Nearly two years since receiving a new liver and fighting a rare form of pancreatic cancer, Steve Jobs, the co-founder and chief executive of Apple Inc., announced to his employees Monday he will take an indefinite leave of absence from the company to focus on his health.
While Jobs did not address specific health reasons, many experts say it's likely that his leave is related to his ongoing treatment for pancreatic cancer.
"I love Apple so much and hope to be back as soon as I can," Jobs wrote in the latest message to his team.
Jobs, who was diagnosed with pancreatic cancer in 2004, initially underwent surgery to remove the tumor. A few months after announcing a leave of absence in January 2009, Jobs had a liver transplant.
Experts say the length of his survival is mainly because he had a slow-growing, rare cancer called a pancreatic neuroendocrine islet cell tumor. While experts say neuroendocrine cancers are known to spread to the liver, Jobs remained private about his condition and the reason behind his transplant.
Methodist University Hospital in Memphis, Tenn., where Jobs underwent his transplant, confirmed that Jobs is no longer a patient at the hospital and declined to comment to ABC News about Jobs' transplant.
"You hear the term pancreatic cancer and you immediately think Pavarotti or Patrick Swayze, but this is a completely different animal," Dr. Andrew Warshaw, surgeon-in-chief at Massachusetts General Hospital, told MedPage Today and ABC News.
"For adenomas, the timeline is very short. The pace of progress with neuroendocrine cancers can be many years, even with metastatic disease. It has a different biology," said Warshaw.
Only 6 percent of patients with any form of pancreatic cancer live longer than five years, according to the nonprofit organization Pancreatic Cancer Action Network.
"There's a lot we need to learn about pancreatic cancer, but more so on the islet form," said Julie Fleshman, president and CEO of the Pancreatic Cancer Action Network.
Neuroendocrine tumors are slow-growing, but frequently metastasize to the liver. Researchers say that appears to be what happened to Jobs, who underwent a liver transplant at Methodist University Hospital in Memphis, Tenn., in April 2009.
Dr. Richard Alexander, a surgical oncologist at the University of Maryland who specializes in pancreatic cancer, said this is a very rare treatment, and is usually a last resort when the metastasized cancer doesn't respond to treatments such as chemotherapy.
"It is unusual that people have a liver transplant for neuroendocrine tumor metastases, especially in the U.S.," Alexander told MedPage Today and ABC News.
Even after a transplant, Alexander said, the likelihood of recurrence "is still really high."
Dr. David Metz, a gastroenterologist at the University of Pennsylvania who conducted a 2005 literature review on liver transplant for neuroendocrine tumor metastases, said survival is variable: "Some recur in a year; others, a few years after surgery."
Data in his report noted a 52 percent survival rate two years after liver transplantation -- but he cautioned that accurate outcome data are hard to come by because the procedure is so rare.
"We can't begin to have meaningful numbers," Warshaw said.
If the tumor has recurred, Alexander said treatment options would include chemotherapy or embolization, although performing surgery "on a transplanted liver is high-risk."
Patients who undergo transplants are typically put on immunosuppressant medication to help their bodies accept the new liver. However, suppressing the immune system can allow cancer cells to grow more quickly, said Alexander.
Warshaw said patients who get immunosuppressants can respond very differently to regular cancer treatments.
"All bets are off" for understanding what type of treatment will be successful, Warshaw said.
Still, it is not clear why Jobs is taking this leave, his third since 2004. Aside from tumor recurrence, he could also be dealing with organ rejection or a possible hormone imbalance if the tumor is active.
In general, Metz said neuroendocrine pancreatic tumors are a relief "because patients stick around for a long time and we can use various modalities to treat them. The downside is that the likelihood of 'curing' people once they've metastasized is very low."