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. Trastuzumab deruxtecan (T‐DXd) frequently induces interstitial lung disease (ILD) more than other anti‐human epidermal growth factor receptor 2 therapies. Here, we report two cases of ILD induced by T‐DXd. To the best of our knowledge, this is the first such report in clinical practice, making it highly significant.