Plasmacytoid mastitis is a rare aseptic-like specific type of benign breast lesion characterized by marked dilatation of collecting ducts at the areola, periductal fibrosis and massive inflammatory cell, especially plasma cell, infiltration. The incidence is low, accounting for approximately 1.41% to 5.36% of benign breast diseases in the same period. The disease is difficult to differentiate from breast cancer and is often misdiagnosed and mistreated, or treated as general mastitis with repeated incisions and drains, with long-lasting wounds and recurrent attacks. It is a difficult disease of benign breast disease. It is also known medically as ductal dilatation, occlusive mastitis, chemical mastitis, and acne mastitis. The etiology of this disease is not well understood, but generally speaking, there are two theories: the early stage may be caused by anaerobic bacterial infection, and common antibiotic treatment is ineffective. The other theory is that it may have started as an autoimmune disease. The disease has two distinctive features: localized ductal dilatation and massive diffuse plasma cell infiltration around the lobular ducts. In general, ductal dilatation is named after the stagnant drainage of the dominant duct near the nipple. When the lesion has progressed to a certain point, inflammation with a predominantly plasma cell infiltrate around the ducts at all levels is called plasmacytoid mastitis. The clinical features of plasmacytoid mastitis include the following: 1. The age of onset is young, mostly seen in 30-40 year old non-pregnant lactating women. The lumps are often located deep in the areola or in the various quadrants of the breast. The lump is often located in the deep areola or in all quadrants of the breast. Its long axis is consistent with the course of the breast ducts, and its texture is hard with unclear borders. Most patients have non-periodic breast pain. In the acute stage, there may be redness, swelling, heat and pain, accompanied by enlarged ipsilateral axillary lymph nodes, often with tenderness, which can be easily misdiagnosed as acute mastitis. 4, can be accompanied by nipple overflow, mostly watery plasma or purulent, there may be bloody overflow. The smear of the overflow can show a large number of plasma cells or inflammatory cells. The affected breast often has nipple invagination or deformation. 5. The duration of the disease varies, and can be between several months or years, with most of them located between 3 months and 1 year. 6.Mammary ductography shows dilated ducts. 7. Fine needle aspiration cytology reveals hyperplastic and inflammatory cells. 8. Mammography and near-infrared examination of the mammary gland show a uniform dense mass in the subareolar area. The surgical treatment principles for plasmacytoid mastitis: Plasmacytoid mastitis is a benign disease that rarely resolves itself without treatment, often extending over several years and lacking a specific effective approach. The disease is a non-bacterial inflammatory disease, its anti-inflammatory efficacy is not obvious, surgery has the following modalities: 1, conservative treatment: for acute inflammatory phase plasma cell mastitis, first treated with anti-anaerobic drugs, to be treated with triamcinolone acetonide (tamoxifen) for those who still have breast lumps after the acute phase of inflammation has improved. For those with more fluid accumulation in the breast, puncture to drain the pus can improve the efficacy. The development of female breasts is mainly due to the action of estrogen. Taking triamcinolone blocks the stimulating effect of estrogen on breasts in women’s bodies, leaving the breast tissue in a relatively dormant period of inactivity. This can make the tissue of plasma cell mastitis caused by autoimmune diseases in a dormant state and make the exudation reduce and the inflammation subside. 2. Simple mastectomy: huge masses, diffuse lesions, sinus tract formation with recurrent infections, or older patients can be considered. 3, local, segmental or quadrant excision of the lesion: local excision of the mass is performed for small masses, but it is easy to recur after surgery, so segmental excision is mostly advocated; segmental or quadrant excision can be considered for dilated milk ducts, forming fistulas and sinus tracts. The surgical treatment of plasmacytoid mastitis should include the complete excision of the lesion, including the surrounding normal breast tissue, and no grayish lesion tissue to prevent recurrence. The trauma is flushed with saline and the suture wound should not be left dead. If there is an abscess, the abscess should be excised and drained, and then the lesion should be removed after the inflammation has subsided. 2. Most of the patients with this disease have recurrent attacks or have undergone multiple surgical treatments, and their conditions are more complicated. The scope and depth of the lesions involved are large, and some appear to have a small surface area of redness and swelling, but invade the posterior space of the breast. If the inflammation invades the breast up to 3/4-4/5, the whole mastectomy should be considered. 3.Breast ultrasound should be done before surgery, and sinus imaging should be done if necessary, so as to have a general understanding of the extent, number and depth of the lesions. 4. Pathological examination must be done to provide a reliable clinical basis for diagnosis. In order to prevent breast cancer from being missed.