Breast Lesions of Uncertain Malignant Potential and Risk of Breast Cancer Development: A Single-Center Experience on 10,531 Consecutive Biopsies

<i>Background and Objectives</i>: Breast lesions of uncertain malignant potential identified on biopsy, known as "B3 lesions," constitute a significant portion of diagnoses in numerous published studies. These lesions are associated with a variable risk of coinciding malignant tumors, and current guidelines recommend complete excision, which can occasionally lead to an upgrade in the resection specimen. However, alternative, less invasive treatment strategies, such as clinical follow-up, may be considered. In this study, we retrospectively analyzed diagnostic biopsies from our institution to determine the upgrade rate of each B3 lesion subgroup to breast malignancy following complete excision. <i>Materials and Methods</i>: All breast biopsies conducted at our institution from 1 January 2018 to 30 November 2022 and classified as B3 lesions were included in this study. The lesions were categorized into groups and subgroups based on their growth pattern and histopathological features. To determine the upgrade rate to ductal carcinoma in situ (DCIS) or invasive breast cancer (IBC) for each B3 lesion subgroup, we assessed the histological concordance between the biopsy and the resection specimen. <i>Results</i>: During the study period, 10,531 biopsies were performed, of which 1045 (9.93%) were classified as B3 lesions. Among these, 795 (76.08%) were subsequently resected, either through surgical procedures (98.32%) or using the Vacuum-Assisted Excision technique (1.68%). Histological examination revealed that 89 (11.19%) of the resected B3 lesions were upgraded to breast malignancy, with 59 cases (7.42%) progressing to DCIS, 22 cases (2.76%) to IBC, and 8 cases (1.01%) to borderline or malignant phyllodes tumor. The upgrade rate varied among histopathological subgroups, being lowest in complex sclerosing lesions without atypia (4.95%, 95% CI: 2.5-8.7%) and highest in intraductal papillomas with atypia (58.82%; 95% CI: 32.9-81.6%). <i>Conclusions</i>: Statistically significant differences were observed between B3 lesion subgroups, with a higher risk of upgrade in lesions exhibiting atypia. As our understanding of B3 lesions evolves, there is potential to implement therapeutic strategies tailored to the specific risk associated with each subgroup. This approach could allow for less invasive management options, such as clinical or radiological follow-up, thereby sparing patients from unnecessary invasive procedures when appropriate.