Metastasis is a secondary malignant tumor - one that has spread from a primary site to affect other parts of the body. A metastasis in the liver may arise, for example, as a result of the spread of a cancer from another part of the body (e.g., colon).
When a cancer metastasizes from its original site to another area of the body, it is termed metastatic cancer. Virtually all cancers have the potential to spread this way. Whether metastases do develop depends on the complex interaction of many tumor cell factors, most of which are not completely understood.
The treatment of metastatic cancer depends on where the cancer started. When breast cancer spreads to the lungs, for example, it remains a breast cancer and the treatment is determined by the tumor's origin within the breasts, not by the fact that it is now in the lung.
About 5 percent of the time, metastases are discovered but the primary tumor cannot be identified. The treatment of these metastases is dictated by their location rather than their origin.
Metastases spread in the three ways - by local extension from the tumor to the surrounding tissues, through the bloodstream to distant sites, or through the lymphatic system to neighboring or distant lymph nodes. Each kind of cancer may have a typical route of spread.
The liver is a common site of metastatic disease. The portal vein drains the abdominal viscera (internal organs) and is presumably the conduit for metastases from tumors of the colon and rectum, stomach, pancreas, biliary tree, and small intestine. Breast cancer commonly involves the liver as do lung cancer and lymphoma.
The diagnosis of liver metastases in patients at high risk is straightforward. An enlarged or tender liver on physical examination, abnormal alkaline phosphatase (a blood test) or abnormal liver scan, ultrasound, or CT (computed tomography) scan of the liver may suggest liver metastases.
Multiple metastatic lesions are the rule, but single metastases may be seen. In patients without a known primary site of cancer, it may be impossible to distinguish metastatic liver disease from multicentric hepatocellular carcinoma (cancer arising in the liver). The diagnosis should be confirmed with percutaneous (through the skin) liver biopsy if treatment will be changed on the basis of biopsy results.
For most metastatic tumors to the liver, systemic chemotherapy directed at the tumor type is offered. But metastases from some cancers may be approached in a different way.
Occasionally, metastases to the liver are localized in one part of the liver or exist as solitary masses. Such tumors may be surgically removed with a possibility of achieving a cure, particularly with metastases from colon, kidney cancers or sarcoma. Generally, no more than four tumors are removed from the liver, however. Removing more than four metastases rarely results in a cure and exposes the patient to significant surgical risks.
Liver metastases from colon cancer may be treated with combination chemotherapy, but because of the peculiar biological properties of some colon cancers, patients may develop liver metastases without ever having any other sites of tumor involvement.
Patients with such limited cancer may benefit from chemotherapy infused directly into the liver. Delivered by an implanted pump connected to the hepatic artery, this therapy takes advantage of the liver's ability to metabolize some drugs, meaning that the tumor may be exposed to high concentrations of chemotherapy while the rest of the body is spared the side effects.
This approach initially requires surgery for the implantation of the pump and often the removal of the gall bladder, but the rest of the therapy can be done on an outpatient basis. The treatment may cause more tumor regression than standard chemotherapy.
Under investigation are embolization, tumor vaccine, biological response modifiers, palliative radiotherapy, hepatic artery ligation with or without chemotherapy, and cryotherapy.