Dilation of mammary duct is a chronic breast disease with a long course and complex and diverse lesions. Bloodgood (1923) called it “dilation of mammary ducts” because of the dilated ducts that were often palpable under the skin in the areola area in the form of strips, resembling spaghetti-like worms or brownish-red tubular masses. Ewing (1925) found a large number of plasma cell infiltrates in the lesions under the microscope; Adai (1933), after a more detailed study, found that in the later stages of the disease, the ductal secretions not only stimulate the ducts to dilate, but also can overflow out of the ducts, causing an inflammatory reaction with a predominantly plasma cell infiltrate around the ducts, and named it “plasma cell mastitis”. “Dockerty (1941) found many thick gray secretions in the dilated ducts that were congested or secreted, and called the disease “acantholytic mastitis”. In 1956, Haagensen and Stout called the disease “ductal dilatation of the breast” based on its pathological features. It is believed that plasma cell infiltration is only an inflammatory reaction in the later stages of the disease, and that the primary lesion and its pathological features are the dilatation of the breast ducts as its basic lesion. This has clarified the nature of the disease and is widely recognized. Recently, it has been suggested that plasmacytic mastitis is not an inevitable process of ductal dilatation of the breast, but that plasmacytic mastitis has its characteristic morphology and clinical manifestations, and it is treated as a special type of mastitis. The pathological changes of ductal dilatation of the breast are: 1. Gross morphology There are distorted and dilated milk ducts and large ducts in the nipple and subareolar area, some of which form cysts. The involved milk ducts are often 3 to 4, and in many cases up to a dozen are involved at the same time. The dilated ducts can be 3-4 mm in diameter or larger. The dilated ducts and sacs are seen to be filled with yellowish-brown, creamy or tofu-like mucus. The ducts are surrounded by fibrous tissue hyperplasia and hyaline degeneration, forming thick white translucent fibrous walls. The adjacent fibrous thick walls adhere to each other to form a yellow-white hard nodule or a solid mass with unclear borders. The epithelial cells of the dilated duct are shrunken and thinned to a single layer of cuboidal epithelium or flattened epithelium, and some of the epithelial cells of the duct are necrotic and fall off, and the shedding epithelial cells and lipid-like material fill and block the lumen. If the contents of the dilated duct overflow or part of the duct wall is destroyed. In the later stage, a large number of plasma cells, histiocytes, neutrophilic leukocytes and lymphocytes infiltrate in the peritubular tissue, or foreign body giant cell reaction, tuberculosis-like nodules or pseudo-abscess formation can be seen. At this point, it should be distinguished from tuberculosis and breast cancer. The diagnosis of plasma breast can be made by experienced physicians with clinical findings. Clinically, plasma breast shows the following characteristics: 1. the condition is recurrent, often for a long time; 2. irregular breast masses, skin redness and swelling or even rupture, but less fever; 3. yellowish thick liquid flows out after rupture, the rupture does not heal for a long time, or this place is healed, and then another place rupture occurs; 4. the wound is not easy to grow after incision and drainage or local excision, and is prone to recurrence; 5. The application of antibiotics may reduce the size of the breast mass for a short period of time, but recur soon after stopping the medication. Overall, pulpy breasts are not life threatening to patients, but they are very threatening to breasts. About 60% of patients end up with severe breast deformation due to repeated surgeries and end up with simple mastectomy. Considering the breast deformation and high recurrence rate associated with local excision, for plasma breast with a wide lesion that exceeds two quadrants, Dr. Guobing Yin recommends that patients undergo total mastectomy after a clear diagnosis and then undergo a mastectomy about 6 months after surgery.