Long-term Outcome of Breast Cancer Patients Diagnosed ≤40 Years According to Breast Cancer Subtype in the Absence of Adjuvant Systemic Therapy: the PARADIGM Initiative.

535 Background: Young age at breast cancer diagnosis is considered a poor prognostic factor. As a result, many treatment guidelines advice adjuvant systemic treatment for young patients. Answering prognostic questions on young patients has therefore become a challenge. The PARADIGM (PAtients with bReast cAncer DIaGnosed preMenopausally) project aims to assess the long-term outcome of women diagnosed with breast cancer ≤40 years in the absence of adjuvant systemic therapy, using real world data from the nationwide Netherlands Cancer Registry (NCR) coupled with tissue biobanking. Methods: All women ≤40 years, diagnosed in the Netherlands between 1989-2000 with a primary invasive, histologically proven, TanyN0M0 breast cancer, without adjuvant systemic treatment were identified through the NCR. Back then N0 patients were considered low risk and did not receive adjuvant systemic treatment. Tissue specimens were revised by a team of dedicated breast pathologists. Cox regression was performed to estimate hazard ratios for recurrence-free (RFS) and overall survival (OS) according to immunohistochemical (IHC) subtype. Analyses were adjusted for grade, pathological T-stage, histological subtype and radiotherapy. Results: We included 2310 patients with a mean follow-up of 15.4 years (range 0-25 years). OS for the whole cohort was 68% and RFS 58.4% at 25 years. In total 740 deaths and 1043 recurrences were observed. Hormone receptor (HR)+/HER2+ patients had a significantly worse OS when compared to HR-HER2+ patients (adjusted Hazard Ratio 1.58; 95% confidence interval 1.05-2.38; p=0.029). No difference was observed between HR-HER2+ and the triple negative and HR+/HER2- subgroups at 25-years. RFS was similar for all IHC subtypes. Conclusions: In this large cohort of non-adjuvant systemically treated young breast cancer patients with long-term follow-up HR+/HER2+ patients have a significantly worse survival when compared to triple negative, HR-/HER2+ and HR+/HER2- patients. The latter three subtypes have similar OS at 25 years. Future molecular studies have been planned to distinguish the favorable from the unfavorable prognostic patients.