Is a shadow on the lung cancer? — Not necessarily. Pulmonary shadow refers to a high-density area in the lung parenchyma, which often appears as a mass or nodule on chest X-ray or CT. The general criterion for a mass is more than 3cm in diameter, while a nodule usually refers to a diameter of less than 3cm. Common causes of lung shadows: infections: specific infections such as tuberculosis, pulmonary aspergillosis, etc., non-specific infections such as pneumonia, etc., and tumors, including benign and malignant tumors.