“Going forward, plaintiffs still face a substantial hurdle of proving liability,’’ Gertner wrote in the 56-page order. “But based on the record before me, plaintiffs have demonstrated that they are able to do so as a class.’’

Her ruling allows thousands of other Massachusetts smokers to join the suit, which covers people 50 or older who have smoked at least one pack of Marlboro cigarettes a day for at least 20 years. If a jury sides with the smokers, Philip Morris could be required to pay for each patient’s low-dose computed tomography scan, which can detect early-stage lung cancer.

The tests typically cost $400 to $500 a year, but many health plans do not cover them, according to Christopher Weld Jr. of Boston, one of the lawyers for the plaintiffs.

Lawyers for Philip Morris did not immediately respond to requests for comment.

Weld said there is a similar suit pending in New York, however, this is the first one in the country that he knows of that has been certified as class action.

“It’s a terrific decision for our class,’’ Weld said. “It allows the case to go forward on a group basis, which is critical. And in many respects, she has accepted our understanding or interpretation of applicable law that will be favorable to the class.’’

The case differs from other tobacco lawsuits because the plaintiffs have no apparent symptoms of lung cancer and are not seeking conventional damages, Gertner wrote. Instead, they want medical monitoring — regular screenings to detect early signs of lung cancer. The plaintiffs say if they do eventually develop the disease, the screenings will increase their likelihood of survival almost six-fold.

No class member would be eligible if they have been diagnosed with lung cancer or are under a doctor’s care for suspected lung cancer, and all must have smoked Marlboro cigarettes within Massachusetts, she wrote. Marlboro cigarettes are designated because that is the brand the two plaintiffs smoked.

Richard Daynard, a law professor at Northeastern University and chairman of its Tobacco Products Liability Project, said if the plaintiffs win, the case could spawn dozens of similar suits in federal courts across the country. “It’s a tremendously important case,’’ he said.

The case before Gertner, he said, will probably turn on whether the chest scans save lives. A growing body of evidence, he said, indicates that they do.

The plaintiffs named in the suit are Patricia Cawley of Rockland and Kathleen Donovan of Randolph. They say they began to smoke more than 30 years ago and suffered lung tissue damage that greatly increases their risk of developing lung cancer.

Lung cancer is the leading cause of cancer death among men and women and is one of the most difficult cancers to treat, according to the American Cancer Society. It is very hard to detect when it is in the earliest, most treatable stage. About 87 percent of lung cancer deaths are caused by smoking.

Related Posts:

• PAPER: men who screen negative for lung cancer are a bit less likely to try to quit smoking.
• Chewing Tobacco Maker Agrees To $5M Settlement
• RJ Reynolds’ ads urge tobacco pouches for smokers
• Pets – Pets can be harmed by second-hand smoke and/or ingestion of tobacco products.
• Smoking isn’t just bad for your lungs