He was first diagnosed with colon cancer in 2001. In 2005, the disease returned as a brain tumor and lung cancer.
A report on his own chemotherapy treatments in February 2006 was broadcast on "Morning Edition" and prompted an enthusiastic response from the audience. It eventually became a regular series and feature on the network's Web site.
"For the past two years, Leroy shared his life with cancer on the air and online with passion, wit and a kind, brutal honesty that created a safe space for an open and candid dialogue about the disease," NPR Vice President for News Ellen Weiss said in a statement.
His cancer continued to spread during the past few years. After several surgeries, he recently decided to stop treatment.
Sievers worked as a journalist for more than 25 years, including 10 at CBS News and 14 years at ABC News Nightline, four of them as executive producer. He covered more than a dozen wars and was embedded with Ted Koppel to cover the Iraq war and produce "The Fallen," a tribute to soldiers killed in Iraq and Afghanistan.
"Cancer was not in Leroy's plans. But he turned his battle with cancer into the most dramatic, the most moving and the most important story of his life," Koppel wrote on NPR's Web site.
Sievers is survived by his wife, Laurie Singer. Funeral arrangements were pending.