A Case of Lung Adenocarcinoma with an Organizing Pneumonia Appearance Which Demonstrated a Spontaneous Resolution After the Administration of Osimertinib

━━ Background. In recent years, pulmonary lesions with a good prognosis, called transient asymptomatic pulmonary opacities (TAPOs), have been frequently reported during osimertinib administration. Case. A 76-year-old man was treated with osimertinib for lung adenocarcinoma, and his treatment course was good. An asymptomatic pulmonary infiltrative shadow appeared 36 weeks after the start of osimertinib administration, and pathological examination of a transbronchial lung biopsy showed findings consistent with those of organizing pneumonia. However, no worsening of the respiratory status was observed, and the patient strongly wished to continue the therapy. The infiltrative shadow disappeared after 9 weeks of osimertinib administration. Conclu-sion. In this case, TAPOs were suspected due to a spontaneous resolution despite the continuation of osimertinib administration. TAPOs are difficult to distinguish from the early stages of severe drug-induced lung injury and are therefore an important clinical problem that is sometimes associated with patients undergoing treatment for lung cancer.