Mastitis most often occurs during lactation, but mastitis that occurs during non-lactation requires caution for plasmacytosis. It occurs most often between 30 and 50 years of age and can occur after puberty, with another high incidence in menopausal women. This disease accounts for about 4-6% of benign breast diseases. The course of the disease is generally long and the pathological changes are complex and varied. In the later stages of the disease, lipids trapped in the dilated ducts break down and leak out of the ducts, leading to chemical inflammation of the periductal tissue, resulting in foci of necrosis in the adipose tissue and destruction of the affected breast lobules, with a large infiltration of inflammatory cells, mainly plasma cells, around the necrotic tissue, hence the name plasmacytic mastitis. Besides, it has many different names, the most common being called ” ductal dilatation syndrome”, ” occlusive mastitis”, ” non-lactating mastitis”, ” chronic mastitis”, etc. In the early stage, it manifests as brownish-yellow discharge from the nipple or a yellowish overflow, and in a few cases, a bloody overflow involving multiple ducts, which often squeezes out yellowish fluid when squeezed at the areola. In the later stages, near the areola, lumps of varying sizes appear, or adhesions of the lumps to the skin may occur, or they may be accompanied by nipple invagination. In advanced stages of the disease, the inflammation around the ducts increases and a round mass with skin adhesions may appear. The mass is mostly located in the central region, firm in texture, irregular in shape, with poorly defined borders, and often adherent to the skin without chest wall fixation. The skin of the mass may be red, swollen and hot, and the axillary lymph nodes on the affected side may be enlarged. The skin of the breast is edematous and orange peel-like, and after a few days, the breast lump softens and forms an abscess, and the pus that comes out after it breaks down often contains pimple-like or lipid-like material, and the wound does not heal over time to form a fistula, which can last for several years. Severe deformation of the breast. The disease is recurrent and can take up to 10 years to develop. Because of the low incidence of the disease and the similarities between the clinical presentation and breast cancer and acute mastitis, it is easy to miss and misdiagnose the disease, which can lead to delayed treatment. How to differentiate? Plasmacytoid mastitis is also an inflammatory lesion, sometimes with symptoms such as redness, swelling, heat and pain, and is similar to acute mastitis. The former is a chemical inflammatory reaction caused by ductal dilatation due to duct obstruction and the decomposition of lipids collected in the duct and leaking out of the duct, not due to bacterial infection, so the systemic symptoms are mild and the effect of antimicrobial therapy is minimal. If there is an abscess, simple incision and drainage is not effective, and the dilated duct must be removed together to be effective. The latter is a purulent inflammation due to milk retention and bacterial invasion, with obvious chills and fever and pain, and generally elevated leukocyte count in laboratory tests, which is better treated with antimicrobial agents. In the former case, inflammatory cells, mainly plasma cells, can be detected on puncture smear, while in the latter case, mainly pus cells and plasma cells are rare. Plasmacytoid mastitis may present with adhesions to the skin, lumps with non-surface borders, and symptoms resembling breast cancer such as nipple invagination, orange peel-like changes in the skin, and enlarged axillary lymph nodes. In addition, the lack of typical imaging images makes it very easy to misdiagnose as breast cancer. Since the nature of the two is fundamentally different, careful differentiation is important to avoid misdiagnosis and mistreatment. Plasmacytic mastitis has a relatively rapid onset, and sometimes the mass can grow large and painful in a few days, while breast cancer hardly grows large in a few days unless it is necrotic and bleeding, and the pain is usually not obvious; although plasmacytic mastitis can adhere to the skin, it does not adhere to the chest wall and is fixed, and the enlarged axillary lymph nodes are relatively soft and can shrink with time. Breast cancer may not only adhere to the skin, but further development may adhere to and fixate the chest wall. The metastatic axillary lymph nodes are harder and become larger and fused with the prolongation of the disease, and supraclavicular lymph node metastasis may occur. Smears of nipple secretions and lump aspiration cytology are useful for diagnosis. Inflammatory cells, mainly plasma cells, can be detected in ductal dilatation, while cancer cells can be detected in breast cancer. Treatment of plasmacytoid mastitis in the past often required total mastectomy, which greatly affected the patient’s appearance. Since the breast is an aesthetic organ, the surgical treatment of mastopathy should take into account the cosmetic aspect while treating the disease. Based on years of experience, we have determined a unique surgical approach, in which the surgical technique is the key: a parareolar incision is made, the areola is turned, the lesion around the large duct is removed, and all inflammatory necrotic parts are excised up to the normal gland. If necessary, the wound is disinfected with canthaxol and then repeatedly rinsed with water. Nipple reconstruction is done immediately and some of the breast glands are displaced for internal reconstruction. Both radical treatment is achieved and the shape of the breast is preserved as much as possible.