WASHINGTON (CNN) — Sen. Arlen Specter's Hodgkin's disease, which he battled in 2005, has recurred, but doctors said its return was detected early and Specter has an "excellent chance" of once again achieving remission, according to a statement issued Tuesday by the senator's office.

Specter will continue to perform his Senate duties and conduct his campaign for re-election to a sixth term, but will undergo 12 weeks of chemotherapy, the written statement said. Hodgkin's disease is a cancer of the lymphatic system.

The recurrence was found during a routine scan, which flagged small lymph nodes in Specter's chest and abdomen, the statement said. A follow-up biopsy of one of the chest lymph nodes was positive for recurrence, but a bone marrow biopsy was negative.

"I was surprised by the PET scan findings because I have been feeling so good," Specter, 78, said in the statement. "I consider this just another bump in the road to a successful recovery from Hodgkin's, from which I've been symptom free for three years."

Specter had no other symptoms besides the findings in the scan, the statement said. "Based on the location of the recurrence and the absence of symptoms, his lymphoma is considered stage IIIA. This is significantly less advanced than his Hodgkin's disease when it was originally diagnosed in 2005, when it was stage IVB."

"Specter has an excellent chance of again achieving a complete remission of his Hodgkin's disease," the senator's oncologist, John H. Glick of the University of Pennsylvania's Abramson Cancer Center, said in the statement.

"Senator Specter's early diagnosis of his recurrent Hodgkin's disease has a five-year survival rate of 60 percent. He is in superb physical condition, with a normal physical examination and blood work, no symptoms of disease, plays squash regularly and follows a careful diet."

The statement from Specter's office noted he had successful surgery for a brain tumor in 1993. It recurred in 1996 and was successfully treated.

In 1998, during a re-election campaign, he underwent cardiac bypass surgery and suffered post-operative cardiac arrest, from which he fully recovered, the statement said.

"I've beaten some tough medical problems and tough political opponents and I expect to beat this too," Specter said. "I look forward to getting through this treatment and continuing to serve the people of Pennsylvania."