Abstract
Pneumonitis is a rare and possibly life threatening side effect of TKI-treatment. We present a patient with adenocarcinoma of the lung harboring an uncommon EGFR Exon 21 mutation treated with osimertinib as second-line therapy. After nine months of treatment, the patient developed progressing shortness of breath and night sweats. A severe late-onset predominantly eosinophilic pneumonitis was diagnosed, osimertinib treatment was discontinued and immunosuppressive treatment was initiated. This case report highlights late-onset pneumonitis as a side effect of third-generation TKI-treatment and possible options for subsequent tumor treatment.