The Apple Inc logo is seen through raindrops on a window outside of the New York City flagship Apple store in New York, Jan 18, 2011. The health of Apple Chief Executive Steve Jobs was set to overshadow quarterly sales numbers on Tuesday from the consumer electronics powerhouse whose iPhone and iPad excited holiday shoppers. [Photo/Agencies]

SAN FRANCISCO - It would be easier to gauge Apple CEO Steve Jobs' current medical problems if he had said more about the ones he has faced in the past.

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Jobs, who turns 56 next month, said Monday that he would take a third leave of absence- he didn't say how long- to focus on his health. It may not be as serious as many fear, but coming from a man who has had cancer and a liver transplant, the lack of detail is causing concern.

Jobs had a neuroendocrine tumor removed in 2004- a rare and very treatable form of pancreatic cancer- but never said if it had spread to lymph nodes or how extensive his surgery was.

"We don't really know how much of his pancreas was removed. He may just have a remnant," and that may be causing continued digestive difficulties, said Dr. Charles R. Thomas of the Ohio State University's Knight Cancer Institute.

Nor has Jobs revealed why he had a liver transplant in the spring of 2009. It could have been for an unrelated problem, but some experts not involved in his treatment speculated that his cancer had spread.

"These tumors are notorious for recurrence. It may take many years for them to reappear" because they grow so slowly, said Dr. Igor Astsaturov of Fox Chase Cancer Center in Philadelphia.

Since the transplant, Jobs has remained extremely thin and has not appeared at recent Apple events.

"If he's jaundiced, there's a concern the liver may not be working as well as it should," Thomas said. Fatigue and weight loss also are troubling signs, he said.

Jobs could be suffering side effects from immune-suppressing medicines to prevent organ rejection. Even if his cancer has recurred, there are many treatment options, including chemotherapy and newer drugs that target various cancer pathways.

"It would be reasonable to consider surgery" or another procedure to destroy small tumors if the cancer turned up in his new liver, Thomas said. "They may be ready to do some aggressive intervention to get him back in the ballgame."

Dr Jennifer Obel, a spokeswoman for the American Society of Clinical Oncology and a cancer specialist at Northshore University Health System in suburban Chicago, said the prognosis is good for those with pancreatic tumors like the one Jobs had, even if they spread.

"He's done extremely well living with this disease for many years," she said. "I wouldn't assume anything until he has released more information."