By Jose de Castro
SAO PAULO (Reuters) - Brazil's popular former President Luiz Inacio Lula da Silva started chemotherapy treatment for throat cancer on Monday, with his doctors voicing confidence in his chances of being cured within four months.
Tests showed the tumor discovered on his larynx was not an abnormal type of cancer and should respond well to chemotherapy and radiation therapy, said doctors at the Sao Paulo hospital where Lula was being treated as an outpatient.
"The outlook is very good," said Dr. Roberto Kalil Filho, Lula's personal doctor who is part of the team treating him at the Sirio Libanes Hospital, a renowned cancer center.
"There is no other problem that could worsen the prognosis or raise the chance of collateral effects. We think the chances are very good."
Lula's diagnosis on Saturday shocked Brazilians and raised the prospect of political life without the charismatic former union boss, who remains an influential force in Latin America's largest country.
Some analysts and politicians have even speculated Lula could run for the presidency again in 2014 if President Dilma Rousseff were not to seek reelection.
Lula, who turned 66 last week, also had been expected to play a vital role in next year's municipal elections, helping stump for candidates from his left-leaning Workers' Party.
Even if he recovers from cancer, his health raises doubts over his ability to continue playing such an active role in domestic politics, especially if the illness damages his voice box and limits his ability to speak publicly.
Lula has been a key advisor for Rousseff this year as she has dealt with rebellious allies in her ruling coalition and weathered the resignation of six ministers, five of them over corruption scandals.
A prolonged absence for Lula, who canceled all national and international trips for the rest of this year, could give Rousseff more of a chance to put her stamp on the government. Most of the ministers involved in the corruption scandals were Lula appointees or allies.
Rousseff, who served as Lula's chief of staff, has taken a tougher line than her mentor on corruption. Before taking office in January, Rousseff was herself treated successfully for cancer at the same medical center that is treating Lula.
Lula left office with high approval ratings after leading the country between 2003 and 2010 -- a period of robust economic growth in which more than 20 million Brazilians were lifted out of poverty into the middle class.
He has been a major force since the late 1970s, when he was a fiery union boss who led massive protests against the military dictatorship at the time. In 1980, he helped found the Workers' Party, now the dominant force in Brazilian politics.
The tumor was localized and had not spread through Lula's neck or to his lymph nodes, his medical team said. He will have three chemotherapy treatments 21 days apart, which will cause him to lose his hair and his trademark beard.
After that he will have radiation therapy. The doctors expect the treatment to end in February.
Brazilian media reported Lula, a charismatic speaker who can electrify rallies, expressed worries to doctors about losing his famous gravelly voice. But they have discounted the possibility of treating his cancer with surgery, which would have been a bigger risk to his voice.
Lula was a smoker and is also known as a drinker, both habits that boost the chances of throat cancer, according to the U.S. National Cancer Institute.
(Writing by Stuart Grudgings; Editing by Todd Benson and John O'Callaghan)