Frequency and outcomes of new or suspicious MRI findings on breast MRI for patients on neoadjuvant therapy

To assess the frequency and outcomes of indeterminate enhancing findings on breast MRI unrelated to the index primary tumor(s) in patients on neoadjuvant therapy (NAT). This retrospective review identified all diagnostic breast MRIs performed to evaluate response to NAT at our institution between 2017 and 2020. All exams with indeterminate enhancing findings (BI-RADS 3-5) unrelated to the index tumor(s) for which follow-up imaging or tissue diagnosis was recommended were included. Cases lacking a pre-treatment MRI or those with insufficient follow-up were excluded. Imaging of all post-NAT breast MRIs were re-reviewed. The electronic medical record was reviewed to evaluate patient and lesion characteristics and outcomes. Between 2017 and 2020, 614/4042 (15.2%) breast MRIs were performed to evaluate response to NAT. After exclusions, 38 of these exams (6.2%) identified 42 indeterminate enhancing findings unrelated to the index tumor for which follow-up imaging (15 exams) or tissue diagnosis (23 exams) was recommended. Fifteen of 42 (35.7%) of the findings were new compared to the pre-treatment baseline MRI, 8/42 (19.0%) increased and 19/42 (45.2%) were present on pre-treatment MRI. Most findings were contralateral to the index tumor (28, 66.7%). Findings were masses (17, 40.4%), focus (15, 35.7%), and non-mass enhancement (10, 23.8%). Of the 42 findings, 19 (45.2%) underwent percutaneous biopsy, 10 (23.8%) underwent surgical biopsy or mastectomy, and 13 (31%) were followed by imaging. Thirty-seven were benign, 4 were high-risk lesions without upgrade (atypical ductal hyperplasia, atypical lobular hyperplasia, and papillomatosis with atypical ductal hyperplasia), and one (2.4%) was malignant (invasive lobular carcinoma), which was non-mass enhancement present on the pre-treatment MRI. Indeterminate enhancing lesions unrelated to the index tumor(s) on breast MRIs performed to assess response to neoadjuvant chemotherapy are uncommon. When present, these lesions have an extremely low likelihood of malignancy, especially when new compared to the pre-treatment MRI. Because incidental indeterminate enhancing lesions on breast MRI at post-NAT follow-up are rarely malignant, radiologists should be judicious in recommending follow-up or tissue diagnosis for such lesions after NAT.