Dikembe Mutombo is undergoing treatment for a brain tumor, the NBA announced Saturday on behalf of the Hall of Fame center and his family.

The NBA said in a statement that Mutombo -- who ranks second in NBA history for career blocks -- is beginning treatment in Atlanta and is in "great spirits."

"Dikembe and his family ask for privacy during this time so they can focus on his care," the league said. "They are grateful for your prayers and good wishes."

Mutombo, 56, played 18 NBA seasons for the Denver Nuggets, Atlanta Hawks, Philadelphia 76ers, New Jersey Nets, New York Knicks and Houston Rockets before retiring after the 2008-09 season.

He was the league's top defensive player four times, earned three All-NBA selections and played in eight All-Star Games over 18 seasons. Mutombo ranks 17th in rebounds (12,359) and finished with 3,289 career blocks, second to Hakeem Olajuwon (3,830). Mutombo followed most blocks with a playful wag of his right index finger, a gesture that became his enduring signature.

Following his playing career, Mutombo has worked extensively for charitable and humanitarian causes. He has served as an ambassador for the sport, particularly in the development of the Basketball Africa League, which completed its second season in May.

Mutombo was inducted into the Naismith Memorial Basketball Hall of Fame in 2015. He had recently appeared at Hall of Fame enshrinement events in Springfield, Massachusetts, as well as a pair of preseason games in Saitama, Japan. Mutombo also appeared with U.S. Secretary of State Antony Blinken at an event in the Congo, Mutombo's native country, in August.

Blinken lauded Mutombo when they were together, telling him, "You've done so much to bring the world together."

Mutombo speaks nine languages and founded the Dikembe Mutombo Foundation in 1997, concentrating on improving health, education and quality of life for the people in the Congo. His foundation led the building of a 170-bed hospital in Kinshasa, the capital city, and that facility has treated nearly a half-million people regardless of their ability to pay for care.

He also has served on the boards of many organizations, including Special Olympics International, the CDC Foundation and the National Board for the U.S. Fund for UNICEF.

The Associated Press contributed to this report.