The involvement of lymph nodes by metastatic tumors signifies the start of a new phase in the progress of a cancer. It indicates that through a succession of molecular changes, the cancer cells have acquired phenotypes that enable them to invade, colonize, and disseminate.

Establishing the presence of metastatic tumor in lymph nodes is essential for the management and prognosis of cancer. In human solid cancer, lymph node status is the most important indicator of clinical outcome (1). The vast majority of lymph node biopsies, particularly those on frozen sections, are performed to confirm or exclude tumor metastases. The anatomic location and the number of lymph nodes involved are also important indicators of the process, and new methods, such as sentinel lymph node biopsies, have been devised to answer questions of staging. Not infrequently, a lymph node metastasis is discovered before an occult primary tumor is detected. In such cases, to identify the unknown primary tumor, extensive studies of the lymph node metastases, including immunohistochemistry and electron microscopy in addition to detailed histopathology, are often necessary.