Abstract P1-02-10: Breast tumor measurement after neoadjuvant chemotherapy using the 8th edition of the American Joint Committee on cancer staging system: Impact on estimates of tumor size and discrepancies with residual cancer burden class

Abstract

Abstract Background: In the 8th Edition of the American Joint Committee on Cancer Staging System (yAJCC) for breast cancer after neoadjuvant chemotherapy (NAC), tumor size is based on the largest focus of residual tumor, excluding treatment-related fibrosis. The goal of this study was to assess the impact of strict adherence to yAJCC criteria on residual breast tumor size and discrepancies between yAJCC stage and Residual Cancer Burden (RCB) class. Design: Breast cancer resections after NAC from 2016-2020 were identified in the Anatomic Pathology laboratory information system. Stage IV, pT4b, pT4d, pCR, neoadjuvant endocrine therapy alone and no response to NAC were exclusion criteria. Tumor size, ypT category and focality were reassessed using current yAJCC criteria and compared to the original reports. A median of 29 slides per case were reviewed (range, 9-113). Results: A total of 189 cases met inclusion criteria, including 58 (31%) TNBCs, 50 (26%) HER2+ and 81 (43%) hormone receptor-positive/HER2-negative (HR+/HER2-). The reassessed tumor size and ypT category differed from the original report in 113 (60%) and 107 (57%) cases, respectively. In all of these cases the tumor size or ypT category was smaller/lower on review. The 107 cases with lower ypT categories included 41% of TNBCs, 62% of HER2+ and 64% of HR+/HER2- (p=0.02) In the original report, 36 (19%) cases were classified as multifocal versus 131 (69%) after review (p=0.00001). yAJCC staging based on the reassessed ypT categories differed from the original report in 38 (20%) cases (all down-staged). There were 77/189 (41%) cases with yAJCC/RCB Class discrepancies in the original reports versus 95/189 (50%) after slide review (p=0.08). The yAJCC/RCB discrepant cases in the original reports included 47% of TNBCs, 38% of HER2+ and 38% of HR+/HER2- versus 47% of TNBCs, 48% of HER2+ and 54% of HR+/HER2- after review (p=0.62). The higher proportion of discrepancies among HR+/HER2- after review did not reach statistical significance (p=0.06). Conclusions: Strict adherence to 8th Ed. yAJCC criteria for measurement of the residual breast tumor after NAC resulted in smaller tumor size, lower ypT category, lower yAJCC stage and more tumors classified as multifocal. Changes in ypT category occurred more often among HER2+ and HR+/HER2- than TNBC. There was no significant change in the frequency of yAJCC/RCB discrepancies. Citation Format: Dawn Harter, Siobhan M O'Connor, Johann D Hertel, Benjamin C Calhoun. Breast tumor measurement after neoadjuvant chemotherapy using the 8th edition of the American Joint Committee on cancer staging system: Impact on estimates of tumor size and discrepancies with residual cancer burden class [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P1-02-10.