WASHINGTON - Does an 80-year-old really need a Pap smear? Do mammograms in the 70s find dangerous breast cancer or tumors that are too slow-growing to threaten women's last years?

The question of when you're old enough to quit routine cancer checks is a tough one. And new research suggests it's one that many doctors and seniors avoid tackling: Tens of thousands of elderly women in California alone are getting mammograms and Paps even though their overall health is too poor to benefit.

Are doctors scared to tell such patients to stop? "I think so," says Dr. Louise Walter of the University of California, San Francisco, who led the study in that state. "You don't want anyone to ever think you're giving up on them."

But it's an important decision, because at some point the benefits of cancer screening can be outweighed by the harms - unnecessary treatment, or additional testing and anxiety from false alarms.

"In older people, (screening) is very much a choice," Walter says. "These discussions need to be had."

Most cancer groups are pretty specific on when Americans should start getting screened: Pap smears to detect cervical cancer should start within three years of sexual activity and no later than age 21. Mammograms to spot breast cancer start at 40. Colon cancer tests start at 50. PSA or "prostate specific antigen" tests for prostate cancer usually are offered then, too. Screening is encouraged earlier if cancer runs in the family or people have other risk factors.

When to quit is murkier.

Guidelines now recommend ending Paps at age 65 or 70 if the woman has no history or recent signs of cervical cancer. It's typically a slow grower that's almost always caused by a sexually transmitted virus.

The other malignancies have no definitive cutoffs. But specialists are rewriting guidelines to reflect that seniors should continue getting routine cancer screening as long as their life expectancy makes it likely they'll benefit - about another five years for mammograms, 10 years for men's PSAs. (No word yet on colorectal screening.)

Most screening is important for vibrant seniors, says Dr. Robert Smith of the American Cancer Society. After all, a 70-year-old without serious medical problems has a life expectancy of 15 more years.

"If she could live to be 85, we don't want her to die of breast cancer at 79," he says.

Yet life expectancy is difficult to predict, and to discuss.

"Primary-care physicians tell us of sitting there with a person in heart failure who suddenly announces that isn't it time for their mammogram," Smith says. "It's very awkward for them to say that wouldn't be a good use for scarce resources."

The California research, published this month in the Annals of Internal Medicine, highlights the confusion. It estimates that 97,000 healthy California women ages 70 to 84 are skipping mammograms - although they could benefit - while 81,000 unhealthy women 80 and older still get them.

Walter derived the estimates by analyzing health survey data from 4,700 elderly women. She found 72 percent of the 80- to 84-year-olds surveyed reported a recent Pap smear, despite guidelines that suggest most were unnecessary.

Some women even reported recent Paps though earlier hysterectomies for noncancerous reasons had left them with no cervix and thus no need for the test. Walter estimated that number could reach 200,000 Californians.

Medicare pays for elder cancer screening, and some people are reluctant to quit. They think, "'This is something I do when I'm healthy. If I don't do it, it means I'm not healthy,"' Walter says.

While risk of cancer increases with age, heart disease and other age-prone killers increasingly take a bigger toll. Breast cancer causes 12 percent of deaths among 50-year-old women, but just 3 percent of deaths among 75-year-olds, Dr. Gilbert Welch with the Veterans Affairs Medical Center in Vermont notes in an editorial accompanying Walter's research.

Yet it's hard for many people to understand that cancer can grow slowly enough that an elderly person will die of another disease before the tumor ever becomes threatening.

Walter urges seniors to have a candid talk with their doctor. Ask: Am I a good candidate to continue screening? What happens if suspicious signs are found? Will I need a biopsy or other testing? What does treatment entail?

And remember, it's reasonable to shun aggressive treatment at 80 that you might have chosen at 50, says the cancer society's Smith - so consider all your options.

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EDITOR'S NOTE - Lauran Neergaard covers health and medical issues for The Associated Press in Washington.