Incidental solitary knee mass as the first manifestation of hidden lung cancer.

Hypertrophic pulmonary osteoarthropathy (HPOA), also known as secondary hypertrophic osteoarthropathy, is a clinical syndrome characterized by proliferative periostitis of the long bones especially in the distal and periarticular aspects, proliferation of the synovial membranes, causing painful and swollen joints, and often with finger clubbing. It is associated with various underlying causes, including pulmonary, pleural, cardiac, abdominal and miscellaneous conditions. Its pulmonary causes include bronchogenic carcinoma, tuberculosis, pulmonary abscess, bronchiectasis, emphysema, etc. Its radiographic presentation involves periostitis in the lower extremities. We report one case that had an incidental finding of HPOA with initial complaint about an incidental solitary knee mass with painful swelling of right knee, leading to early diagnosis of occult bronchogenic carcinoma. The radiographs and magnetic resonance imaging (MRI) revealed periosteal reactions without definite intraosseous lesion. Chest radiography and CT scan disclosed an infiltrating right upper lobe lesion suspicious malignancy. Patient received right S2 segmentectomy of lung with pathological confirmation of adenocarcinoma of lung cancer. It is important for the clinician to be aware of the radiographic findings of periostitis of HPOA, which may be the clues leading to early detection of lung cancer without significant pulmonary symptoms and to avoid possible tumor progression and distant metastases.