Are All Prognostic Stage IB Breast Cancers Equivalent?

Background/Objectives: The 8th edition of the American Joint Committee on Cancer integrates histology and biomarker status with anatomic extent in breast cancer (BC) pathologic prognostic staging (PPS). However, PPS IB includes anatomic locally advanced hormone-receptor-positive/HER2-negative (LA-HR+/HER2-) and early-stage triple-negative BC (ES-TNBC). Previous research shows that increased nodal involvement is a critical predictor of worse prognosis, raising questions about whether biological subtype or anatomic stage has a greater influence on outcomes in these discordant cases. We hypothesized that overall survival (OS) remains worse for LA-HR+/HER2- BC compared to ES-TNBC, despite both being classified as PPS IB. Methods: Using the National Cancer Database, we identified patients with LA-HR+/HER2- BC (pT3N1 or pT1-3N2, grade 1-2) and ES-TNBC (T1N0, grade 2-3) treated between 2004 and 2017. Patients without complete primary tumor stage, biomarker status, grade, TNM staging, or treated with neoadjuvant therapy were excluded. The primary endpoint was OS. Multivariable Cox regression evaluated OS between LA-HR+/HER2- BC and ES-TNBC. Results: Among 45,818 patients (17,359 LA-HR+/HER2- BC and 28,459 ES-TNBC), LA-HR+/HER2- BC had significantly worse 6-year OS (86.1% vs. 90.4%; HR = 1.63; p < 0.0001). Among patients receiving appropriate therapies, patients with LA-HR+/HER2- BC had 35% relatively higher risk of death (HR = 1.35; 1.24-1.48; p < 0.0001). These results highlight that LA-HR+/HER2- breast cancer has worse survival compared to ES-TNBC, despite both being classified as PPS IB and receiving all appropriate treatments. Conclusions: Anatomic disease extent remains an important factor in patients with discordant AS and PPS. Future iterations of PPS should re-classify LA-HR+/HER2- breast cancer from PPS IB to ensure more accurate prognostic and survival information.