1.Definition of isolated pulmonary nodule (SPN) With the increasing attention to health and the improvement of physical examination means (by chest CT instead of ordinary chest plain film), more and more SPN are detected early. Definition of isolated pulmonary nodule: any intrapulmonary or pleural lesion that appears on imaging as a round or round-like shadow with a diameter of 2 to 30 mm and well-defined or indistinct margins. Nodules can be divided into two main categories: benign and malignant. According to clinical statistics, approximately 40% of SPNs are malignant, usually bronchopulmonary carcinoma. In addition are carcinoid tumors or isolated metastatic lung cancer. The majority of benign nodules are tuberculosis, sarcoidosis, malignant tumors or AVMs. Others are spherical pneumonia, pulmonary infarction, bronchial cysts, and pleura-related lesions such as discoid pneumonias and extrathoracic lipomas. 2, Clinical risk factors There are some clinical factors that play an important reference role in determining the benignity and malignancy of SPN: (1) Nodule diameter >3CM greatly increases the likelihood of the lesion being malignant. (2) A history of >400 years of smoking is a high-risk risk factor for lung cancer. (3) History of extra-thoracic malignancy (e.g., liver, rectal, breast cancer, etc.) suggests the need to consider the possibility of intrapulmonary metastases. (4) Family history of tumor. 3. morphological analysis of isolated pulmonary nodules (1) nodule size: most benign nodules are less than 2 cm in diameter. however, with the promotion of minimally invasive thoracic surgery in recent years, many early lung cancers >8 mm and less than 2 cm have been identified. a recent screening study by MayoClinic identified 2832 nodules, 89% of which were less than 7 mm. only 1% of these small nodules were malignant. nodules. Another study found that none of the nodules smaller than 5 mm were malignant. Therefore, if the nodule is less than 8 mm, surgery is generally not considered. SPNs usually have a constant growth rate, and we refer to the time it takes for a lesion to double in size as the “doubling time”. The DT of benign nodules is usually longer than 400 days; DT within 30-400 days is usually malignant; if DT is less than 20 days, the likelihood of malignancy is reduced. When measuring the size of the nodule, it should be noted that 2D CT measures the diameter of the lesion, and 3D volumetric measurement software helps to measure the nodule volume more accurately. (2) Nodule density: soft tissue density, calcification, fatty density, and hairy glass-like density The presence of calcification within a nodule is usually a characteristic sign of a benign nodule. Mass calcifications, scattered calcifications, laminar calcifications, central or concentrated calcifications, and “popcorn”-like calcifications within the lesion are all suggestive of benign lesions. However, calcification is not only a feature of benign nodules, but a few malignant tumors may also show the presence of calcification. Oddly shaped calcium salt deposits or dotted or clustered calcifications do not exclude the possibility of malignancy. Certain characteristic CT findings may identify or specifically suggest a benign nodule, such as fatty densities found in intrapulmonary nodules, which may be diagnosed as a malignant tumor, or if the tumor contains fat on or outside the pleura, it is likely to be an extrapleural lipoma. High-resolution CT (HRCT) may show some nodules that are not substantial, exhibit hairy glass-like or mixed density, or have small air-containing vacuoles within the lesion. Studies suggest that mixed density nodules with a diameter >1.5 cm have an increased likelihood of malignant nodules (63%); on the other hand, if malignant nodules show hairy glass-like density or small air-containing vacuoles within the lesion, the nodules are still relatively early and stable, and most lung adenocarcinomas contain an alveolar cell carcinoma component (see Figure 3). Suzuki reported 69 cases of lung cancer with predominantly gross glassy density, 47 (68%) of which were bronchoalveolar cell carcinoma. All nodules were in stage I, and there was no recurrence 3 years after surgery. (3) Nodal margins: smooth, lobulated, irregular, burr, pleural depression signs Irregular nodal margins, burr, and entanglement of peripheral structures to the nodule, as well as the finding of involvement of one bronchus and three or more vessels, are highly suggestive of malignancy. The presence of burrs in 90% of these nodes suggested malignant nodes, but 10% of nodes with burrs were benign. And 21% of nodules with smooth margins are malignant nodules. (4) Nodules containing cavities: Cavities can occur in both benign and malignant nodules. Benign nodules tend to show smooth thin-walled cavities, while the typical cavity in malignant nodules is an irregular thick-walled cavity. The inner edge of the cavity wall of benign nodules is smooth, while the inner edge of the cavity wall of malignant nodules is nodular and not smooth. This may help in the differential diagnosis. However, there is more overlap between the two. (5) Satellite foci of nodules: The presence of satellite foci of nodules strongly suggests that the lesion is a benign nodule. The positive predictive value of benign nodules reaches 90%. If the lesion is larger than 1CM and close to the chest wall, CT-guided percutaneous lung puncture and isotope lung scan with blood tumor markers can be considered to exclude the possibility of lung cancer, and whole-body PET-CT can be considered if there is evidence of intrapulmonary infection, anti-inflammatory first, and chest CT will be repeated after 2 weeks. If the lesion cannot be examined invasively due to its location or for other reasons and is accompanied by multiple high-risk factors, surgery or close follow-up (once every 3 months) may be an option. Lesions less than 8 MM are not considered for surgery. If the lesion is less than 5 MM, follow up every 6 months. Each patient’s situation is different, so it is recommended to go to a specialized hospital for expert guidance. Due to the current high incidence of lung cancer, it is important to pay high attention to isolated nodules in the lung to avoid missing or misdiagnosis.