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Most women know that getting a mammogram is an important step they can take to protect their health. Two new studies show just why this breast cancer screening tool is so critical.

One, published in the journal Cancer, shows that, on average, women are being diagnosed with smaller tumors now than in past decades. This is in large part thanks to earlier detection by mammograms. The other study, published in the Journal of the National Cancer Institute (JNCI), finds that women whose breast cancer is first spotted with a mammogram tend to live longer than those who find their breast cancer some other way (by feeling a lump, for instance). And this held true even when the researchers looked at women whose tumors were found at the same stage.

"These findings provide additional support for the importance of adherence to regular screening," said Robert Smith, PhD, director of cancer screening for the American Cancer Society. ACS recommends that women begin getting yearly mammograms at age 40 if they're at average risk of developing breast cancer. Women at high risk may want to start screening before 40.

Although mammograms can't always find every problem in a breast, it's well known among doctors that these x-rays can find smaller tumors than a physical exam can. Finding a small tumor has many advantages; for one thing, a woman who has a small tumor may be eligible for breast-conserving surgery, rather than mastectomy, to remove it.

But smaller tumors are also more likely to be early-stage tumors -- meaning they're less likely to have spread to surrounding tissue or lymph nodes or other parts of the body. Tumor stage can range from stage 0, the earliest form of breast cancer (called carcinoma in situ), to stage IV, the most advanced form (where the breast cancer has spread to other organs).

But tumor characteristics can vary even within a stage.

"For example, within stage II disease there is a range from 'barely a stage II' to 'nearly a stage III,' Smith explained, and prognosis depends on which end of those extremes one is at.

The research published in Cancer shows that mammograms not only find smaller tumors, but also tumors that are more favorable within their stage, Smith said.

In that study, researchers from Memorial Sloan-Kettering Cancer Center in New York reviewed the size of breast tumors diagnosed between 1975 and 1999. The information came from SEER (Surveillance, Epidemiology, and End Results), a huge National Cancer Institute database of US cancer statistics.

Tumor size "decreased substantially" over those 25 years, Elena Elkin and colleagues wrote. From 1975-1979, fewer than 10% of localized tumors (those confined to the breast) were under 1 cm in size. But by 1995-1999, about 25% of tumors were that small. Even tumors that had spread beyond the breast into surrounding tissue or lymph nodes got smaller over time. About 20% of these more advanced tumors were smaller than 2 cm in 1975-1979; by 1995-1999, that proportion was up to 33%.

Breast cancer survival also improved over that time period, both for women with localized tumors and for those with more advanced breast cancer. Much of that improvement was due to smaller tumor size, the researchers determined. Especially in women older than 65, smaller tumor size provided a big survival advantage.

The study published in JNCI suggests that there's even more to the story than just improvements in tumor size and stage. In that study, researchers from MD Anderson Cancer Center in Houston tried to compare survival between women who got mammograms and those who didn't, but were diagnosed with the same stage of tumor.

They used data from three large studies conducted in the 1960s and the 1980s, before mammograms were routine. As expected, women who got screened were more likely to be diagnosed with earlier stage tumors. But even when they compared women with the same stage of tumor, those whose tumor was found by a mammogram did better.

The researchers, led by Donald Berry, PhD, say clinical trials should start collecting information about how a breast cancer was detected so doctors can learn how important this factor may be in predicting a woman's outcome. But it's too soon for treatment decisions to be made based on that information, Smith and other experts say.

The studies Berry and his colleagues used were older and didn't have the same level of detail about the tumors, including the precision concerning size, that is routinely collected today. This information would also include whether the cancer has hormone receptors, for instance, or expresses other biological markers that could influence treatment.

When you take all those factors into account, Smith said, the biggest advantage that mammography offers is that it gives women an opportunity to start treatment earlier.

In an editorial published along with the study, other breast cancer experts from Canada's Princess Margaret Hospital and the University of Toronto say there haven't been enough studies to evaluate whether how a tumor was found should influence treatment. Until more research is done, they say, doctors and patients should rely on the established methods of deciding treatment.

Citations: "The Effect of Changes in Tumor Size on Breast Carcinoma Survival in the US: 1975-1999." Published online August 8, 2005 in Cancer (Print issue: Vol. 104 No. 6). First author: Elena B. Elkin, PhD, of Memorial Sloan-Kettering Cancer Center, New York.

"Role of Detection Method in Predicting Breast Cancer Survival: Analysis of Randomized Screening Trials." Published in the Journal of the National Cancer Institute (Vol. 97, No. 16: 1195-1203). First author: Yu Shen, PhD, of M.D. Anderson Cancer Center, Houston.

"Should Screen-Detected Breast Cancers Be Managed Differently?" Published in the Journal of the National Cancer Institute, (Vol. 97, No. 16: 1170-1171). First author: Monika K. Krzyzanowska, MD, of Princess Margaret Hospital and the University of Toronto.