PS2-05-22: Assessment of the tumor bed area in patients with breast cancer and RCB class I after neoadjuvant therapy: comparison of original and modified method

Abstract Background: One of the important prognostic and predictive markers in the histological assessment of breast cancer (BC) after neoadjuvant therapy (NAT) is the Residual Cancer Burden (RCB) index. In the original formula developed by MD Anderson Cancer Center the linear dimensions of the tumor bed are taken into account, which makes it impossible to assess fragmented material, for example, when performing vacuum-assisted biopsy. Considering this limitation, an attempt was made to modernize the method for evaluating tumor response to NAT by recalculating the RCB index using the area of the tumor bed based on digital images (scans) of histological slides obtained from sectoral resections in patients with early-stage BC. Methods: A single-center retrospective study included 20 patients with ypT1a-cN0 stage breast carcinoma who had undergone neoadjuvant systemic therapy (FAC-T — 3 patients, AC-T — 8, AC — 2, DHP — 1, T — 2, EC+D+TRAST — 1, TCARB — 1, TCHP — 2) and were classified as RCB class 1. Digital scans of histological slides were obtained using the 3D HISTECH PANORAMIC 1000 scanner. Measurements of the tumor bed were carried out using two methods: traditional MD Anderson technique, which defines the tumor bed area with the assessment of the two largest dimensions of the tumor bed, and a modified in-house technique, which implies manual delineation of the entire area of tumor bed on the histological scans using a stylus within the 3D HISTECH SlideViewer 2.5.0 software. The measured areas were then used to recalculate the RCB index according to the original formula, which includes extracting the square root of the area. Results: The mean tumor bed area calculated using the MD Anderson method was 76.67 mm2 (range: 14.2-258.7), whereas for the modified method, the mean was 153.65 mm2 (range: 35.6-535). The mean RCB index for the standard method was 1.034 (range: 0.804-1.316), and for the modified method — 1.102 (range: 0.829-1.461). The obtained data was normally distributed in both groups. A high correlation was found between the two methods, indicating a high consistency between the methods (Pearson's Correlation r = 0.96, p<0.001; Spearman's Correlation ρ = 0.95, p<0.001). The mean difference in RCB index between two methods was 0.068 (95% CI: +0.040 to +0.096). According to the t-test, the difference between the two methods was statistically significant (p < 0.01). The modified method demonstrated systematically higher values of RCB index. Moreover, when applying the modified method, 2 out of 20 patients (10%) were reclassified from RCB class 1 to RCB class 2. Conclusion: The proposed modified method of tumor bed assessment correlates highly with the classical approach in calculating the RCB index. Larger studies are needed to determine the clinical significance of a persistent slight overestimation of the RCB index using the modified method. Citation Format: N. Amirov, N. Mikhaskova, P. Krivorotko, A. Kudaybergenova. Assessment of the tumor bed area in patients with breast cancer and RCB class I after neoadjuvant therapy: comparison of original and modified method [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS2-05-22.