6589 Background: Trastuzumab (T) is now approved for the treatment of HER2-positive early breast cancer (EBC) in most European countries. Approval was based on the results of the HERceptin Adjuvant (HERA) trial, in which distant recurrence-free survival was reduced by half. The model assesses the long-term impact of T treatment in EBC on the annual number of patients entering the metastatic setting between 2005 and 2015 in five European countries. Methods: Annual EBC incidence for 2005–2015 was projected by applying stage-specific proportions for stages I-III, according to regional cancer registry data, to published female breast cancer incidence rates from 1990–2002. Age-specific rates for 2002 were then applied to United Nations population projections for 2003–2015. The annual number of patients with HER2-positive metastatic breast cancer (MBC) includes annual de novo MBC incidence plus patients with breast cancer recurrence. The baseline 10-year recurrence rate for standard treatment was estimated as 37%, based on 4-year follow-up in the control arm of a combined trial analysis in patients with HER2-positive breast cancer and the long-term timing of recurrence in all patients with breast cancer. To model recurrence in T-treated EBC patients, the hazard ratio (HR) from the HERA trial (0.49; 95% CI: 0.38, 0.63) was applied, resulting in an estimated 10-year recurrence rate of 18.1% (95% CI: 14.0, 23.3). Results: In 2004, prior to approval of T in EBC, the pool of de novo and relapsed MBC patients in the five countries was estimated to be 16,156. Between 2005 and 2015, the model predicts that the use of T will result in an annual decline of 2.5% (95% CI: 1.7, 3.2). The total number of patients saved from developing metastases over 10 years is projected to be 27,727 (95% CI: 20,116, 33,709). Conclusions: T treatment of HER2-positive EBC is expected to prevent nearly 28,000 women from developing metastases over a 10-year period in five countries alone, which may result in a similar number of breast cancer deaths being avoided. Author Disclosure Employment or Leadership Consultant or Advisory Role Stock Ownership Honoraria Research Expert Testimony Other Remuneration F. Hoffmann-La Roche Ltd F. Hoffmann-La Roche Ltd