Anti-Her2 therapy patterns in metastatic breast cancer-Real-world data suggest undertreatment.

Treatment efficacy of anti-HER2 therapies for metastatic breast cancer (mBC) has been demonstrated in clinical trials, but real-world data are lacking. In particular, it is unclear whether patients in clinical practice receive treatment as recommended. We took advantage of population-based registries in Norway to assess anti-HER2 therapy patterns in real-world data, with specific attention to the treatment of vulnerable groups. We included 715 patients with HER2+ mBC diagnosed from 2012 to 2021. Median age was 60 years, 473 (66%) had relapsed from early-stage BC, and 440 (62%) had hormone receptor positive (HR+) disease. Anti-HER2 therapy patterns aligned with national recommendations. Median treatment duration for first line therapy was 7.2 months, where 261 patients (55%) used trastuzumab and pertuzumab (±chemotherapy), followed by monotherapy with trastuzumab (195 patients, 41%). Second line therapy was initiated by 206 patients (43%), with a median duration of 7 months, where trastuzumab emtansine was the most prevalent therapy, used by 84 patients (41%). Third line therapy was initiated in 119 patients (25%) and 55 patients (11%) proceeded to fourth line therapy. The 182 patients (28%) who did not receive any anti-HER2 therapy were older (74 years vs. 55 years) and had more comorbidity compared to treated patients. Among patients ≤75 years and healthy, 15% did not receive anti-HER2 therapy. Patient characteristics strongly influence anti-HER2 treatment patterns, and although numerous treatment options are available, a substantial proportion of HER2+ mBC patients did not receive targeted therapy. Undertreatment may be present.