Isolated pulmonary nodules (SPN) refers to the only round-like isolated lesion in the whole lung with a diameter of 2 to 30 mm. About 150,000 cases of SPN are detected by screening worldwide every year, and among the detected SPN, 10% to 70% are malignant tumors, 80% of benign lesions are inflammatory granulomas, and 10% are malformations. The percentage of benign lesions is 80% for inflammatory granuloma and 10% for malignant tumor. 1. The detection rate of scanning radiographs is significantly lower than that of CT, and small nodules of less than 1 cm are often missed. Some studies have shown that the number of lesions detected by low-dose spiral CT can be 8 times higher than that of plain radiographs; others have found that 76% of small subclinical lung cancers detected by low-dose CT cannot be shown on radiographs. Therefore, CT should be chosen as the screening tool instead of X-rays. Of course, with the development of tumor immunoassay technology, the detection of tumor immune expressions will play an increasing role in screening. Spiral CT with a layer thickness of 1 to 3 mm is appropriate for routine lung cancer screening. Shingo et al. showed that a layer thickness of 1 mm is more accurate for the identification of benign and malignant isolated lung nodules. Therefore, the current screening with 10 mm layer thickness will have a large number of misses. With the development of multilayer spiral CT, the layer thickness is getting thinner and thinner, and now the layer thickness of 16-layer CT routine scans has been reduced to 1 mm, allowing retrospective personalized reconstruction as needed. With regard to scanning conditions, the optimal dose at constant voltage (120-140 kV) has been reported by different authors and varies from 20 mA to 120 mA. Schoepf et al. recommend scanning the entire chest at 120 kV with a dose of 10-40 mA (adjusted for individual body size). However, Zwirewich et al. compared images obtained at 40 mA with those obtained at 400 mA and found that while low-dose CT showed the lung parenchyma relatively satisfactorily in most cases, it failed to show subtle ground-glass signs (20%) and emphysema (11%) in some cases, and they proposed using 80 to 90 mA for the first high-resolution (HRCT) scan and a low dose of 40 to 50 mA for subsequent reviews. A low dose of 40 to 50 mA. 2, SPN imaging characteristics size, SPN diameter less than 5 mm, the possibility of malignancy is 0%-1%; 5-10 mm, the possibility of malignancy 6-28%; greater than 20 mm, it is 64%-82%. In ground glass opacities (GGO), the likelihood of SPN malignancy is 59%-73%. Nodule balls are commonly found in the posterior superior and dorsal lower lobe segments; metastatic nodules are often located subpleural. Morphologically, there is a high rate of display of malignant SPN burrs or burr margins, but a lower rate of appearance of lobar signs and pleural depression signs, for example. Secondary lung lobular structures are about 1 to 3 cm, and the basis of lobular formation is firstly the obstruction of intrapulmonary framework structures, and uneven differentiation of tumor at larger size is also involved in lobular formation. Therefore, if the tumor does not fill the whole lobule when it is small, it is not easy to form the lobar sign. As for the pleural depression sign, the occurrence of pleural traction is also low because the small nodules have not yet developed sufficient traction. Second, density is another basic sign of SPN. Since many early-stage lung cancers have not yet become dense, there is a high occurrence of the vacuolar sign and bronchial inflation sign, and a significant proportion of nodules show ground glass-like density (GGO). In a lung cancer screening study, 19% of non-calcified nodules were GGO, of which 18% were simple GGO nodules and 63% were complex GGO nodules (i.e., with small nodular dense areas), while only 7% of solid nodules were confirmed to be lung cancer, suggesting that screening nodules, especially heterogeneous GGO nodules, require special attention. Nakata et al. found 34 cases of lung cancer and 9 cases of adenomatous hyperplasia in 43 GGO cases. Another reason for special attention is that since lung cancers presenting as GGO are early stage lesions and have a relatively long multiplication time, with 100% surviving for more than 5 years after surgical resection, the correct identification of GGO is crucial to save patients’ lives. In addition, the change in nodule size is also very important in determining the nature, in most cases, nodules with a doubling