Though the topic of never-smokers with lung cancer is a particular focus of my interest and research, and there has been a greater focus on this topic in the last few years, most lung cancer is still related to smoking. However, we have clearly reached a point where the majority of people diagnosed with lung cancer are not current smokers at the time of their diagnosis, and about 60% have quit at some point. Many of these patients, and their families and friends, express surprise that they developed lung cancer, especially if someone had quit 20 or 30 years prior. How much does the risk for developing lung cancer decline over time, and does the amount smoked before quitting make a difference?

The short answer is that, as you would probably expect, smoking more "pack-years" (the product of average number of packs of cigarettes smoked per day X number of years smoking) is associated with a higher risk than smoking fewer, and that the risk of lung cancer declines over time but doesn't get to the level of a never-smoker (which we know isn't zero risk of lung cancer either). To illustrate, here are the "case-control" results that compare the smoking histories of 521 patients with lung cancer to over 76 thousand people without lung cancer in western Washington state (abstract here):

(click to enlarge)

The table highlights the following conclusions:

1) Over 60% of lung cancer is diagnosed in former smokers.

2) The lung cancer population appears to be skewed toward current smokers and ex-smokers as over-represented vs. people without LC, and more who quite within the last decade

3) The number of pack-years is greater in patients who developed lung cancer, both among current and former smokers.

But research does clearly indicate that there is a value to quitting, and quitting as early as possible (and that it's never too late). For instance, research from the behavior of over 34,000 British physicians (see full article here, which is very complex and full of details) revealed that smoking is associated with a reduction in survival of about 10 years on average and that the the earlier a person quit smoking, the less of a negative impact smoking had on their survival:

The same themes are supported by another study that characterized the relative risk of lung cancer based on how much a person had smoked and when they quit (relative risk being the likelihood of developing LC compared with a never-smoker, so a relative risk of 5 is 5 times the rate of LC development seen in never-smokers) (abstract here). Specifically, the risk of LC is still quite high within the first five years, especially among those patients who have smoked extensively, but the risk declines steadily over time, regardless of how much someone had smoked before quitting. And while risk never gets down to never-smoker levels, the risk of LC declines toward that ballpark after a few decades.

Interestingly, one thing that is observed is that there can be a higher rate of lung cancer seen in people who just quit smoking in the previous year. This isn't really because quitting smoking causes lung cancer, but rather almost certainly because a person with undiagnosed lung cancer may experience symptoms that finally motivate them to quit, but the symptoms continue, with a diagnosis made a few months after the person quit smoking.

Overall, this work is consistent in saying that the risk conferred by smoking cannot be completely undone, but that the risk of LC declines with time since quitting, along with an improvement in overall survival compared to continuing to smoke (related to a wide range of tobacco-related detrimental effects on health). Although there's no question that it's hard thing to do, it's never too late to quit.