Breast Cancer Screening: A Conversation with Mette Kalager, MD

In its 2024 update, the US Preventive Services Task Force (USPSTF) revised its recommendation on breast cancer screening, now advising biennial mammography for women aged 40 to 74 years. But what is the evidence behind the change—and what should primary care physicians know about communicating the benefits and risks to patients?

In a conversation with Patient Care at ACP Internal Medicine 2025, Mette Kalager, MD, professor of medicine and epidemiology at the University of Oslo and a general practitioner in rural Norway, explained the data behind the new guidelines, the challenge of overdiagnosis, and why shared decision-making is more difficult—and more essential—than ever.

The following transcript has been edited for clarity, style, and length.

Patient Care: What is the evidence behind the 2024 USPSTF updated recommendation for breast cancer screening?

Mette Kalager, MD: There have been several randomized trials on mammography screening, though most were conducted before the era of modern adjuvant treatments. These trials showed that mammography screening can reduce deaths from breast cancer. However, they did not demonstrate a reduction in all-cause mortality, meaning that, on average, screening doesn’t increase overall life expectancy.

This could be because the trials—about 660 000 women total—weren’t large enough to detect differences in all-cause mortality. Or it could be that screening also causes harm.

One of the most significant harms is overdiagnosis. That’s when a tumor—either invasive breast cancer or ductal carcinoma in situ (DCIS)—is detected that would never have caused symptoms or death during a woman’s lifetime. The problem is we can’t distinguish between tumors that are harmless and those that are not, so we end up treating all of them. Treatments include surgery, radiation, chemotherapy, and hormone therapy—all of which come with risks.

Another potential reason that screening doesn’t prolong life is that some women may die earlier due to treatment side effects, including coronary heart disease or neurotoxicity. Even though overdiagnosis is the most significant harm, numerically, false positives are more common. More than half of women who undergo screening will have at least one false positive result. These cause anxiety and often require follow-up tests or biopsies that ultimately turn out to be unnecessary.

There’s also the issue of interval cancers, which are cancers that appear between screenings. About one in three cancers detected through screening fall into this category. So, when deciding whether to be screened, women need to weigh a small reduction in breast cancer mortality against the risks: overdiagnosis, false positives, false negative results, and unnecessary treatment.

Patient Care: How can primary care physicians explain these risks to patients in a way that’s both accessible and meaningful?

Dr Kalager: It’s tricky. Many women believe that mammography is more effective at saving lives than it actually is. Research shows that they often overestimate their risk of dying from breast cancer.

You’re not starting from zero in these conversations—patients often come in with strong beliefs about screening. One method that can help is using pictograms to visually show benefits and harms among 1,000 women screened. Still, it’s hard to communicate these nuances—even to other doctors, let alone patients.

And many patients don’t actively seek your advice. They’re often invited for screening through insurance or public health programs, so you’re not initiating the conversation. When you do, start by asking: What do you know about mammography screening? What do you think its benefits and risks are?

Often, patients are surprised to hear that the side effects of breast cancer treatment are 10 times more common than dying from breast cancer itself. Once you begin to explain this, you’re often met with blank stares. It’s a hard message to convey.

Patient Care: For primary care physicians who have brief but important conversations during routine visits, what key points should they cover when counseling a patient about starting breast cancer screening?

Dr Kalager: Start with the patient’s baseline beliefs. Ask: What do you already know? How do you think screening works? What outcomes do you expect from it?

Then, provide context. For example, for a 40-year-old woman, her risk of dying from breast cancer in the next 10 years is about 0.13% with screening and 0.17% without—a difference of 0.04%, or 4 in 10 000 women.

Overdiagnosis risk is similar. Between ages 40 and 50, the incidence of breast cancer is 1.9% with screening versus 1.5% without, meaning about 4 in 1000 women will be overdiagnosed. Understanding these numbers is difficult, which is why visual aids can be so helpful.

Ultimately, you’re asking patients to weigh the small chance of avoiding breast cancer death against the chance of being harmed by overdiagnosis and overtreatment. That decision depends on their personal values.

Patient Care: Can you walk us through what effective shared decision-making looks like for breast cancer screening?

Dr Kalager: Yes, and we touched on this earlier. Start by understanding the patient’s values, preferences, and baseline knowledge—not just about breast cancer screening but about preventive health more broadly.

Then, present the absolute risk numbers along with visual aids like pictograms. Help them weigh their personal risk tolerance and health goals before making a decision together.

Patient Care: You mentioned that you work in a rural primary care setting. Looking ahead, what are the biggest challenges you see in your practice or in primary care more broadly?

Dr Kalager: That depends on the population. In rural Norway, where I work, the population is aging. Younger people move away, so we mostly care for older adults. That means managing multiple chronic conditions—which is quite challenging.

Also, I think physicians—especially my generation—haven’t always prioritized discussing patients’ values and preferences. We need to ask: What do you want? How do you feel about your care? and start the conversation there.

Shared decision-making is harder in practice than it sounds. Patients often turn around and ask, What would you do? They’re looking for guidance. So while shared decision-making is important, it’s not always easy.

Patient Care: What do you hope primary care clinicians will take away from your session?

Dr Kalager: I hope they learn something new—and understand that even with mammography, there is no free lunch. Just because we’ve done something for decades doesn’t mean it’s without harm. Evidence changes, and we have to be open to that.

For example, many physicians still don’t believe that breast cancer can be indolent, even though we now know that some cancers—like prostate cancer—can be overdiagnosed. This was not something I was taught in medical school, and I think many doctors my age haven’t caught up to this new understanding.

That’s a paradigm shift we need to make.

Patient Care: How do you want people—both clinicians and patients—to think about breast cancer screening?

Dr Kalager: They need to truly consider the harms of overdiagnosis. We’ve ignored that for too long. When we present data, we focus heavily on how much screening reduces relative risk of breast cancer death. But we often fail to mention that this doesn’t translate into longer life for the average woman—likely because of the harms from treatment.

That’s a major shift in thinking that more clinicians need to embrace.