Pathology of MRI and second-look ultrasound detected multifocal breast cancer.

INTRODUCTION: Targeted second-look ultrasound (US) is often performed following MRI of the breast to determine if an MRI-detected lesion is visible on US and thus amenable to US-guided biopsy. This study aimed to assess the pathology of lesions detected and biopsied on the second-look US. In particular, for multifocal cancers, whether the pathology of additional lesions detected by second-look US is different to the index lesion. METHODS: Multicentre single-institution retrospective study of 300 consecutive cases of second-look US biopsies from August 2017 to April 2022 was performed, with their histopathology and imaging characteristics recorded. For multifocal cancers, Wilcoxon Signed Ranks Tests were used to compare differences between the index and additional lesions in the histopathology category (i.e., high-risk benign, precursor or malignant) and BRE grade. RESULTS: 69 multifocal cancers were detected. For the purposes of this study, additional lesions were considered more invasive if they were of a higher histopathological category or BRE grade, or demonstrated lymphovascular invasion when the primary lesion did not. 15/69 additional lesions were not seen on the initial mammogram/tomography or ultrasound, seen on subsequent MRI and second look US, and were less invasive than the index lesion. 3/69 additional lesions were more invasive than their index lesions. Wilcoxon Signed Ranks test showed additional lesions were of either similar or lesser invasiveness compared to index lesions (z= -3.207, p = 0.001) in the histopathological category, and the same or lower BRE grade (z= -2.972, p = 0.003). CONCLUSION: In multifocal breast cancers, additional lesions detected on MRI and second-look US have the same or less invasive histopathology compared to the index lesion.