A Diagnostic Dilemma: New Enhancing Findings on Breast MR Following Neoadjuvant Chemotherapy

Purpose: To evaluate the incidence and outcome of new enhancing lesions on breast MRI following neoadjuvant chemotherapy (NAC).

Materials and Methods: HIPAA-compliant IRB approved retrospective review of all women who underwent breast MRI for evaluation of response to NAC at our institution from January 1st, 1998 through March 3rd, 2021 was performed. Post-NAC MRIs with a BIRADS score of 4 or 5 were identified and those with a new enhancing lesion were reviewed. Patients were excluded if they did not have a pre-treatment MRI, if the lesion was not new or suspicious, if no follow up of the lesion was available, or if the lesion was a satellite of the primary tumor ( <2cm from index cancer). Medical records, imaging studies, and imaging reports were reviewed to identify patient and lesion characteristics and outcomes.

Results: In the study period, 2579 post-NAC breast MRIs were performed. 101 MRIs with a BIRADS 4 or 5 following NAC were identified, and 23 new, enhancing, suspicious lesions were found for an incidence rate of 0.9% (23/2579). Zero of these new lesions were found to be malignant. Twelve lesions underwent image-guided biopsy, 2 had biopsy attempts but the lesion was no longer visible, 1 was surgically excised, 5 were not biopsied because the patient underwent mastectomy, 3 were not biopsied but were stable at 2 years or resolved on follow up MRI. Of the 17 lesions with pathology available, 15 were benign and 2 were benign and high risk (ADH, LCIS/ALH).
New lesions were ipsilateral to the index cancer in 14/23 (61%) of cases and the average distance from the primary tumor was 5.2cm (range 2.5 - 8.0cm). Most new lesions were characterized as non-mass enhancement (11), followed by mass (7), then focus (5) with an average maximum dimension of 1.1cm (range 0.3 – 3.7 cm).

Conclusion: New suspicious breast MR findings following neoadjuvant chemotherapy are uncommon and have a very low likelihood of malignancy. Further study is warranted utilizing multi-institutional data due to low incidence of this finding.

Clinical Relevance Statement: New findings on breast MRI following neoadjuvant chemotherapy create a diagnostic dilemma for radiologists and surgeons. Given the very low likelihood that new lesions on post-NAC MRI represent malignancy, biopsy with associated anxiety for patients and potential surgical delay, may be avoided. Further study is warranted.