Background: Microinvasive breast cancer (T1mi) is considered as a precursor of invasive cancer developing from ductal carcinoma in situ (DCIS). This study discussed the clinicopathological and immunohistochemical features of T1mi by comparing those of ductal carcinoma in situ (Tis) and T1a to assess the nature and causes of the progression of Tis into invasive cancer.
Methods: Three hundred and ninety-two Tis, 32 T1mi, and 141 T1a lesions which were pathologically diagnosed in surgical specimens between 2006 and 2009 were analyzed retrospectively. T1mi was defined as a tumor no greater than 1 mm in its greatest dimensions, and T1a was defined as a tumor larger than 1 mm but no greater than 5 mm.
Results: The frequency of the comedo type was significantly higher in T1mi (68.8%) than in Tis (13.8%) (p < 0.001) and T1a (30.5%) (p < 0.001). Estrogen receptor (ER) negative-HER2 positive type was more frequent in T1mi (46.9%) than in Tis (8.7%) (p < 0.001) and T1a (10.6%) (p < 0.001). There was a significant difference in Ki-67 index between T1mi (35.2 ± 19.2%) and Tis (18.8 ± 14.5%) (p < 0.001). T1mi had higher rates of comedo type and ER negative-HER2 positive type.
Conclusion: Comedo type and ER negative-HER2 positive type were found more frequently in T1mi than in Tis and T1a. There could be different biological mechanisms for promoting Tis into invasive cancer from enlarging invasive cancer.