Mammograms miss between 20% – 60% of tumors.
It means that a women who receives a letter saying her mammogram is normal, she at that moment might very well have breast cancer that the mammogram couldn’t distinguish.
Unless she detects the tumor with another method, it will keep growing in the body, increasing the chances that it will kill her.

Even with this low detection rate, a mammogram is important since it can detect a tumor that is too small to feel. They also can show a cluster of very tiny specks of calcium. These specks are called microcalcifications. Lumps or specks can be signs of cancer.

WHY MAMMOGRAPHY FAILS to find many tumors?
1. Breast Density
    Dense breast and breast tumors both look white on a mammogram. An in-depth explanation is in the “Need to Know” section of this site.

2. Human Error
    Studies indicate that radiologists, those who read more than 300 mammograms per month (high-volume readers), achieved better results than did those who read 101-300 mammograms per month (medium-volume readers) or 100 or fewer mammograms per month (low-volume readers). The length of time since completing medical training (less than 15 years) and the quality of residency training received affects the number of false positive readings by radiologists.
Centers using a double reading process where a mammogram is read by two radiologists has been shown to reduce errors.

3. Poor Quality of Mammogram
If a women moves during the screening process, it can adversely affect the quality of her mammogram. Likewise, the technical quality of equipment and experience of personnel play an important role in obtaining a good quality mammogram.

4. Breast Implants
Breast implants can Mask Tumors
. Both silicone and saline implants appear as a solid white mass on X-rays and can block breast tissue. This obscures the view for a radiologist. A recent study from the Journal of the American Medical Association reported that of the women studied, 55 percent of breast cancers in women with implants were missed in comparison to 33 percent of the women without implants.
That is the reason why a monthly self exam is so important.

5. Hormone Replacement Therapy (HRT)
HRT reduces the sensitivity of mammography screening, because it increases the breast tissue density.

6. Rapid Tumor Growth
Aggressive tumors may develop so rapidly that a previous mammogram (even within a prior few months) would not have detected it.