A 31-year-old patient with HR-positive and HER2-negative non-mass-like enhancement lesion (invasive ductal cancer with extensive carcinoma in situ components). The contrast-enhanced (subtraction) images selected from the same level in pretreatment, F/U-1, and final MRI are shown. The area of the enhanced tumor tissues and the degree of enhancement are decreasing with treatment, indicating a good response to the chemotherapy. The size measured on final MRI is 3.0 cm, but the post-NAC pathological examination shows scattered cancer cells within a 10 cm region. This is a true positive case with a large size discrepancy of 7.0 cm. It is typical for a non-mass lesion to show scattered diseases within the original tumor bed, and it is difficult for MRI to make a correct diagnosis. Furthermore, MRI diagnostic accuracy is the worst in HR-positive and HER2-negative tumors compared to other biomarker types.