[A Case of Interstitial Pneumonitis Induced by Lapatinib plus Letrozole].

Lapatinib is an orally bioavailable dual inhibitor of the intracellular domain of both the HER2 protein and the epidermal growth factor receptor. This dual inhibitor can effectively prevent the downstream signal transduction initiated by a tyrosine kinase, thereby reducing the proliferation rate of tumor cells. Lapatinib was demonstrated to be beneficial in patients with HER2-positive locally advanced and metastatic breast cancer. We present here a case of interstitial pneumonitis that occurred after lapatinib treatment. A 79-year-old woman was diagnosed with Stage III infiltrating ductal carcinoma of the right breast. She underwent a modified radical mastectomy in January 2009, followed by anthracycline and paclitaxel plus trastuzumab administration. In November 2015, lung metastatic disease was detected. Therefore, lapatinib plus letrozole administration was initiated. Twelve days after starting treatment, she developed severe eruptions along with dyspnea. Radiography and a CT scan showed a diffuse ground glass shadow. Both her symptoms and the radiographic findings improved dramatically after the start of high-dose corticosteroid therapy. Clinicians should be aware that lapatinib has the potential to cause lung injury.