How often are breast microcalcifications not cancerous?

Not often.: Apparently 80 % of microcalcifications are benogn. They themselves are not cancerous. Sometimes, if cancer is suspected, and these are seen, a repeat mammogram may be done to see if there are changes. Sometimes tight clusters of these might mean early breast cancer. Sometimes biopsy might be done.

12.7% in one study: Calcifications come in various types, some more suspicious such as clustered or linear branching. One large prospective study found a positive predictive value of 12.7%. Reference is radiology. 2009;250(3):648.

Most are not: An overwhelming majority of women with microcalcifications seen on mammography have no breast disease. A small subset of these women will have microcalcifications that are deemed "suspicious" for cancer, warranting a (stereotactic) biopsy. Of these suspicious microcalcifications, cancer will be found 15-30% of the time.