Carcinoid tumor is a very rare neuroendocrine tumor that can occur in all parts of the body, most of them occur in the gastrointestinal tract, and appendix is more common. It is very rare in the bronchopulmonary region and is of low grade malignancy, accounting for 1%-7% of primary lung tumors and 1%-2% of primary lung malignancies, respectively. Kulchitsky cells, which originate from neuroendocrine cells in the bronchopulmonary mucosa and submucosal glands, are low-grade malignant tumors with diverse clinical and imaging manifestations, and are difficult to differentiate from other occupying lesions in the lung. This study aims to improve the diagnosis and differential diagnosis of this disease. 1. Data and methods (1) Clinical data Seven cases of pulmonary carcinoid tumors, three males and four females, aged 18-52 years, with an average age of 39.5 years, were collected from our hospital from 2009 to 2011 with complete clinical data confirmed by surgery and pathology. Two of them underwent percutaneous lung aspiration biopsy, five underwent bronchoscopy, and all seven underwent open-chest exploration and surgery. Clinical symptoms: cough in 3 cases, including 1 case with bloody sputum; chest discomfort in 2 cases, including 1 case with chest pain; physical examination revealed 2 cases with no clear medical history or positive signs. (2) Examination methods Preoperatively, all cases underwent MSCT plain and enhanced scans. The scanning equipment was a Philips Brilliance 64-layer multi-row spiral CT machine, and the scanning range was from the entrance of the thorax to the level of the first hepatic portal in the epigastrium, with the main scanning parameters: layer thickness of 3.0 mm, reconstruction thickness of 3.0 mm, and pitch of 1.0. The image post-processing mainly used multi planar reformation (MPR) and curved planar reformation (CPR) techniques. (3) Image analysis Two radiologists with the title of attending or above jointly analyzed the CT manifestation of the lesion and reached a consensus. The direct signs included: the location, shape and size of the mass, and the enhancement characteristics of the lesion after enhancement; indirect signs included obstructive changes, lymph node metastasis, intra-pulmonary and other sites of metastasis, etc. 2. Results (1) CT performance All cases in this group underwent plain and enhanced CT scan of the chest. The central mass was small with an average diameter of about 2.5 cm, while the peripheral lesions were round, lobulated and irregular in shape, the largest of which was about 5.6 cm in diameter; except for one case with a solid lung lesion with partially indistinct borders, the remaining lesions had smooth and clear margins; one case had a burr sign, one case had a punctate calcification shadow, and one case had a small amount of ipsilateral pleural effusion. The average CT value was about 31 Hu. After enhancement, there were 4 cases with significant enhancement (CT value increased >40 Hu), 2 cases with moderate enhancement (CT value increased >20 Hu, <40 Hu), and 1 case with mild enhancement, and the average CT value increased 42 Hu. (2) Pathological results All of the cases in this group were sent to pathology after surgery for clear diagnosis by immunohistochemical staining. 5 cases of typical carcinoid tumor and 2 cases of atypical carcinoid tumor were diagnosed. 5 cases of lymph node enlargement were shown by CT, but the pathology only suggested that 3 cases were metastases and the remaining 2 cases were inflammatory enlargement, among which 3 cases of positive lymph node enlargement, 2 cases were typical carcinoid tumor and 1 case was atypical carcinoid tumor. (1) Clinical characteristics Primary bronchopulmonary carcinoid tumor is a rare low-grade malignant tumor in the lung, which originates from Kulchitsky cells in the bronchial and fine bronchial epithelium. In recent years, according to the pathological ultrastructural examination and immunohistochemistry results, they are divided into two types: ① Kulchitsky cell type I (KCC-Ⅰ), i.e. typical carcinoid tumor (TC). This type is mostly of central type, and the lesion grows into the bronchus, forming a polyp-like mass protruding into the lumen with a smooth border and a diameter not exceeding 2.5 cm. Pathologically, there are fewer tumor cells, and the size, morphology and arrangement are more consistent, and there is no nuclear schizophrenia or necrosis; ② Kulchitsky cell type II (KCC-II), i.e. atypical carcinoid (AC). AC is more aggressive and is often associated with lymph node metastasis. The present study found that AC is more prone to lymph node metastasis, which is consistent with its poor prognosis. Pathologically, the disease is characterized by dense, irregularly arranged, nested tumor cells with increased nuclear anisotropy, schizotypy and necrotic areas, and