The lymph nodes, brain, lungs, liver, and bones are common sites of metastasis. A metastasis is a tumor that starts from a cancer cell or cells from another site. Sometimes, however, a primary cancer is discovered only after a metastasis causes symptoms. For example, a patient whose prostate cancer has spread to the bones in the pelvis may have lower back pain before experiencing any symptoms from the prostate tumor itself. In a small number of cancer patients, a secondary tumor is diagnosed, but no primary cancer can be found, in spite of extensive tests.
Doctors refer to the primary tumor as unknown or occult.
It is often possible to treat the metastasis even when the primary tumor cannot be located. The cells in a metastatic tumor resemble those of the primary cancer. When a second cancer is found in a patient who has been treated for cancer in the past, it is more often a metastasis than another primary tumor. By examining the cancerous tissue under a microscope, a pathologist can usually tell whether the type of cell is normally found in the part of the body where the tissue sample was taken. Metastatic cancers may be found at the same time as the primary tumor, or months or years later.