The popular 66-year-old former president spent several hours at Sao Paulo's Sirio-Libanes Hospital before returning to his home in the Sao Paulo suburb of Sao Bernardo do Campo, officials said.

A statement from the hospital said Lula would undergo "daily radiation sessions from Monday to Friday."

The outpatient treatment, expected to last between five and seven weeks, aims to completely eradicate the tumor, which was first detected last October.

Late last year, Lula underwent three chemotherapy sessions, also at Sirio-Libanes Hospital, and doctors said the last one shrank his cancerous tumor by 75 percent.

Brazil's first leftist president led the country from January 2003 to December 2010, leaving office with a soaring 80 percent approval rating after two consecutive terms.