After decades of qualms about lung cancer screening, the American Cancer Society says there now is enough evidence to recommend it, but only for current and former heavy smokers ages 55 to 74 and after a frank talk about risks and benefits.

The new guidelines, announced Friday, are a cautious but exciting step against the world's most deadly cancer, doctors who wrote the advice say.

It is based on a big study in 2011 that found annual, low-dose CT scans — a type of X-ray — could cut the chances of dying of lung cancer by 20 percent and from any cause by nearly 7 percent.

The study only included older people who smoked a pack of cigarettes a day for 30 years or the equivalent, such as two packs a day for 15 years. Whether screening would help others isn't known, so scans were not advised for them.

"We're trying to make sure we restrict harm that might come from screening," such as unneeded biopsies and follow-up procedures when scans falsely suggest cancer, said Dr. Richard Wender, family medicine chief at Thomas Jefferson University in Philadelphia. Lung cancer is fairly rare before age 55, so "the benefits of screening are going to be less if you start at a younger age."

Wender, a former Cancer Society president, led the guidelines panel. Three of its 20 members have ties to companies that make cancer treatment or imaging products. The scans cost $100 to as much as $400 and are not covered by Medicare or private insurers.

"We believe insurance companies should cover this test for the right people — not for everybody," Wender said.

More than 160,000 people die of lung cancer in the United States alone each year, and the vast majority are diagnosed after the disease has spread.

Cancer screening has provoked great argument in recent years, especially over when and how often women should get mammograms and whether men should have PSA blood tests to look for prostate cancer.

Some of the most influential guidelines come from a government-appointed panel — the U.S. Preventive Services Task Force — but it hasn't considered lung cancer screening since 2004, when it said there wasn't enough evidence to recommend for or against it. An update is in the works now.

The Cancer Society used to recommend screening with chest X-rays but withdrew that advice in 1980 after studies showed they weren't saving lives. Since then, the CT scans have come into wider use, and several medical groups backed limited screening with them.

Many private companies also market CT scans directly to the public, including for some who are at lower risk for lung cancer than the people in major studies have been.

WellStar Health System, a network of hospitals and private doctors in suburban Atlanta, has screened nearly 900 people since 2008. Less than 3 percent were referred for lung biopsies because of suspicious findings, and of those, 70 percent turned out to have lung cancer, said screening coordinator Vickie Beckler.

The system generally follows the advice of the National Comprehensive Cancer Network, a group of top cancer centers, but eligibility for scans is "a very fluid area" that's being refined, she said. Patients younger than 50 need a doctor's referral for a scan, but if they want one and have major risk factors, "it should be their prerogative to have access to screening as long as they understand the risks and benefits involved and come to that decision with their physician," she said.

Kathy DeJoseph, 62, of suburban Atlanta, is glad she was screened as part of a study at WellStar. Several years of scans found nothing but last year, one detected cancer.

"I'd have been dead had I not had that scan," she said. "I was very, very lucky."

She also finally quit smoking after 40 years to qualify for lung cancer surgery.

Counseling smokers on how to quit is part of the Cancer Society's guidance. Having a scare from a scan "is a great motivator for people to quit smoking — fear that they might have had lung cancer, that they dodged a bullet, really causes people to change and take a look at their behavior," Wender said.

People also should be told that a normal scan doesn't mean no change is needed.

"The absolute worse thing that would happen" is people thinking "now I'm safe and I can continue smoking," he said.