Anti‐Her2 therapy patterns in metastatic breast cancer—Real‐world data suggest undertreatment

Abstract 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.