01/19/2011 03:24 pm ET Updated May 25, 2011

Will Steve Jobs Return From His Leave Of Absence?

Will Steve Jobs return to Apple?

It's the question many are asking following the CEO's announcement on Monday that he will be taking a medical leave of absence, his second in two years.

Little has been revealed about Jobs' current condition and it is difficult to determine his prognosis with any certainty. Yet from all that can be gleaned from the public record -- what is known about his prior medical history, as well as what little he himself has shared -- it is hard to construct an encouraging scenario.

Jobs has struggled with health problems since 2004, when he was treated for a pancreatic neuroendocrine tumor -- a rare but not necessarily deadly form of cancer. In 2009, he took a six-month leave of absence during which he was treated for cancer in Switzerland, according to former Apple director Jerry York, then received a liver transplant in Tennessee.

By contrast with his leave of absence in 2009, for which Jobs gave a precise end-date, this time the CEO has given no indication of when -- or if -- he will return.

"I love Apple so much and hope to be back as soon as I can," he wrote in a sparse 114-word email to Apple's employees.

Experts speculate that Jobs may now be suffering from complications relating to his liver transplant or from the recurrence of his tumor. In either case, the prognosis may be grim.

Though there is little data on patients with Jobs' medical history -- fewer than 1,000 people suffering from neuroendocrine cancer have received liver transplants as treatment -- a 2004 study by the Mount Sinai School of Medicine found the one-year survival rate for this therapy was 70 percent, but that this number drops considerably over time. The researchers placed the five-year survival rate at between 30 and 40 percent.

Even if the transplant is a success, this form of cancer frequently metastasizes, especially as the medications used to treat transplant patients can increase the likelihood of recurrence by weakening the body's immune system and creating an environment more hospitable to tumor growth. The drug therapy for transplant patients -- a complex cocktail of medications -- can lead to other complications, including diabetes, hypertension and kidney problems.

Timothy Donahue, a physician at the UCLA Center for Pancreatic Diseases, said Jobs' decision to take a medical leave, taken together with what is known of his condition, suggests his cancer may have returned in spite of his treatments in 2004 and 2009.

There are patterns of recurrence with this particular disease to support Donahue's speculative diagnosis. A 2007 study found just 20 percent of patients who had received a liver or multivisceral transplantation to treat neuroendocrine tumors saw no additional metastases after five years, though the Mount Sinai study, which tracked 11 patients, found only one person remained disease-free five years after receiving a new liver.

If Jobs' tumor has metastasized, such a resurgence so soon after his transplant would be cause for serious concern.

"If he is out because of a tumor recurrence, that has a very bad prognostication this early," noted Florman, director of the Recanati/Miller Transplant Institute at the Mount Sinai Medical

Although Jobs' leave of absence was a surprise for some, experts suggest it may have been a foregone conclusion given the risks of transplantation and recurrence.

"Giving anyone with neuroendocrine tumors a liver transplant is usually considered a treatment, not a cure," said Florman.

Though Jobs has struggled with complex illnesses, there is still hope that the technology leader -- who, for the time being, will continue as CEO and oversee "major strategic decisions" -- will recover and return to the helm of the world's most valuable technology company. After all, he has already done it twice before, both times following major medical procedures.

"What [Jobs'] situation has shown is that people can live very productive lives as they live with cancer and complex medical treatments," said Jennifer Obel, a physician with the NorthShore Medical Group specializing in gastrointestinal cancer. "That's really what is remarkable about this."