The precise measurement and detection of a tumor is indispensable in establishing the regular use of minimally invasive procedures for breast cancer treatment. We, herein, focused on ultrasonography （US） and magnetic resonance imaging （MRI）, and evaluated their effectiveness in measuring the tumor size of ductal carcinoma in situ
（DCIS）. Furthermore, we assessed the DCIS subtype and its relationship with the accuracy of the imaging modalities and some critical histopathological characteristics. A total of 102 DCIS cases （that were diagnosed from 2018 to 2021） were retrospectively analyzed. The tumor sizes measured by US and MRI were compared with those obtained by the postoperative histological examination of the tumors. The DCIS subtypes were classified based on the nuclear grade and immunohistochemical （IHC） pattern; namely, estrogen/progesterone receptor expressing status, human epidermal growth factor receptor 2 （HER2） expressing status, and Ki-67 index. Tumors classified as HER2-enriched DCIS exhibited a high nuclear grade and a high Ki-67 index （Ki-67＞20％） when compared with the luminal DCIS tumors （p
＜0.05）. The tumor size measured by US was smaller than the actual tumor size in all DCIS subtypes, while MRI systematically overestimated the tumor size with the exception of the triple-negative DCIS. The median tumor size differences identified between MRI and histopathology were smaller than those identified between US and histopathology, especially in HER2-enriched DCIS cases （p
＝0.01）. The HER2-enriched DCIS tumors were more accurately measured by MRI than by US. MRI could measure the DCIS tumor size more accurately than US, especially in HER2-enriched DCIS cases （p
＝0.01）. These results might be particularly useful in developing a more tailored DCIS management strategy.
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