Thirty percent of ductal carcinoma in situ of the breast in Japan is extremely low-grade ER(+)/HER2(-) type without comedo necrosis.

Background Overdiagnosis in mammography (MMG) is a problem. Combination of MMG and ultrasonography for breast cancer screening may increase overdiagnosis. Most cases of overdiagnosis are low-grade ductal carcinoma in situ (LGD), but no reports have focused on them. Materials and methods We immunostained 169 ductal carcinoma in situ (DCIS) cases for ER, PgR, HER2 and Ki67 and classified them into 4 subtypes: ER(+)/HER2(-), ER(+)/HER2(+), ER(-)/HER2(-) and ER(-)/HER2(+). The Ki67 index was used to evaluate the grade of malignancy and examined for correlations with each ER/HER2 subtype and the nuclear grade (NG), with/without comedo necrosis. Results The Ki67 index correlated significantly with NG, both with/without comedo necrosis, and reliably evaluated the grade of malignancy. The index for ER(+)/HER2(-) (n=117, 69.2%) was 7.45±7.10, which was significantly lower than for each of the other types. The index was 5.71±6.94 for ER(+)/HER2(-) without comedo necrosis (n=52, 30.8%), which was significantly lower than with comedo necrosis. This was considered LGD, characterized by absence of microcalcification in MMG and either presence of a solid mass or cystic lesion or absence of hypoechoic areas in ultrasound. Conclusion In Japan, ER(+)/HER2(-) without comedo necrosis accounts for about 30% of DCIS and is LGD. This may be being overdiagnosed. J. Med. Invest. 63: 192-198, August, 2016.