For the second time in less than a week, we have learned of cancer recurrence in a prominent public figure: Elizabeth Edwards with metastatic breast cancer and now presidential spokesperson Tony Snow with colon cancer.

Tony Snow underwent six months of chemotherapy after having his colon removed for cancer treatment in 2005. When a slowly growing tumor in his right lower abdomen was removed earlier this week, the surgeons found that the cancer had spread to his liver.

This unfortunate finding is not unusual in people with colon cancer. Even if the entire tumor is removed at the time of surgery, the cancer may recur locally in the intestine or nearby. The cancer is termed metastatic when it spreads to other sites or organs, the liver being the most common site for the spread of colon cancer.

In rare cases, surgical removal of one or several discrete liver metastases can lead to a cure of the cancer. Most often, however, the liver involvement is too diffuse to allow the surgeon to remove every speck of it. And surgery on the liver would not be considered when cancer has already been identified at another site, as is the case with Tony Snow's.

So, in almost all cases, metastatic colon cancer cannot be cured. The only available treatment option is systemic chemotherapy, which may alleviate symptoms and prolong life somewhat. Some cancer specialists would begin chemotherapy even before a patient complains of any symptoms. That approach is controversial, however, because chemotherapy is associated with a wide range of side effects, and survival is often extended only modestly.

Experts estimated that about 150,000 Americans would be diagnosed with colon or rectal cancer in 2006, and that approximately 55,000 Americans would die of these cancers, mostly with metastatic tumors.

These grim figures once again underscore the importance of getting a colonoscopy to detect and remove polyps before they become cancerous and to remove cancerous colon tumors at the earliest possible stage.