[Lung cancer with a cystic lesion formed by the check-valve mechanism].

A 55-year-old man was admitted to our hospital with a growing cystic lesion in the left middle field of the lung. After we diagnosed it as non-small cell lung cancer, we performed left upper lobectomy. A series of chest X-ray revealed that the cyst was formed by the check-valve mechanism due to the lung cancer, retrospectively. We should keep in mind the existence of lung cancer and other malignant tumors adjacent to cystic lesions.