To summarize the ultrasound and magnetic resonance imaging (MRI) features of plasma cell mastitis (PCM) and granulomatous mastitis (GM), the two most common forms of non-lactating mastitis.

From April 2015 to August 2017, a total 75 lesions from 75 patients with mastitis were retrospectively analyzed. All of these lesions were pathologically confirmed by surgery or biopsy at Ruijin Hospital, Shanghai Jiao Tong University School of Medicine. All of these patients had undergone ultrasound and MRI before the surgery or biopsy. Ultrasound and MRI features of different types of non-lactating mastitis were compared by the χ² test.

Of the total 75 lesions evaluated in this study, 17 were PCM, 20 were GM, and the remaining 38 were other types of mastitis, such as suppurative inflammation, tubulitis, and tuberculosis. The shape, echo pattern, vascular pattern and extent of the blood supply, and spread to the fat of breast lesions in ultrasound had significant differences between different types of non-lactating mastitis (χ²=7.747, P=0.021; χ²=10.923, P=0.027; χ²=29.609, P<0.001; χ²=10.806, P=0.029; χ²=7.939, P=0.019). PCM always had more blood supply than other types and more likely spread to the fat, with marginal type being the main blood supply pattern. For GM, the blood supply patter were mainly the mixed type. There were also some ultrasound features with no statistical difference, such as the location of the lesion, whether the margin was smooth or not, as well as the detection of axillary lymph nodes in ultrasound. The gland density level, enhancement pattern, time-signal intensity curve type, and detection of abnormal lymph nodes in MRI were statistically different among different types of mastitis (χ²=6.724, P=0.035; χ²=15.576, P=0.016; χ²=12.457, P=0.014; χ²=6.394, P=0.041). Most of the PCM lesions were enhanced like tumors, and the GM lesions were mostly ring enhanced unlike tumors, with more axillary lymph nodes detected after enhancement. There were no statistically significant differences in the location of the lesion, shape, margin difference, or detection by non-enhanced scan. The diagnostic accuracy of ultrasound was 52.0% (39/75), and the rate of misdiagnosis was 48.0% (36/75). The diagnostic accuracy of MRI was 26.7% (20/75), and the rate of misdiagnosis was 73.3% (55/75). The diagnostic accuracy of ultrasound combined with MRI was 57.3% (43/75), and the rate of misdiagnosis was 42.7% (32/75).

The imaging features of mastitis have unique characteristics, and the combination of ultrasound and MRI is helpful in diagnosing mastitis.