[A case of interstitial pneumonitis induced by S-1].

S-1, an oral fluoropyrimidine derivative, has been identified as an effective agent for the treatment of breast cancer. We present here a case of interstitial pneumonitis that occurred after S-1 treatment. A n 80-year-old woman was diagnosed with stage III infiltrating ductal carcinoma of the left breast and underwent a modified radical mastectomy in November 2001, followed by six courses of paclitaxel. In October 2008, metastatic disease was detected in her skeletal system. Therefore, S-1 chemotherapy was initiated (100 mg/body). Five days after starting S-1, she developed severe eruptions along with dyspnea. X-rays and CT scan showed diffuse ground glass shadow. Both her symptoms and the radiographic findings resolved dramatically after the start of high-dose corticosteroid therapy. Clinicians should be aware that S-1 has the potential to cause lung injury when it is included in chemotherapy.