What is plasmacytoid mastitis? Plasmacytoid mastitis is more accurately called periductal inflammation. Periductal inflammation is most commonly seen in older women and occurs at the base of the nipple or in the areola area, which is the central region of the breast. The inflammation is slow to develop and may start with a painful nipple and a small hard nodule under the breast, but as the inflammation continues the nodule may gradually expand and even the entire breast may become hard with a large mass. It may be related to an irritant in the duct that causes a chronic inflammatory response by damaging the duct interstitium and then seeping around the duct and repeatedly irritating it. It is not a bacterial inflammation, but a chemical inflammation. The end of the inflammation may all cause localized necrosis of the ducts as well as the adjacent glandular lobes, and the necrosis will slowly produce pus, which may produce sinus tracts around the areola with constant pus flow. Plasmacytoid mastitis and periductal inflammation can usually only be cured by surgery and will occasionally resolve if treated conservatively, but may recur.