Breast-Conserving Surgery After Neoadjuvant Chemotherapy Guided by a Supine Magnetic Resonance Imaging Projection Mapping System Visualizing Original Cancer Extent Running title: Breast-Conserving Surgery after NAC Guided by Projection Mapping System

It is ideal to remove areas where breast cancer existed before neoadjuvant chemotherapy (NAC) in breast-conserving surgery (BCS) after NAC to ensure a pathological response. However, this becomes challenging, particularly when NAC is effective. It is also important to determine the appropriate resection line to maintain breast cosmesis in the absence of residual cancer cells. We aimed to identify magnetic resonance imaging (MRI) findings that enable the removal of the original cancer extent without additional margins outside the cancer boundary in BCS after NAC using an MRI projection mapping system (PMS). We enrolled 36 women with breast cancer in the study. Contrast-enhanced MRI was performed in the prone and supine positions before NAC. Supine unenhanced MRI with skin markers for registration was performed after NAC, one day before surgery. A maximum intensity projection image visualizing both the tumor extent before NAC and skin markers was projected onto the breast using PMS, and BCS was performed. We compared the clinical, pathological, and MRI findings between patients who required additional margins and those who did not undergo BCS using MRI-PMS. Nineteen of the 36 patients did not require additional margins for BCS after NAC using MRI-PMS. These patients had a higher tumor diameter reduction rate and a lower signal ratio between the tumor bed and normal tissue after NAC than those who required additional margins. This study indicates that MRI-PMS allows BCS without additional margins and may facilitate the conservation of normal breast tissue in patients with a greater diameter reduction rate or a lower signal ratio between the tumor bed and normal tissue after NAC.