Apple Inc. co-founder Steve Jobs’s death today at age 56 follows years of health struggles that began in 2003, when he was diagnosed with a rare form of pancreatic cancer.

Jobs had a neuroendocrine tumor, which is less aggressive than some other types of pancreatic malignancies. He eventually resigned as chief executive officer on Aug. 24, saying he could “no longer meet my duties and expectations.”

Neuroendocrine tumors can grow slowly and be treated successfully with early removal. Those that spread to other organs, such as the liver, can be life-threatening. Without specifying a reason, Jobs had a liver transplant in 2009, a treatment that can prolong the lives of patients with his type of cancer.

Not all doctors endorse the approach. No more than 60 percent of patients who had transplants for this purpose survived five years after surgery, according to an analysis published in the World Journal of Gastroenterology in 2005. The author, David Metz, is associate chief of gastroenterology at the University of Pennsylvania Perelman School of Medicine in Philadelphia. He has said that his data was of varying quality and that survival odds may be better today.

Jobs had been on medical leave since Jan. 17. It was his third leave since 2004, when he first announced he had a tumor removed. Both he and Apple closely guarded his medical information.

A patient who had a liver transplant after a neuroendocrine tumor might suffer from a number of complications, Simon Lo, director of endoscopy at Cedars-Sinai Medical Center in Los Angeles, said after Jobs resigned in August.

Among the possibilities was a return of the original cancer, leading to pain, fatigue, and disruption of digestion, Lo said then.  Transplant patients also take immune-suppressing drugs that can raise the risk of infections and new cancers, Lo had said.

Two new drugs won U.S. clearance as neuroendocrine tumor treatments this year: Pfizer Inc.’s Sutent and Novartis AG’s Afinitor. Doctors in Europe are also using new radiation treatments for the disease.

Neuroendocrine tumors are uncommon, with new cases occurring in no more than five people in every 100,000 each year, according to Rodney Hicks, professor of medicine and radiology at the University of Melbourne. Doctors are still trying to understand these rare tumors, which in August Hicks said range from benign to very aggressive.