Exemestane , a steroidal aromatase inhibitor , reduced invasive breast cancer incidence by 65 % among 4,560 postmenopausal women randomly assigned to exemestane ( 25 mg per day ) compared with placebo in the National Cancer Institute of Canada ( NCIC ) Clinical Trials Group MAP .3 ( Mammary Prevention 3 ) trial , but effects on quality of life ( QOL ) were not fully described .