Following some recent changes to the American Cancer Society's (ACS) guidelines for breast cancer screening, MRI as a screening tool has made many headlines. In the 2 minute news segment, the consumer often misses out on the complete facts. Let's spend a bit more time discussing the new recommendations and what they mean for all women.
Many people often ask why x test is not used on everyone to detect y disease. It is important to understand the criteria for a screening test to see why not every test makes a good screening test for all people. There are basically six criteria that must be met for a good screening test:
We know that mammogram is a relatively good screening test for detecting breast cancer in early stages. It is not fool-proof, misses some cases, and for some women, leads to a biopsy but only to find that the spot was not cancerous. Overall, it is a pretty reliable, accurate, and relatively inexpensive test. Some ask, why don't we routinely do mammogram on women younger than 35-40? Good question, and the criteria above can address this. Younger women are far less likely to develop breast cancer, making the disease less common in this group (going against the fourth criteria above). In addition, younger women have denser breast tissue, making it much harder for the radiologist to interpret the results of the mammogram, in turn, decreasing the reliability (criteria #2).
There have been quite a few studies looking at MRI for screening in women at high risk for developing breast cancer. Let's look at how these results fit the criteria. These studies have found that MRI (with a contrast agent) has a high sensitivity (reliability) for finding cancers in high-risk women, with or without symptoms of breast cancer, and finds smaller, earlier stage tumors. But, the studies also found that MRI tends to have a lower specificity than mammogram, meaning more women were called back for repeat images or sent on for a biopsy that ended up having a benign (non-cancerous) lesion. This can also be called a false positive; the MRI was positive, but the biopsy turned out to be negative. This process can be extremely anxiety provoking for the woman and her family. While this increase in repeat studies and biopsies may be acceptable for women at highest risk, it would not be appropriate for women in lower risk categories.
While it would seem reasonable that any radiologist running an MRI machine could do breast screening, this is not so. The ability of MRI to detect breast cancer is dependent on a high quality image and an experienced radiologist reading the image. In addition, if a cancer is suspected, an MRI guided biopsy is preferred. These technologies are not yet available in many areas, limiting access for many women who could benefit from the test. The American College of Radiology is currently developing an accreditation for institutions performing MRI screening and biopsy, which it hopes to have available later this year. This accreditation will not be mandatory for sites performing MRI screening, but it will help patients in choosing a qualified provider. To sum it up, the test seems to pass muster for high risk women, but availability is still an issue.
Given the widespread availability of mammogram, the lower specificity, higher cost and lack of access for MRI, the average risk woman is not considered a candidate for screening MRI. The ACS breast cancer screening guidelines recommend annual screening MRI, in conjunction with mammogram, for the following women:
There are some women who researchers feel the studies do not provide a clear recommendation for. They are at higher risk than the general public, but do not fall into the highest risk category, making it difficult to generalize recommendations for this group. These include women with an estimated lifetime risk of less than 20%, those with lobular carcinoma in situ (LCIS), atypical lobular or ductal hyperplasia, high breast density on mammography and those with a personal history of breast cancer. These women should discuss their particular case and concerns with their healthcare provider. MRI is not meant to replace mammogram for any woman; it is to be used in conjunction with mammogram. Some recommend alternating the tests every six months, while others see a benefit to reading them together.
Many studies categorize women based on their estimated lifetime risk of developing breast cancer. There have been several models developed to calculate this risk, and each has its own faults. The models consider age, family history, age at first menses and age of first pregnancy. Factors that contribute to being classified as high risk include:
It is important to point out that the majority of women with one relative with breast cancer are not at increased risk at all or only a small increase in risk. While cancers such as ovarian, endometrial and colon are sometimes related to breast cancer, the majority of cancers are not. So, for example, a family history of lung cancer and leukemia does not necessarily increase the risk of breast cancer. It is important for each woman to have an accurate understanding of their family history and to discuss this and how it affects their risk with their healthcare provider. For many families, discussing illnesses such as cancer has long been considered forbidden, but these taboos must be broken to best estimate your own risk.
MRI is significantly more expensive than mammogram. Coverage by insurance companies will vary and will likely depend on the woman's individual risk. The updated ACS guidelines will likely assist women at high risk from having the test covered for screening.
MRI has been in the news for newly diagnosed women as well. After a new diagnosis of breast cancer, there is a chance that there is a second tumor in the opposite breast. Traditionally, after a diagnosis, the woman would have a clinical exam by her healthcare provider and a mammogram of the opposite breast to detect such a tumor. Despite this, 10% of women are later found to have a tumor in the opposite breast (called contra lateral breast cancer). The study used MRI in 969 women who had been diagnosed with breast cancer and had a clinical breast exam and mammogram that were negative for cancer. Based on the MRI results, 135 women (13.9%) were referred for a biopsy for a suspicious area. Of these, 30 resulted in the diagnosis of a contra lateral breast cancer (3.1% of the 969 women). Three cases were diagnosed within 1 year that had not been detected on MRI, mammography or clinical exam. Newly diagnosed women should discuss the use of MRI to screen for contra lateral breast tumors in their specific cases.
Lehman, CD et al (2007) MRI evaluation of the contra lateral breast in women with recently diagnosed breast cancer. New England Journal of Medicine:356(13) p.1295-1303.
Saslow, D et al. (2007) American Cancer Society guidelines for breast screening with MRI as an adjunct to mammography. CA: Cancer Journal for Clinicians: 57(2) p.75-88.