Practical Pathological Assessment of Residual Cancer Burden (RCB) After Neoadjuvant Therapy in Human Epidermal Growth Factor Receptor 2 (HER2)-Positive Invasive Breast Carcinoma: A Case Report

Neoadjuvant therapy is routinely used in human epidermal growth factor receptor 2 (HER2)-positive breast carcinoma and permits the assessment of treatment response in the surgical specimen.Although pathological complete response (pCR) is an important endpoint, it is binary and does not quantify the extent of residual disease in patients with non-pCR.The Residual Cancer Burden (RCB) index provides a standardized quantitative assessment incorporating the dimensions of the treated tumor bed, residual cancer cellularity, in situ component, and nodal involvement.We report a 48-year-old woman with HER2positive, estrogen receptor/progesterone receptor (ER/PR)-negative invasive breast carcinoma who received six cycles of neoadjuvant TCH (docetaxel, carboplatin, and trastuzumab) followed by modified radical mastectomy.Gross examination showed no residual mass but identified a fibrotic tumor bed measuring 7 × 5 × 5 cm.Microscopy demonstrated a single residual invasive focus measuring 0.4 × 0.4 cm with 10% overall cancer cellularity, 1% in situ component, and no metastatic tumor in 13 axillary lymph nodes.The RCB index was 1.891 (RCB-II), with final staging of ypT1aN0, and adjuvant ado-trastuzumab emtansine (T-DM1) was initiated.This case is not novel in RCB or HER2-directed therapy, but it demonstrates the importance of meticulous tumor-bed evaluation after neoadjuvant chemotherapy.Treatment-related fibrosis and inflammatory change may obscure scattered residual carcinoma, and a small residual invasive focus may coexist with a substantially larger treated tumor bed.Accurate RCB assessment therefore requires correlation of gross findings, microscopy, nodal evaluation, and pre-treatment disease location.In this setting, RCB complements pCR by providing a reproducible description of residual disease that can support multidisciplinary risk assessment and management.