Mastitis is an infection of the breast tissue.
Acute mastitis causes an area of erythema and firmness in the breast, commonly during lactation. The most common infecting organism is S. aureus. The breast is often biopsied to differentiate the condition from inflammatory carcinoma, another painful breast condition. Microscopically there is acute and chronic inflammation with abscess formation in some cases.
Fat necrosis is often related to trauma or prior surgery, and it may produce a palpable mass or a discrete lesion with calcifications on mammography. Microscopic changes include fat necrosis, chronic inflammation, hemosiderin deposits and fibrosis with calcification.