which lymph nodes and the thyroid gland originate. Mesothelial inclusions occur preferentially in the mediastinal lymph nodes of patients who are affected by pleural or pericardial effusions. Aggregates of melanocytic cells are generally found in the lymph node capsule. The explanation for this occurrence is uncertain, but it is believed to be a consequence of incorrect migration of neural crest cells, or because of "benign metastases" of nevi present in the skin. Studies on benign inclusions in lymph nodes take on importance through assisting in correctly diagnosing the presence of metastases.