Does anyone know why 10% to 15% of biopsy proven breast cancers have negative mammograms? I believe I do. It is due to MASKING of the cancer by the hormone estrogen. I know that Tamoxifen is known as the antiestrogen drug, which implies that estrogen is the culprit that makes breast cancer grow more rapidly. Tamoxifen is a wonderful drug, giving many extra years of life to breast cancer patients. But I question the "fact" that it works by neutralizing estrogen. In my experience, estrogen appears to have the ability to "mask" the cancer and make the cancer tissue feel (by palpation) like normal breast tissue, and causes the mammogram to be interpreted as negative. Breast malignancy may appear as a mass before the mammogram becomes positive. If a mass appears, it does not feel like the usual malignancy (i.e., hard, fixed, with skin retraction, etc.), but may just feel different from the surrounding normal breast tissue. Breast malignancy may appear shortly after discontinuing estrogen, whether for birth control or for postmenopausal reasons. However, the "mass" may appear while a woman is still on the estrogen.

What kind of study is needed to prove or disprove my findings? Either a radiology group or a medical student or a radiology resident could undertake such a study. Every patient having a mammogram must have a thorough hormone history, including use of estrogen in any form, brand, dosage, length of hormone use, and whether hormones have been used in the recent past. The next step would be to identify the cases in which there was a biopsy proven breast cancer with a negative mammogram. The case histories would need to be checked to see how many of these women were taking estrogen or had recently discontinued use of estrogen. I came across 20 estrogen "masking" cases in my years of practice. I think a study such as this may answer why there is a relatively high percentage of false negative mammograms.