time of less than 1 month or a stabilization time of more than 24 months should be considered benign, so to speak, if SPN is found on examination and the nodule does not appear on the contrast photograph 2 months ago or the nodule found has basically no change in 24 months, then it is less likely to be malignant. The follow-up interval is usually based on the size and density of the nodule, generally the larger the nodule the shorter the follow-up time, and the more soft tissue density in the nodule the shorter the follow-up time. 5 mm or less nodules can be reviewed half a year, 5-10 mm nodules can be reviewed 3 months, and nodules above 10 mm should be reviewed once a month; according to the density, simple GGO should be followed up half a year, mixed GGO 3 months, and solid Solid nodules are followed up at 1 month. In a recent study of 156 isolated pulmonary nodules and masses (1-14 cm in diameter), 74 cases had old film controls, 26 of which were nodules without growth, but 9 were malignant lesions, and the diagnosis of benign based on no significant growth over 2 years was only 65% accurate. Therefore, extra caution should be exercised when using the rule that “no growth within 2 years of chest radiograph comparison suggests a benign course”. In addition to this, the accuracy of the conclusion of the two comparisons is governed by the method used and experience, most of which is manual measurement, but several factors limit the accuracy of this method: (1) whether the conditions used for the two examinations are the same, including magnification; (2) there is manual error in each measurement and it is not repeatable; (3) the purely two-dimensional measurement does not reflect the volume change of the nodule, because the nodule The size of the nodule is roughly estimated based on the diameter, which is based on the assumption that the lesion is spherical, and a 30% increase in diameter is considered a 1-fold increase in volume. Most of the newer spiral CTs now have volume measurement methods (lungcare or volume software), which are more accurate and reliable with automated software and can address the limitations of less accurate measurements. After a series of non-invasive tests, if still not characterized, an invasive test may be considered. The sensitivity of bronchoscopy for detecting malignant SPN ranges from 20% to 80%, depending on the size of the nodule, its proximity to the bronchial tree, and the difference in the incidence of lung cancer in the study population. Its sensitivity is 10% for nodules less than 1.5 cm in diameter and 40% to 60% for nodules 2.0 to 3.0 cm in diameter. When CT shows that a bronchus is connected to the lesion, the sensitivity of bronchoscopy can reach 70%. Microfiber bronchoscopy can reach the 8th level bronchial branches, allowing us to directly observe peripheral lesions, but this technique is still in the experimental stage. Transthoracic needle aspiration cell biopsy can determine the benignity and malignancy of 95% of peripheral lesions, and its sensitivity for malignant lesions is 80% to 95%, and its specificity is 50% to 88%. 3. Treatment The method used to treat SPN must be based on an estimate of its probability of malignancy, which varies in size depending on the patient’s age, smoking history, nodule volume, and differences in CT signs. When the likelihood of malignancy is small, the nodule should be followed up with CT. When the likelihood of malignancy is high, surgical resection should be performed at an acceptable risk. In patients with a 10% to 60% chance of malignancy, additional testing is necessary. Positron emission tomography (PET), percutaneous needle aspiration biopsy, and bronchoscopy are available. If the nodule is 1. 5 cm in diameter, PET should be performed if financially possible, but a negative PET does not exclude a tumor, and PET-CT has a sensitivity of 80%-100% and a sensitivity of 40%-100% for the diagnosis of malignant SPN. If the lesion is peripheral, needle aspiration cell biopsy is performed; if the nodal signs strongly suggest malignancy, televised thoracoscopic surgery can be performed if it is determined that the patient is at low risk for surgery. According to the results of studies, the operative mortality rate for lobectomy in patients with malignant tumors is 3% to 7% or less, and the operative mortality rate for resection of benign nodules is less than 1% (mainly because only a small wedge resection is required). Therefore, surgery for indeterminate nodules should be performed as aggressively as possible.