CHICAGO — When Alpa Patel’s grandfather was diagnosed with cancer, he seemed healthy: The 64-year-old had been training for a triathlon when doctors found a lemon-size tumor in his brain. He died almost a year to the day from when he was diagnosed.

Patel, only a teenager at the time in Daytona Beach, Fla., said that experience "really got me to thinking what causes most cancer."

She became an epidemiologist and is now the principal investigator — and one of the participants — in the American Cancer Society’s third generation study on cancer prevention, which researchers hope will help them solve one of medicine’s most perplexing puzzles: why some people never get cancer.

To take part in the study, a person must be 30 to 65 and never have had cancer. Participants fill out comprehensive surveys about their health and habits and give blood samples and waist measurements. Researchers will track participants’ progress over the years, sending short follow-up surveys every two years or so that can be filled out at home in about 15 to 20 minutes.

The American Cancer Society has already enrolled 200,000 people and hopes to find 100,000 more people nationwide.

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Enrollment in Chicago’s western suburbs began Thursday and runs until April 26. Enrollment will take place in Springfield in May, in Chicago’s north suburbs in June and in Chicago in August.

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When the cancer society started tracking cancer-free participants in its first study in the 1950s, a cancer diagnosis was "like a death sentence," Patel said. But that study, by following participants over the years, established the link between smoking and lung cancer — a no-brainer today, but groundbreaking research at the time, Patel said.

Findings from the second study, which started in the 1980s, helped link obesity with increased cancer risk.

That’s one area upon which researchers are hoping to expand with new data from today’s participants, whose health will be tracked for at least the next 20 years, Patel said.

"We haven’t really studied people who have been very heavy their entire lives, which wasn’t the case in previous generations," said Lauren Teras, an epidemiologist at the cancer society.

Teras, 36, who has enrolled in the study along with several of her cancer-free family members, also intends to focus on what happens to people living in a more sedentary society.

"More people are in their cars, in front of an iPad, at their desk in front of a computer all day," Teras said. She hopes to explore whether increased sitting time has adverse health effects even for people who exercise regularly, Teras said.

That research question wasn’t the point of focus in generations past, when Frances Kent’s parents took part in the first study in the 1950s. Kent, 62, of Chicago’s North Shore, said her parents "were always active physically."

Both have since passed away, but Kent’s mother never had cancer. She died at 85 simply of "old age, didn’t have any particular disease," Kent said. Her father had prostate cancer, but only in his late 70s, and "he was never in the hospital or getting chemo or all that." He died of Alzheimer’s disease and pneumonia at 86.

"They always watched things that we know now are not so good for us — salt, they didn’t eat red meat. They kept their diet simpler, like vegetables, rice, grains and nuts," Kent said.

But Kent’s oldest sister passed away at 47 of colon cancer, a motivating factor for Kent, who remains cancer-free and intends on enrolling in this generation’s cancer study.

"I think it’s so important to participate because it gives (the researchers) patterns," Kent said. "It gives them information to press for a cure."

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FOR MORE INFORMATION ON CANCER STUDY

For information, and to enroll in the study, you can go to cancer.org/cps3 or call 888-604-5888. Organizers suggest scheduling an appointment to avoid waiting but said that walk-ins are possible.