Case 12 Answer
Hormone replacement therapy. About 20% of patients on HRT show some increased density at mammography. Not all cases are this pronounced. Pregnancy can also cause such an effect, although most patients of screening age are more likely undergoing HRT.
Menopause usually leads to fatty replacement of the breasts. Mastitis may cause increased density but is usually unilateral, and is often associated with skin thickening. PLUS, patients with mastitis are usually symptomatic. Since this was a screening mammogram, the patient should be asymptomatic. Finally, an estrogen-producting tumor elsewhere in the body would be exceedingly rare.