By Ilene Barth

I'm participating in the Philadelphia Marathon this weekend because - I can.

Strictly speaking, I won't be running; I'll be walking fast. And I'm not trying to cover 26.2 miles; my husband, our three kids, and I are participating in the related Rothman Institute 8-kilometer run the day before.

Frankly, I'm closer to 60 than 50, and I've never been in a footrace before.

One other thing: I'm missing a good part of one lung.

I'm a lung cancer survivor. My family calls our showing this weekend "Plan B." Plan B is surviving lung cancer.

As you might suspect, I used to smoke. I even wrote a book about the allure of tobacco.

You can quit cigarettes, but they don't quit you. You can lower the threat of a stroke or heart attack, but you can't completely undo the damage smoke did to your lungs.

Sixty percent of lung cancer cases diagnosed in 2006 were in people who had quit smoking, according to the Centers for Disease Control and Prevention's most recent report on the subject. (Another 21 percent were in people who had never smoked.) Given prevailing trends, ex-smokers' share of cases could be even higher now.

In other words, people who were once smokers are at high risk for lung cancer. Giving up tobacco is good for you many times over, but it's not a free pass.

Federal health officials do not publicize this fact. They fear that former smokers' inconvenient vulnerability to lung cancer will undermine stop-smoking campaigns.

On a lovely fall day seven years ago, I was feeling as good as the weather. Then a doctor friend advised me to get a CAT scan of my chest. He knew I'd smoked for a long time and understood - despite the lack of official guidelines - that I remained at high risk.

I decided to get scanned, and a tumor measuring an inch across was discovered in my right lung. A biopsy showed it was cancer.

The youngest of my children was only 10. I thought about how to say goodbye.

I was lucky, though. Thanks to my doctor friend's advice, I'd been tested before I experienced any symptoms - which, with lung cancer, often mean it's too late. My cancer was small and hadn't metastasized.

It was removed by surgery. I had chemo. I continued being a mother and wife and went back to work. My family spent summer vacations, as usual, in Colorado's San Juan Mountains. For a few of those summers, living at 9,000 feet was unusually difficult - scary, really. But I kept pressing myself to regain breathing power, and I did. Now I ski and play doubles tennis.

The National Cancer Institute recently ended a huge, long-term study of early detection of lung tumors by annual CAT scans or X-rays. The test was stopped early because the CAT scans' lifesaving potential was so clear that ineffectual X-rays could no longer be justified.

The institute, however, is in no rush to promulgate early-detection guidelines for lung cancer. Instead of unequivocally lauding the life-affirming discovery, institute director Harold Varmus emphasized that smoking remains unsafe. And several public-health officials insisted that more study is needed.

That might be good science, but you or someone you love could be dying in the meantime. Twice as many women and men die from lung cancer as from all the gender-specific cancers combined.

I think lung cancer screening will eventually become a near-universal recommendation, just as it is for many other cancers. Chest scans are not nearly as uncomfortable as mammograms. But the average cost of a CAT scan is $300, and health insurers may resist covering them when nothing is found. (Cheaper detection methods are being explored.)

One argument against testing people at high risk for lung cancer is that it will reveal tiny tumors that might never grow dangerous, and there is no sure way to distinguish between potentially lethal and relatively benign growths. On the other hand, the recently concluded study seems to have found that CAT scans also saved lives by discovering abnormalities that were noncancerous but life-threatening.

Since the price of wait-and-see is often death, I am glad I was able to make the call for myself. My family is running with a larger group called Lung Love, and most of our teammates are running in memory of a relative or friend who didn't survive the prevailing lung cancer strategy: late detection.

Full disclosure: My family and I will be in the Philadelphia Marathon because:

We hope it will draw attention to lung cancer issues and raise money for the Lung Cancer Alliance, a Washington-based advocacy group not funded by Big Tobacco.

Some of us, myself included, have experienced acute Philadelphia withdrawal symptoms since our middle child graduated from Penn in May.

We guessed that telling my story might save lives - and how often do you get to do that?

We could use the exercise.