Mammogram & sonogram found circumscribed, round, equal-density mass 4x2x3. 6 mos later rescan shows 3x4x1. told low risk/prob benign. 6 mos later it is no longer seen on mammo or ultrasound. can that be possible to miss on both?

If the mammogram and sonogram were done at the same imaging location, certain kinds of breast cysts can resolve spontaneously. The important finding for potential breast cancer is a serious of light calcifications, which do not "disappear", and do require biopsy. It is unlikely that with the former location of the mass "known", that a tech or physician would "miss" this finding.