Histopathology of non-mass-like breast lesions on ultrasound

There have been several investigations of non-mass-like (NML) lesions on ultrasound (US) since Uematsu first described this approach, and it is a relatively new concept for breast examination. However, the results have varied, and there have been only a few studies related to the detailed histopathology of NML lesions on US. Here, we review the histopathology of NML lesions. NML lesions are pathologically benign, atypical, or malignant. There are two major findings of NML lesions on US: architectural distortion and calcifications. Architectural distortion pathologically indicates a fibrous change with ductal proliferation, invasive breast carcinoma, and carcinoma in situ. Histopathologically, microcalcifications are seen in both benign and malignant lesions, and it is important to distinguish between these lesions among NML lesions, particularly fibrocystic changes including adenosis and hyperplasia in the case of benign lesions and carcinoma in situ (ductal and lobular) in the case of malignant lesions. The differential major points may be whether NML lesions are associated with abundant hyperechoic foci, which indicate comedo necrosis on histology. They are usually high-grade carcinoma in situ that may be positive for HER2 or triple negativity. A recent report indicated that low-grade carcinoma in situ showed better survival than higher-grade carcinoma in situ, which is often accompanied by comedo necrosis on histology, reflecting visible microcalcification on US. NML lesions are considered to include a certain rate of low-grade carcinoma in situ. Therefore, more caution may be needed when detecting and managing NML lesions to avoid overdiagnosis and overtreatment as a result of this recent "low-risk ductal carcinoma in situ" concept.