Metastasis is one of the complicated biological properties of tumor cells. Usually, metastasis occurs in an orderly sequence: local invasion, breakthrough of the basement membrane, intravasation and survival in the blood or lymph stream, extravasation, colonization, and proliferation. Most metastases follow a predictable route of dissemination on the bases of the circulatory map. Locoregional metastasis usually occurs first, with a pattern corresponding to the blood or lymphatic drainage of the primary neoplasm; distant metastasis usually occurs later. The common destinations of distant metastases are the vascular organs, such as the liver, lungs, and brain. Bone is another preferential target of distant metastasis, although it is not a particularly vascular site. Notably, metastasis occurs more frequently in these organ sites than do primary tumors, and sometimes, metastatic tumors can cytologically resemble primary tumors of these sites. For example, adenocarcinoma of the pancreaticobiliary tract in the lungs often shows a bronchioloalveolar or “airway” growth pattern, mimicking primary bronchioloalveolar carcinoma of the lungs (Fig. 1.2).

In general, carcinomas tend to metastasize via the lymphatics and initially involve the lymph nodes, whereas sarcomas tend to metastasize hematogenously to visceral organs such as the liver and lungs. However, for gastrointestinal epithelial malignancies, the liver is the most common recipient due to the characteristic anatomic venous pathway. The common metastatic pattern of various tumors and the common primary origins of various metastatic sites are listed in decreasing frequency, on the basis of published studies, in Tables 1.1 and 1.2. Familiarity with the common metastatic pattern helps to predict the location of the primary tumor during the workup of FNA cases.