Clinical and Pathological Characteristics of the Mammary Paget's Disease: A Single-Center Retrospective Study in Japan.

Mammary Paget's disease (MPD) is a rare breast malignancy often associated with ductal carcinoma in situ or invasive carcinoma. However, its diagnosis remains challenging owing to the subtlety or absence of findings on conventional imaging. In this study, we retrospectively analyzed 12 Japanese patients with MPD. All patients showed uniform overexpression of the human epidermal growth factor receptor 2 (HER2; immunohistochemistry score = 3+), with 92% exhibiting associated ductal carcinoma in situ. Magnetic resonance imaging (MRI) revealed skin and nipple enhancement in 78% of patients, along with non-mass enhancement and nipple thickening that correlated with the pathological findings. Moreover, Ki-67 proliferation index was high in most cases (median, 67%), indicating the presence of biologically active tumors. No recurrence or death was observed during the median follow-up period of 96 months. Overall, our findings suggest that HER2-positive MPD exhibits aggressive biological behaviors despite a subtle clinical presentation and highlight the importance of MRI in its detection. Furthermore, integration of imaging with pathological and molecular marker assessment is essential for accurate MPD diagnosis and treatment. This study on a Japanese cohort provides valuable insights and highlights the diagnostic utility of MRI for MPD, especially HER2-driven MPD.