Katherine Heigl on <I>Grey's Anatomy</i> Katherine Heigl on Grey's Anatomy

Dr. Izzie Stevens could be in a coma or even dead when Grey's Anatomy resolves its Season 5 cliffhanger on Sept. 24, says Katherine Heigl.

"You last saw Izzie flat-lining, so... I'm either there as a ghost 'on the other side,' or I survived a disease [stage IV metastatic melanoma] no one survives," the actress said during a Monday visit to Late Show with David Letterman.

Video: Katherine Heigl shows some skin

When Letterman asked how she could be pulling 17-hour workdays — as Heigl had shared earlier in the sit-down — if Izzie isn't alive and kicking, she countered, "What if I was in bed in a coma? They could do that."

Heigl shared that it was "a little weird" reporting to the Grey's set last week without pal T.R. Knight around. (Last month, the actor, who played George O'Malley on the show, negotiated an early release from his contract with the ABC drama.) When asked for how much longer she will stick with Grey's, Heigl said with a trace of sarcasm, "That's what I keep wondering."

Heigl, who is contractually bound to Grey's for another two seasons, said that while she hopes the show enjoys the longevity of, say, ER, which ran 15 years, she'd like to continue playing Izzie "maybe not quite that long."

Heigl stars opposite Gerard Butler (300) in the romantic comedy The Ugly Truth, in theaters July 24.

Crave scoop on your favorite TV shows, from this summer or the upcoming fall season? E-mail senior editors Matt, Mickey and Tim at mega_scoop@tvguide.com and follow TV Guide on Twitter.