Two cases of trastuzumab deruxtecan‐induced interstitial lung disease in advanced breast cancer

Trastuzumab deruxtecan (T-DXd) frequently induces interstitial lung disease (ILD) more than other anti-human epidermal growth factor receptor 2 therapies. We diagnosed two cases of ILD induced by T-DXd in patients with advanced breast cancer. The first case is that of a 57-year-old Japanese woman who complained of dyspnoea and fever after 4 cycles of T-DXd. Chest computed tomography (CT) showed diffuse consolidation and a reticular shadow. The second case was that of a 72-year-old Japanese woman who complained of dyspnoea after 3 cycles of T-DXd. Chest CT showed a reticular shadow predominantly in the left lung. Both patients were treated with corticosteroids, including pulse methylprednisolone; however, their general condition weakened due to ILD, long-term corticosteroid therapy and breast cancer progression. Subsequently, the patients were unable to continue chemotherapy for breast cancer. To the best of our knowledge, this is the first report in a real-world clinical setting.