Organizing pneumonia secondary to lung cancer of unknown primary site

Secondary organizing pneumonia (OP) is associated with other pathological conditions, such as infections, drugs, cancers and radiotherapy. Lung cancer-associated secondary OP has rarely been reported. In this study, we reported on a case of secondary OP caused by lung cancer. The patient was initially diagnosed with community-acquired pneumonia and then cryptogenic organizing pneumonia by CT scan-guided and transbronchial lung biopsy. Poor response to anti-infection or corticosteroid therapy prompted us to search for underlying disease. A TBNA biopsy of the 4R mediastinal lymph node revealed the diagnosis of lung cancer. OP secondary to lung cancer of unknown primary site are rare. When OP patients have lymphadenopathy or poor response to glucocorticoid, a more differential diagnosis should be considered, especially for avoiding the misdiagnosis of a malignancy.