Does preoperative MRI accurately stratify early-stage HER2 + breast cancer patients to upfront surgery vs neoadjuvant chemotherapy?

PURPOSE: HER2 +- amplified breast cancer patients derive benefit from treatment with anti-HER2-targeted therapy. Though adjuvant treatment is based on final pathology, decisions regarding neoadjuvant chemotherapy are made in the preoperative setting with imaging playing a key role in staging. We examined the accuracy of pre-operative imaging in determining pathological tumor size  (pT) in patients undergoing upfront surgery. METHODS: Early (cT1-T2N0) HER2 + breast cancer patients who underwent upfront surgery between 2015 and 2016 were identified from a prospective institutional database. We compared data for both clinical and final pathologic stage. Only those who underwent magnetic resonance imaging (MRI), mammography, and ultrasound in the preoperative setting were included in the analysis. Adjuvant treatment regimens were reviewed. RESULTS: We identified 87 cT1-2N0 patients with invasive HER2 + breast cancer who underwent upfront surgery. Median age was 52 years (IQR 43, 58) and median tumor size was 1.1 cm (IQR 0.5, 1.6). Fifteen patients (17%) were upstaged to stage II/III based on final pathology. Thirty-seven patients were T1cN0 on final pathology; 8 were cT1a-bN0 preop and 12 had pT overestimated by MRI by an average of 1.5 cm (> 0.5-1.5 cm). Compared to both mammography and MRI, the imaging modality most predictive of pT was ultrasound (p = 0.000072 ultrasound vs mammography and 0.000042 ultrasound vs MRI). CONCLUSION: For small HER2 + cN0 tumors undergoing upfront surgery, ultrasound was the imaging modality most predictive of pT. MRI overestimated tumor size in approximately 40% of patients. MRI may not accurately discriminate low-volume tumor burden in the breast and carries the potential of overtreatment in the upfront setting.