Organizing pneumonia secondary to lung cancer of unknown primary site

Background: 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. Case review: 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. Conclusion: 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.