Pathological Analysis

The status of pathologic response (to NACT) for each breast tumor was determined by pathological analysis of the post-NACT resection specimen. The pathology parameters measured from the resection specimen under light microscopy include: cross sectional tumor size in two dimensions (d₁, d₂, measured [in mm] grossly and confirmed microscopically), estimated invasive tumor cellular density (f_inv), number of involved lymph nodes (LN), and the greatest tumor dimension (d_met) in the largest involved node. The Residual Cancer Burden (RCB) index value was calculated using Equation (4) published by Symmans et al [9]:

A complete pathologic response (pCR) is defined as the absence of residual invasive tumor (RCB = 0). A pathologic non-response (pNR) is defined as tumor cell density in resection specimen equal to or greater than that in core biopsy specimen. Pathologic partial response (pPR) is defined as findings intermediate between pCR and pNR. Non-pCR includes both pPR and pNR and can be further stratified into RCB classes based on RCB index values [9]: RCB-I: 0 < RCB ≤ 1.36; RCB-II: 1.36 < RCB ≤ 3.28; RCB-III: RCB > 3.28. Since the MRI metrics were measured from the primary breast tumor only, the in-breast component of RCB (the first term on the right hand side of Equation (4)) [23] was also computed for correlation with the MRI results. The same RCB index value ranges were used for defining in-breast RCB classes.