"If it's left untreated, it will lead to premature death," said Dr. Shlomo Melmed, dean of Cedars-Sinai Medical Center, who is not involved in Kosen's treatment. "There's a 30 percent increased mortality in patients who are uncontrolled."
Gigantism and acromegaly also put people at increased risk for other conditions.
"They are prone to arthritis and diabetes," Melmed said. "Growth hormone is a powerful antagonist to the action of insulin."
He added that there are generally three treatment options -- medication, surgery and radiation.
"Patients can have cosmetic surgery for their large jaws or orthopedic surgery for their joints," Melmed said.
He said the gamma knife radiosurgery procedure can be effective depending on the type and size of Kosen's tumor.
Sheehan believes the surgery will be a success.
"It could take six to 24 months for the tumor to shrink," he said. "But he has a very favorable prognosis."
The procedure should take about three or four hours. Sheehan expects him to be discharged this afternoon or Friday morning.
Kosen is scheduled to leave Virginia on Sunday and will head to California, where a dentist has offered to fix his teeth. He'll return to Turkey on Sept. 1.
He has a parting message for everyone in the U.S.
"He sends his heartfelt thanks to everyone who's gone out of their way to give him some kind of help," said Garrett.