Jorge Araujo, Associated Press
In this photo taken Dec. 10, 2011, Argentina's President Cristina Fernandez, right, gestures as her Vice President Amado Boudou smiles after their swearing-in ceremony at the government house in Buenos Aires, Argentina. Fernandez has thyroid cancer, but it has not spread and will be surgically removed on Jan. 4, and then she will take 20 days of medical leave, during which Boudou will run the country, Alfredo Scoccimarro, Fernandez's spokesman said Tuesday, Dec. 27, 2011.

BUENOS AIRES, Argentina — President Cristina Fernandez has thyroid cancer, but test results showed Tuesday that it remains limited to a lobe in the right side of her neck and has not spread into her lymph nodes, her spokesman said.

Fernandez, 58, will undergo surgery on Jan. 4 and then take 20 days of medical leave, during which Vice President Amado Boudou will run the country. Meanwhile, she will keep up her normal routine, spokesman Alfredo Scoccimarro suggested, noting that she will appear at several events on Wednesday as planned.

Scoccimarro said the cancer was discovered during a routine exam on Dec. 22, and that Fernandez received the results from follow-up tests hours before the announcement.

This kind of thyroid cancer is highly survivable, with more than 95 percent of patients living at least 10 years after detection, according to the U.S. National Institutes of Health.

The usual treatment is to surgically remove as much of the cancerous material as possible, and then follow up with radioactive iodide treatments, taken orally. This substance helps to destroy any remnants of the cancerous gland and provide for clearer images showing any additional cancer, the NIH said on its Web site.

After surgery, patients usually must take medicine — levothyroxine sodium — for the rest of their lives to replace a hormone that the thyroid glands produce. Blood tests every six to 12 months to measure thyroid levels also are recommended.

Fernandez is only the latest South American leader to be diagnosed with cancer. Presidents Fernando Lugo of Paraguay, Hugo Chavez of Venezuela and Dilma Rousseff of Brazil all have undergone treatments recently.

Presidential doctors Luis Buonomo and Marcelo Ballesteros said the operation will be led by Dr. Pedro Saco, chief of the surgery department at Hospital Austral and chief of the Head and Neck Service of the oncology institute at the University of Buenos Aires. Saco also trained in cancer centers in Houston and New York, the hospital said.

Thyroid surgery is not without risk: the NIH says a nerve that controls the vocal cords can be damaged, and doctors sometimes accidentally remove the parathyroid gland, which helps regulate blood calcium levels.