Abstract Background: The integration of Residual Cancer Burden (RCB) and post-treatment Ki67 (Residual Proliferative Cancer Burden [RPCB] index) has been proposed as a stronger predictor of long-term outcome in unselected breast cancer (BC) patients undergoing neoadjuvant chemotherapy (NACT), as compared to RCB. However, no specific analysis in the subgroup of patients with hormone-receptor (HR)-positive/HER2-negative BC is available so far. We investigated the prognostic value of RPCB in an independent cohort of hormone-receptor (HR) positive/HER2 negative BC patients treated with NACT at our Institution. Methods: A cohort of 130 stage II-III, HR+/HER2 negative BC patients who underwent NACT between 2000 and 2014 was included. Archival surgical specimens were evaluated for RCB (Symmans et al. J Clin Oncol 2007). RPCB was calculated by combining RCB and Ki67 as previously described (Sheri et al, Ann Oncol 2015). Patients were categorized in 4 RCB categories (pathological complete response -pCR- and RCB tertiles) and 4 RPCB categories (pCR and RPCB tertiles). Disease-free Survival (DFS) and overall Survival (OS) estimates were determined by Kaplan-Meier analysis and compared using the log-rank test. The c-index was used to compare the performance of the prognostic models. Results: Patients characteristics were: ductal histology 70%, Grade 3 41%, Stage III 54%, PgR negative (≤10%) 35%, median Ki67 at diagnosis 25%. Neoadjuvant chemotherapy included both anthracycline and taxane in 92% of cases. The rate of pCR was 8%. The vast majority of patients received adjuvant endocrine therapy (96%), whereas 30% received further adjuvant chemotherapy. Median follow up was 8.5 years (95% CI 8.0-9.0). RCB was calculated for 105 patients and RPCB was calculated for 85 patients. RPCB was better than RCB in predicting both DFS and OS (5-year DFS and 8-year OS according to RPCB and RCB are reported in the Table). The c-index for DFS prediction was numerically higher for RPCB vs RCB (0.4042316 vs 0.3396437, p=0.08). Similar results were observed for OS prediction: c-index 0.3099490 (RPCB) vs 0.2372449 (RCB), p=0.08. Conclusions: This is the first study evaluating RPCB specifically in HR+/HER2- BC patients treated with NACT. In this independent cohort, after a median follow up of 8.5 years, RPCB was a strong predictor of DFS and OS. The better performance of RPCB vs RCB was in part due to the ability of RPCB to discriminate a subgroup of patients with a particularly worse prognosis after NACT, who may be candidate for clinical trials evaluating novel adjuvant treatments. Five-year DFS and eight-year OS according to RPCB and RCB5-year DFSp-value8-year OSp-valueRPCBpCR100%0.041100%<0.001I tertile73.2%84.6%II tertile75.6%82.6%III tertile47%36.8%RCBpCR100%0.155100%0.023I tertile75.7%87.0%II tertile67.5%83.9%III tertile65.2%51.0% Citation Format: Federica Miglietta, Vassilena Tsvetkova, Maria Vittoria Dieci, Gaia Griguolo, Grazia Vernaci, Alice Menichetti, Cristina Falci, Giovanni Faggioni, Tommaso Giarratano, Simona Frezzini, Eleonora Mioranza, Marcello Lo Mele, PierFranco Conte, Valentina Guarneri. Validation of residual proliferative cancer burden (RPCB) as a predictor of long-term outcome following neoadjuvant chemotherapy in hormone-receptor positive/HER2 negative breast cancer patients [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P5-06-12.