Since I was able to feel a lump in my breast, why didn't it show up on the mammogram?
The most likely explanation for this occurrence is that the tumor was hidden in the middle of glandular tissue or that it was glandular dense breast tissue. Another explanation is that the radiologist did not see it because it was very small or subtle. Another possibility is that some cancers do not form lumps which would show up as a spot on the mammogram. Infiltrative cancers are more spread out. Many, many cells are spread out over an area and are not visible on mammogram. Another explanation is that some tumors are confined to the skin of the nipple and won't show up on mammogram.