I. Introduction
Breast cancer, as the most common cancer among women in developed countries, is one of the leading causes of deaths [1], [2]. Fortunately, among all the cancers, breast cancer is relatively easy to cure once it is detected at the early stage. According to the 2017 AJCC cancer staging table [3], the tumors with the diameter of less than 2 cm can be classified as early-stage tumors. During the last decades, great efforts have been devoted to detect the breast cancer in a convenient and cost-effective way [4], [5] so that the death rate can be reduced.