small neuroendocrine granules. It is generally believed that the disease develops equally in both sexes, and some literature reports that pulmonary carcinoid tumors mainly occur in adults aged 40-50 years with a median age of 47 years, which is earlier than lung cancer and rare in children and adolescents. Bronchopulmonary carcinoid tumor has no obvious clinical specificity and depends mainly on the location and type of lesion. TC is mainly of central type, and common respiratory symptoms such as cough, sputum and blood in sputum may appear in the early stage, while AC is of peripheral type, and there may be no symptoms in the early stage, and the two patients with peripheral type in this study were found accidentally during physical examination. (2) CT manifestation Carcinoid tumors mainly appear on CT as smooth, sharp round nodular lesions, which may be lobulated, and the lesions may be completely located in the bronchial lumen. Because the tumor mainly originates from the base of the bronchial epithelium or the deep layer of the bronchial wall, the surface covering mucosa is mostly intact, therefore, the CT presentation lacks the small burrs or infiltrations that are often found around lung cancer tumors. In this group of cases, six nodules had neat margins, and only one case had rough margins, which was basically consistent with the main CT manifestations reported in the literature. In terms of tumor size, the average diameter of the tumors measured in this study was 1.8 cm-5.6 cm, and the average diameter of the central type was 2.5 cm, while the maximum diameter of the peripheral type was about 5.6 cm, which was close to the tumor diameter of 3.6 cm (central type)-4.3 cm (peripheral type) reported in the literature. Bronchopulmonary carcinoid tumors have a more homogeneous internal density, and necrosis and cystic changes are rare. However, according to the literature, the incidence of calcification in bronchopulmonary carcinoid tumors was higher, about 30%, and the rate of TC calcification was higher than that of AC, while only one case of calcification was found in this study, accounting for about 14.3%, which was lower than that reported in the literature, probably due to the small sample size of this study. In addition, the CT enhancement scans of this group showed that 6 lesions were significantly enhanced after enhancement, and the CT values increased by more than 40 Hu, which is related to the rich blood supply of carcinoid tumors and the blood supply from bronchial arteries, thus suggesting that the CT enhancement scans of bronchopulmonary carcinoid tumors have certain characteristics. (3) Differential diagnosis Typical carcinoid tumor TC is mostly of central type and its clinical and CT manifestations are very similar to those of central type lung cancer, so the differential diagnosis is quite difficult; TC usually progresses slowly, with smooth and clear borders, uniform density, uniform enhancement, few lymph nodes enlargement, and the onset is not related to smoking; the mass is mostly located in the lumen of the affected tracheobronchial tubes, and the walls are mostly soft. It has been suggested that it causes a smooth and limited lesion in the bronchial lumen, which forms the so-called "iceberg sign" with large irregular lesions outside the lumen, and can be used to distinguish between central carcinoid tumors and central lung cancer; whereas central lung cancer masses progress rapidly, with rough margins, lobar signs and eccentric cavities, and often have enlarged hilar and mediastinal lymph nodes, and their onset is mostly related to smoking. The lesions often show infiltrative growth in the bronchi, causing irregular thickening of the bronchial walls, luminal narrowing or even occlusion, and distal formation of obstructive pneumonia, and the lesions often show inhomogeneous intensification on enhanced scans, with non-enhancing necrotic areas visible within them. Atypical carcinoid tumor (AC) should be differentiated from peripheral lung cancer, tuberculosis spheres and metastases. Patients with tuberculosis spheroid have characteristic clinical manifestations and are more likely to be under 30 years of age, with CT manifestations mostly located in the upper lobe of the lung, with uneven density, calcified foci or translucent areas, and scattered "satellite foci" around them. "The lesions are usually multiple, often located in the lower and middle lungs, with a cotton ball-like appearance, uniform margins and density, and varying sizes. In addition, this disease should be differentiated from spherical pneumonia, inflammatory pseudotumor and malignant tumor. Primary bronchopulmonary carcinoid tumors still lack distinctive features in terms of clinical symptoms and CT manifestations, and have many similarities with other occupying lesions in the lung.