Lung cancer needs to be considered with shadows on CT of the lungs, and sometimes there is a higher likelihood that a shadow on the lung is cancer. There are two kinds of lung shadows, which may be benign diseases or malignant tumors. Benign tumors are characterized by clearer boundaries, complete envelope, no spread or metastasis, and can usually be cured by surgical resection. It may also be caused by tuberculosis, pneumonia, bronchiectasis or pulmonary aspergillosis. Malignant tumors are characterized by unclear borders, burrs, no complete envelope, faster growth, easy to spread and metastasis, and easy to be accompanied by enlarged lymph nodes. Most early-stage patients can usually be cured by radical surgical resection. Lung shadows need to be confirmed by puncture biopsy and pathologic diagnosis, which is the gold standard for cancer diagnosis. People with lung CT shadows need to do further examination in time, and should actively consult the doctor, do not self-medication to avoid delaying the condition.