Interval Breast Cancers Are More Dangerous

Let's begin by defining interval breast cancers. These are breast cancers that are found between routine mammogram screenings. For example, a woman has a mammogram that is normal in May, but then feels a lump in October and goes to the doctor and the lump turns out to be malignant. I have heard many such stories through the years, and the woman is inevitably shocked and confused. Aren't annual mammograms supposed to protect us from such experiences?

... interval breast cancers are more likely to be high-grade (grade 3) and to have a more aggressive natural history.

A recent retrospective Canadian study, published in JAMA Network Open, found that these women had a higher risk for aggressive disease and death. With a median follow up of seven years after diagnosis, these women were more than three times more likely to die of breast cancer than women whose cancers were discovered at routine screening mammograms.

There are several explanations. First, these interval breast cancers are more likely to be high-grade (grade 3) and to have a more aggressive natural history. Breast cancers seen on annual mammograms were more likely to be indolent, lower-grade cancers that were less likely to require chemotherapy. Interval cancers were more likely to be triple negative or HER2-positive, ensuring that chemotherapy would be part of the recommended treatment. Sometimes the mammogram itself was the problem. Films of dense breasts are more difficult to read; the mass itself and the surrounding healthy tissue may both look white. Technically this is referred to as a masking effect, but I have always liked the it's like looking for a polar bear in a snowstorm reason. It seems to come down to either the study itself was imperfect or the cancer is faster growing than some others.

Other than being alarmed, what can we do about this? Women who are known to be at high risk, those with a gene mutation or a strong family history or who have had more than one breast cancer themselves, should be having annual breast MRIs in addition to annual mammograms. The tests should be scheduled six months apart, so a screening is happening twice each year. Women who have had breast cancer should speak with their doctors about their personal situation and risk. Does it make sense to add a breast MRI to their routine or is the annual mammogram sufficient?

Since the recommendations for monthly breast self-examinations (BSE) have been mostly abandoned, this does leave us with a choice. Over the years, the belief has become that women do often find their own breast cancers, but they usually don't do so during a monthly self-exam. Instead, they feel a lump in the shower, or a partner discovers it during love making. As a regular self-examiner, I found my first breast cancer while awakening one morning; it had nothing to do with the monthly ritual. However, whatever the statistics say, it seems to make sense to examine your own breasts regularly. By doing so, we learn the natural contours and feels and will know if something is different.

If you find yourself in this situation, understand that any study is referring to large groups of women and that your doctor will help you make the best appropriate treatment decisions. My personal experience, though knowing many women to whom this happened, does not support the study's conclusion. These women were distressed and scared but received the best possible treatment and did as well as someone else with a different story.