Steve Jobs' resignation as Apple's CEO has shaken the tech world and is stirring rumors of severe health problems.
"I have always said that if there ever came a day when I could no longer meet my duties and expectations as Apple's CEO, I would be the first to let you know," he wrote in the letter to the company's board of directors. "Unfortunately, that day has come."
Jobs did not elaborate on his reasons for stepping down. While the tech guru has remained tight-lipped on his health status since his pancreatic cancer diagnosis seven years ago, many worry the recent development is a sign his health has taken a turn for the worse.
E-mails to Apple requesting comment were not immediately returned.
Jobs was diagnosed with a pancreatic neuroendocrine tumor in 2004. The disease is known to progress more slowly than the more common form of pancreatic cancer, but it can be no less devastating.
"Some people have described them as cancer in slow motion," Dr. Jonathan Strosberg, attending physician at the Moffitt Cancer Center in Tampa, Fla., told ABC News in January. "Patients tend to live longer, even if it's in its later stages. The average survival is six years from diagnosis."
Treatment options for the disease include hormone therapy, chemotherapy, surgical removal of the tumor and liver transplant.
Jobs' cancer spread to his liver, as this type of tumor often does, and in April 2009, he underwent a liver transplant at Methodist University Hospital in Memphis, Tenn. During that same year, the CEO traveled to Switzerland to receive a treatment that is not yet approved in the U.S., Fortune magazine reported.
But researchers say even after a liver transplant, the likelihood of disease recurrence in the organ "is still really high," said Dr. Richard Alexander, a surgical oncologist at University of Maryland.
Despite taking three medical leaves in the past seven years, Jobs remained Apple's CEO. In March, he received a standing ovation after making a rare appearance to unveil the iPad 2. Clad in his staple black turtleneck, blue jeans and glasses, many people attending the event reported Jobs seemed energetic, but frail.
Steve Jobs Resigns; Rumors Swirl that Health Worsened
There is not a large amount of data on these pancreatic tumors, but about half of the people diagnosed with them are still alive after five years, said Dr. Simon K. Lo, director of the Pancreatic and Biliary Diseases Program at Cedars-Sinai Medical Center.
Nevertheless, Dr. Otis Brawley, chief medical officer of the American Cancer Society, warned against speculating on a person's life expectancy.
"There is a spectrum of pancreatic cancer," said Brawley. "Some are very slow-growing and indolent, so indolent that they never kill the person. Some are very fast-growing and very aggressive, so aggressive that the median life expectancy is six to eight months after diagnosis.
"I have seen people who have pancreas cancer who have been told they have six to eight weeks to live alive six to eight years later to tell me about it," continued Brawley. "I caution against any kind of speculation."
Despite his resignation, Jobs expressed optimism for the future of Apple.
"I believe Apple's brightest and most innovative days are ahead of it," wrote Jobs. "And I look forward to watching and contributing to its success in a new role. I have made some of the best friends of my life at Apple, and I thank you all for the many years of being able to work alongside you."