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Abstract

The few publications on <10-mm invasive breast carcinomas have reported worse outcomes for women with human epidermal growth factor receptor 2 (HER2)-positive cancer compared with HER2-negative cases and indicated that the high risk of recurrence in HER2-positive cases is related to the high grade, hormone receptor negativity, and high proliferation index of the invasive tumor component. We studied the subgross morphology of such tumors in a consecutive series of 203 cases documented in large-format histology slides and worked up with detailed radiological-pathological correlation. The invasive component was associated with a diffuse in situ component in 78% of the HER2-positive and 26% of HER2-negative tumors <10mm in size (odds ratio [OR], 11.3936; P<.0001). The in situ component was of high grade in 75% of HER2-positive and 9% of HER2-negative cases (OR, 29.6000; P<.0001). Significant associations were also found between the HER2 positivity of the invasive component and diffuse combined lesion distribution (P>.0001), invasive tumor grade 3 (P=.0004), presence of vascular invasion (P=.0026), extensive disease (P=.0170), "not special" (ductal) histological tumor type (P=.0302), estrogen receptor negativity (OR, 7.8846; P<.0001), and high Ki67 proliferation index (OR, 5.0000; P=.0007). The HER2-positive tumors tended to be multifocal (OR, 2.000) and lymph node-positive (OR, 3.0147), but the tendency was not statistically significant. The vast majority of <10-mm HER2-positive breast carcinomas exhibited a high-grade, diffuse, and extensive in situ component, which may explain the high risk of recurrence among these tumors.