Biopsy-Guided Pathological Response Assessment in Breast Cancer is Insufficient: Additional Pathology Findings of the MICRA Trial

Background: Neoadjuvant systemic treatment (NST) leads to pathologic complete response (pCR) in 10–89% of breast cancer patients depending on subtype. The added value of surgery is uncertain in patients who reach pCR; however, current imaging and biopsy techniques aiming to predict pCR are not accurate enough. This study aims to quantify the residual disease remaining after NST in patients with a favorable response on MRI and residual disease missed with biopsies. Methods: In the MICRA trial, patients with a favorable response to NST on MRI underwent ultrasound-guided post-NST 14G biopsies followed by surgery. We analyzed pathology reports of the biopsies and the surgical specimens. Primary outcome was the extent of residual invasive disease among molecular subtypes, and secondary outcome was the extent of missed residual invasive disease. Results: We included 167 patients. Surgical specimen showed residual invasive disease in 69 (41%) patients. The median size of residual invasive disease was 18 mm (interquartile range [IQR] 12–30) in hormone receptor-positive (HR+)/human epidermal growth factor receptor 2-negative (HER2−) patients, 8 mm (IQR 3–15) in HR+/HER2-positive (HER2+) patients, 4 mm (IQR 2–9) in HR-negative (HR−)/HER2+ patients, and 5 mm (IQR 2–11) in triple-negative (TN) patients. Residual invasive disease was missed in all subtypes varying from 4 to 7 mm. Conclusion: Although the extent of residual invasive disease is small in TN and HER2+ subtypes, substantial residual invasive disease is left behind in all subtypes with 14G biopsies. This may hamper local control and limits adjuvant systemic treatment options. Therefore, surgical excision remains obligatory until accuracy of imaging and biopsy techniques improve.