Something may be missing in the new breast cancer screening guideline

This year, more than 26,000 women in Canada will be diagnosed with breast cancer, and 5,000 will die of it. A new guideline published in the Canadian Medical Association Journal are meant to save lives through early detection. But critics have their doubts.

The new guideline comes from the Canadian Task Force on Preventive Health Care. It contains recommendations for women aged 40 to 74 years who are not at increased risk of breast cancer through family history and other factors.

• For women ages 50 to 74, the task force recommends screening mammography every two to three years.
• For women ages 40 to 49, the task force recommends against screening women for breast cancer. That's because they have a lot of abnormalities seen on mammography that turn out not to be breast cancers yet have to be biopsied to make sure.

The task force also recommends against screening with magnetic resonance imaging, tomosynthesis (a type of three-dimensional mammogram) and ultrasound in women unless they're at high risk of breast cancer. It also recommends against regular breast self-examination.

The guideline explains the rationale for the rather tepid recommendations for screening. The task force says that in women ages 40 to 49, breast cancer screening saves very few lives. According to the evidence quoted in the document,  you have to screen 1,724 women to save one life over a seven year period. 

The task force adds that the risk of overdiagnosis and overtreatment is substantial. It says that for every 1,000 women ages 40 to 49 who are screened for breast cancer, 294 will get at least one mammogram that gives them a false positive result, and 43 of them will undergo a biopsy with the risk of harm.

In women ages 50 to 74, the benefit is somewhat greater. A total of 1, 333 women in that age group need to be screened to prevent one death, and the risk of false positives leading to breast biopsies is almost as great as in younger women.

Ask about a woman's preference

The 2018 guideline is substantially different from previous editions in one big way. The previous guideline dictated screening based solely on a woman's age and other risk factors. Women either qualified for mammography and other screening tests or they did not. There was no place in the guideline for patient input.

The new guideline says that women and their doctors should share in the decision to screen for breast cancer or not.  The change is based on 29 studies that looked at the value women place on being informed of the benefits and harms from breast cancer screening.

In other words, while the task force recommends against screening women ages 40 to 49, it adds that women who really want to be screened should talk to their doctor. Similarly, while the task force says women ages 50 to 74 should be screened, it acknowledges that some women in this age group don't want screening and should not be persuaded to do so.

Critics such as Dr. Paula Gordon, Dense Breasts Canada's medical adviser, applaud the notion of shared decision making, but that's just about the only thing they like about the new guideline. They say the task force has based its conclusions on studies that used outdated mammography. They argue that breast cancer screening beginning at 40 saves the most lives and dispute the notion that the harm (false positives and unnecessary biopsies) outweighs the benefit.

They say a breast biopsy is an office procedure that is not particularly traumatic. In experienced hands, the risk of complications is very low. 

That's just for starters. Critics say the biggest thing that's missing from the guideline is the impact of dense breasts on the accuracy of mammography.

Earlier this season on White Coat, Black Art, our show on dense breasts quoted a study saying that over 40 per cent of all women ages 40 to 74 have dense breast tissue. Dense breasts camouflage breast cancers, making them invisible on mammography.

I spoke with women who had cancers detected by ultrasound after a normal mammogram. Dense breasts are more likely to develop cancers, something the task force acknowledged in its update. However, it says the U.S. Preventive Services Task Force concluded the benefit of supplemental screening with ultrasound or MRI in women with dense breasts and a negative mammogram is unknown.

Dense breast camouflage

That the task forced commented on dense breasts at all tells me it is keeping an open mind.

• Breast cancer more common in denser tissue

Women should talk with their health professional about their individual risk of breast cancer. Women at increased risk need to be watched carefully. For those at low risk, the new guideline is a starting point for discussion. All women should be informed of the risks and benefits of screening. However, shared decision-making means that some women prefer more screening and others less. The system needs to accommodate both.

For women with dense breasts, things are changing. This fall, the BC Cancer Agency became the first in Canada to inform women if they have dense breasts so they can consider other options for screening. Other provinces have followed.

The next task force guideline may look a lot different for the many women with dense breasts.