When a mammogram is conducted prior to breast reduction, a false positive result 
can frequently occur, according to a recent study conducted by doctors at Virginia Mason Medical Center in Washington.
The use of mammography before breast reduction varies. Some surgeons may recommend it and some may not. According to Dr. M.J. Campbell and the authors of this study, between 31% and 72% of patients do not undergo mammograms before breast reduction surgery. The authors suggest that the role of mammography in breast reduction is “ill defined” and there are “no formal guidelines for surgeons.”
When mammograms are conducted, the likelihood of receiving a false-positive result is relatively high. In a study group of 207 breast reconstruction patients, 32 of them presented abnormal mammographic results, and all were false positives. While the authors say the reason for this unexpectedly high number is unknown, they do offer a theory:
“The high incidence of false-positive results may be the result of poor image clarity, as obese women, typically with larger breasts, might have a thicker volume of breast tissue compressed between the mammography plates, which can lead to more scattered radiation and decreased image contrast and quality.”
You can read the entire study, “The role of preoperative mammography in women considering reduction mammoplasty: a single institution review of 207 patients” in the May 2010 issue of The American Journal of Surgery.