Unusual manifestations of metastatic tumors to the lungs.

Because of the lung's rich capillary bed and central location in the circulatory system, this organ is one of the most frequent recipient sites for metastatic spread of disease. The histopathologic diagnosis in the vast majority of cases is straightforward and usually supported by clinical findings and a prior history of a known primary tumor elsewhere. However, unusual circumstances may arise: a primary tumor may remain occult at the time of pulmonary metastases, the metastasis may develop after a long period of latency, or the histopathologic appearance of the metastasis may assume a form that is uncharacteristic or inconsistent with the suspected primary lesion, which raises the possibility of the emergence of a second primary tumor in the lung. Herein we review a variety of unusual manifestations of metastatic disease to the lungs that may pose difficulties for diagnosis and interpretation. Emphasis is made on the histological differential diagnosis and the clues that may alert the pathologist to recognizing the process as metastatic in origin.