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BACKGROUND: Of the most common imaging modalities for breast cancer diagnosis – mammogram (MAM), ultrasound (US), magnetic resonance imaging (MRI) – it has not been well established which of these most accurately corresponds to the histological tumor size. OBJECTIVE: To determine which imaging modality (MAM, US, MRI) is most accurate for determining the histological tumor size of breast lesions. METHODS: A retrospective study of 76 breast cancers found in 73 female patients who received MAM, US, and/or MRI was performed. 239 charts were reviewed and 73 patients met inclusion criteria. Analysis was performed using signed rank tests comparing the reported tumor size on the imaging modality to the tumor size on pathology report. RESULTS: Mammography and ultrasonography underestimated tumor size by 3.5 mm and 4 mm (p-values < 0.002), respectively. MRI tends to overestimate tumor size by 3 mm (p-value = 0.0570). Mammogram was equivalent to pathological size within 1 mm 24% of the time and within 2 mm 35% of the time. CONCLUSIONS: No one single modality is the most accurate for detecting tumor size. When interpreting the size reported on breast imaging modalities, the amount of underestimation and overestimation in tumor size should be considered for both clinical staging and surgical decision-making.