Borderline breast lesions: diagnostic challenges and clinical implications.

Breast cancer remains a global public health problem and is currently the most polarized cancer in the world. Attention to this disease, public awareness, and advances in breast imaging have made a positive impact on breast cancer screening and detection. The growing use of image-detected biopsies has led to increased diagnosis of ductal carcinoma in situ and high-risk proliferative breast lesions. This progress, however, has created a challenge for pathologists. In lieu of the fact that these entities are difficult to diagnose even in tissue sections taken from surgically excised lesions, pathologist are now expected to diagnose them in small and often fragmented tissue/cellular samples obtained from image-guided biopsies. In addition, some proliferative lesions are associated with an increased risk of finding neighboring malignancy when diagnosed on minimally invasive procedures. Therefore, classifying these lesions in small biopsies is difficult and risky. Some of the most challenging areas in diagnostic pathology includes the differentiation between atypical ductal hyperplasia and low-grade ductal carcinoma in situ, lobular neoplasia versus solid low-grade ductal carcinoma in situ, the correct interpretation of papillary lesions with atypia, and classifying the spectrum of columnar cell changes. Although these issues have been recognized for years, consensus criteria and uniform terminology for the diagnosis of these problematic lesions are far from being achieved. The purpose of this study is to review these "borderline" proliferative lesions in an effort to clarify some criteria and prompt the most needed discussion for consensus.