The goal of this observational study is to investigate the disease progression rate and timing in patients with mild Non-Lactational Granulomatous Lobular / Periductal Mastitis (NL-GLM/PDM) during follow-up with observation alone strategy. The main questions it aims to answer are: What is the rate of disease progression in mild NL-GLM/PDM patients under observation, and what clinical and pathological factors are associated with this progression? Participants with diagnosis of mild NL-GLM/PDM will be monitored over a specified period, with data collected on their clinical outcomes and relevant factors influencing disease progression.

Non-Lactational Granulomatous Lobular / Periductal Mastitis (NL-GLM/PDM) is an inflammatory breast disease of unclear etiology, primarily characterized by the presence of breast masses. A variety of treatment approaches, including intralesional injection, ductal lavage therapy, or corticosteroids had been proposed. Recently, an increasing number of studies have demonstrated that an observation and follow-up strategy for mild cases can yield comparable efficacy. An international consensus recommends that this observation strategy may serve as a first-line treatment approach for mild NL-GLM patients, with additional therapies implemented only upon disease progression.However, in clinical practice, we have observed that some patients with mild NL-GLM/PDM experience disease progression during the observation period, necessitating further treatment. The proportion of this population and the specifics of their disease progression remain unknown. Additionally, the clinical and pathological factors associated with disease progression during observation have not been previously studied. This research aims to conduct a single-center, single-arm, prospective cohort study to investigate the rate and timing of disease progression in mild NL-GLM/PDM during observation, as well as to explore the clinical and pathological factors related to progression. The findings will provide further guidance for subsequent research and clinical practice.

Inclusion Criteria:

1. Female, aged 18 to 65 years;
2. Clinically diagnosed with possible non-lactational (more than 6 months since stopping breastfeeding) granulomatous lobular mastitis or periductal mastitis.
3. Mscore ≤ 4 points;
4. Patients with ultrasound findings suggesting localized breast abscess formation must have undergone abscess drainage before enrollment;
5. After education, participants should have a strong willingness for observation and follow-up, and, after discussing with the primary physician, jointly decide to proceed with observation and follow-up;
6. Signed informed consent form.

Exclusion Criteria:

1. Patients with confirmed or suspected breast malignancy;
2. Patients with bilateral mastitis (including those with bilateral simultaneous onset and those with sequential onset of non-lactational granulomatous mastitis);
3. Pregnant patients who are currently in the gestation period;
4. Patients who have had non-lactational mastitis in the same breast within the past year and have undergone treatments such as surgery, oral corticosteroids (for more than 2 weeks), anti-tuberculosis treatment (for more than 2 weeks), or ductal lavage therapy.
5. Patients who received surgery, oral corticosteroids, or anti-tuberculosis treatment for the non-lactational mastitis of the ipsilateral breast within 2 weeks before enrollment.

Patients with non-lactational mastitis (defined as more than 6 months since stopping breastfeeding) who meet the eligibility criteria at the Breast Tumor Center of Sun Yat-sen Memorial Hospital, Sun Yat-sen University.

IPD could be shared if the studies were completed for two years, and approvals were obtained from the administrators.