WASHINGTON (CNN) -- Potential GOP presidential candidate Fred Thompson has disclosed he has a treatable form of lymphoma, a source familiar to the "Law and Order" star's deliberations tells CNN.

Lymphoma is a type of cancer that originates in the lymphatic glands -- part of the body's immune system. Nearly 500,000 Americans are currently living with the disease according to the Lymphoma Research Foundation.

Thompson, a former Republican senator from Tennessee, has been discussing his condition privately "for sometime," the source close to Thompson said, and his decision to disclose the information signals he is seriously considering a White House bid.

"This disclosure should indicate to everyone interested in seeing Fred enter the race for the White House how serious he is in taking the next steps toward launching a campaign," the source said.

Another source close to Thompson said the former senator has been living with the disease for two and a half years.

The Georgetown University Medical Center oncologist handling Thompson's case will be holding a press conference on the former senator's health later Wednesday.