It turns out that catching cancer early isn't always as important as we thought.

Some tumors are too slow-growing to ever threaten your life. Some are so aggressive that finding them early doesn't make much difference. And today's treatments are much better for those somewhere in the middle.

Those complexities are changing the longtime mantra that cancer screening will save your life. In reality, it depends on the type of cancer, the test and who gets checked when.

"We can find cancer early. We can reduce the burden of the disease. But along the way, we're learning our tests are not as perfect as we'd like," says the American Cancer Society's Dr. Len Lichtenfeld, a longtime screening proponent. "We're learning that we're now finding cancer that would in fact never cause harm."

Now cancer specialists are struggling to find a new balance: to quit over-promising the power of early detection and to help people understand that the tests themselves have risks - while not scaring away those who really need it.

Least controversial are cervical and colorectal cancer screenings. They can spot pre-cancerous growths that are fairly easy to remove, although even some of those tests can be used too frequently. More serious questions surround other cancers - like which men, if any, should get a PSA blood test to check for prostate cancer, and whether women should start mammograms in their 40s or wait until they're 50.

Also in question is whether doctors will be able to head off another looming controversy: Just which smokers and ex-smokers should get a pricey CT scan that can detect lung cancer but also is prone to false alarms? A recent study found the scans could save some lives. But guidelines aren't due out until early next year that would decide who is at enough risk to outweigh the test's potential harm - such as a risky, invasive biopsy to tell if a suspicious spot is cancer or just an old smoking scar.

Yet already people like 80-year-old Fred Voss of Sunderland, Md., are seeking out the tests.
"It was a big relief, and it gave me something to watch," says Voss, who participated in the CT study but wanted to get tested again to make sure nothing had changed.

Today, guidelines for how to handle some of the most common cancer screenings conflict. And, they're written for the average patient when many people may need a more customized decision, says Dr. Jeanne Mandelblatt of Georgetown University. She has studied breast cancer risk for a government panel that recommends most women not begin screening for the disease until age 50.

Consider this, she says: The average woman has a 3 percent lifetime risk of dying of breast cancer, a low risk for a disease that women find so scary. But the chances of getting breast cancer do gradually increase with age and other circumstances.

So if you're 40 and have several risk factors - like dense breasts and close relatives with the disease - then you have the same risk as an average 50-year-old, not an average 40-year-old, and might consider earlier mammograms, Mandelblatt says. Few primary-care doctors have the time to go into that kind of detail.

Adding to the confusion are testimonials from cancer survivors that a screening saved their lives. Dartmouth researchers recently studied how often that's true for mammograms, and estimated that about 13 percent of women in their 50s whose breast cancer is detected by the tests survive as a result.

What else plays a role? Treatments have dramatically improved in recent years, saving more lives. Also, increasingly powerful mammograms are detecting more low-risk tumors, the kind that probably wouldn't have threatened a woman's life in the first place.

Still, mammograms are "not perfect, but they're the best we have," cautions Mandelblatt. She thinks the Dartmouth estimate is somewhat low.

PSA tests for prostate cancer are a much tougher call. Last month, a government panel recommended an end to routine PSA screenings, a step further than other major medical groups that urge men to weigh the pros and cons and decide for themselves. But the U.S. Preventive Services Task Force found limited, if any, evidence that screening average men improves survival. That's largely because so many men are diagnosed with slow-growing tumors that never would have killed them; still, they have treatments that can cause incontinence, impotence or even lead to death.

"We really - underline the word `really' - have to pull back the messaging on prostate cancer," says the cancer society's Lichtenfeld, who himself isn't sure of the test's net worth. PSA testing took off on the basis of "blind faith" that they would work, not science, he says.

What really worries Lichtenfeld is that ever more powerful cancer screenings are being developed, before doctors have a way to tell exactly which early tumors should be removed.

"We have cells in our body that are abnormal all the time, and our bodies deal with it," he says. "Our technology takes us further and further down the early-detection path, and we need to sort through all this."