Pittsburgh Classification and Treatment Algorithm for Idiopathic Granulomatous Mastitis: A Multicenter Cohort Study

Idiopathic granulomatous mastitis (IGM) is a rare inflammatory breast condition lacking standardized treatment and with unpredictable outcomes. To address these issues, using clinical and ultrasound findings from an initial subset, we created the Pittsburgh Classification to stratify severity and developed a corresponding treatment algorithm for IGM, then evaluated its effectiveness in a larger cohort of IGM patients.

This retrospective multicenter study reviewed clinical and sonographic findings and outcomes of women with biopsy-proven IGM treated at multiple breast centers between 2020 and 2025. The Pittsburgh clinical classification ranges from Type 1 (minimal skin irritation) to Type 5 (widespread involvement); ultrasound classification spans Type A (localized mass ≤2 cm) to Type D (diffuse disease). Treatments were assessed utilizing the Pittsburgh algorithm, with responses classified as full response (CR), near-complete response (nCR), or no response (NR). Chi-square tests assessed associations (p<0.05).

Of 522 patients included (mean age 37.0±8.8 years), 86.4% (n = 451) received algorithm-concordant treatment, achieving CR in 68.7% (n = 310), nCR in 35.3% (n = 159) and NR in 11.8% (n = 53). Among these, 65.4% (295/451) of patients with CR were concordant with the Pittsburgh treatment algorithm, whereas 13.6% (n = 71) patients received discordant treatments, with a significantly lower CR rate of 21.1% (15/71) (p<0.001). Multifocal disease was significantly more prevalent in NR (83.0%, 44/53) and nCR (70.4%; 112/159) patients compared to CR (20.6%; 64/310) (p<0.001), although lesion-based response rates were similar (CR 56.8%, nCR 57.0%, NR 56.6%). Regarding concordance with treatment algorithm, clinical Type 4 IGM was more prevalent in NR (67.9%; 36/53) and nCR (72.9%, 116/159), whereas in clinical Type 1 IGM, NR, nCR, and CR were 1.8% (1/53), 4.4% (7/159), and 30.6% (95/310), respectively (p<0.001). Surgery at presentation was preferred in 16.9% (n = 88) of patients, with 6% (n = 30) requiring subsequent surgical treatments to treat residual disease.

Concordance with the proposed IGM treatment algorithm based on clinical and ultrasound findings resulted in significantly higher CR rates. Multiple foci and stratified clinical types correlated with outcomes. Prospective global research is needed to validate these findings.