I. Imaging examination methods and selection
1.Chest X-ray: Because of its wide popularity, simplicity and low cost, chest X-ray is still the preferred imaging screening method for suspected lung cancer, but the sensitivity and accuracy of chest X-ray for lung cancer detection are lower than CT scan. However, chest X-ray is less sensitive and less accurate than CT scan in detecting lung cancer.
2.CT: CT has become the main and most commonly used method for early detection, diagnosis and differentiation, staging, efficacy evaluation and lifelong follow-up of lung cancer. The application of low-dose spiral CT for lung cancer screening in high-risk groups can improve the early detection rate and surgical cure rate of lung cancer, and its false-positive rate and cost effectiveness have not yet been determined, which is the focus of clinical research debates at home and abroad. Various computer-aided detection and diagnostic analysis software (CAD) for small lung nodules are also maturing and gradually put into clinical application. For example, for lung nodules that are difficult to characterize, CT can be reviewed after 1-3 months to calculate the doubling time and help determine the benignity and malignancy. For peripheral lung nodules and diffuse lung lesions (such as suspected lymphatic metastases), attention should be paid to high-resolution CT scanning (HRCT); for central lung cancer, enhanced scanning should be performed, and 2-dimensional or 3-dimensional post-processing techniques such as multiplanar reconstruction (MPR) should be applied as much as possible to judge the relationship between the tumor and surrounding structures to help judge the resectability of surgery and formulate treatment plans.
3.MRI: It is a supplementary means to CT scan. MRI may be preferred for patients with supraglottic sulcus tumor, lung cancer with close relationship with chest wall and diaphragm, and patients with iodine contrast allergy but want to show the relationship between the lesion and large blood vessels in the hilum and mediastinum; MRI may be superior to CT for differential diagnosis of some lung masses (such as silicosis nodules), fibrosis and tumor recurrence after more than 1 year of radiotherapy. MRI is the preferred method for suspicion or exclusion of central nervous system metastasis; for focal bone metastasis, MRI may be helpful for diagnosis when X-ray, CT and bone scan cannot be characterized.
4.Positron emission tomography (PET) and PET-CT: Currently, F-18FDG whole-body imaging is mainly used in clinical practice, which has high specificity and accuracy for lung cancer diagnosis, more comprehensive and accurate staging, and is also of great value for lung cancer efficacy observation and early detection of residual and recurrent tumors after treatment. The weaknesses of PET or PET-CT include: there are still some false positives and false negatives, small lesions (less than 1 cm) are easily missed, not sensitive enough to central nervous system metastasis, the anatomical details provided are not as good as CT scan, more expensive, and the equipment is not popular.
5.Image-guided lung biopsy and treatment: fluoroscopy, CT or ultrasound-guided puncture lung biopsy and physical therapy (microwave, freezing, etc.) can be chosen according to the size and location of the lesion.
II. Imaging performance
Usually, lung cancer is divided into central type and peripheral type according to the site of occurrence. Supraglottic sulcus tumor occurs at the tip of the lung and has the tendency to directly invade the local chest wall, ribs, vertebrae, lower brachial plexus nerve, sympathetic chain and subclavian vessels. Less than 1% of cancers occur in the trachea.
Different histological subtypes of lung cancer have certain characteristics, for example, squamous carcinoma and small cell carcinoma are mostly of central type, while adenocarcinoma is mostly of peripheral type.
1.Central type lung cancer
The imaging manifestation of central lung cancer includes direct and indirect signs of the primary tumor.
The direct signs are nodules in the lumen of the bronchus above the segment or segmental bronchus, limited wall thickening or masses growing inside or outside the lumen; the secondary signs mainly refer to obstructive pulmonary changes distal to the tumor.
CT is significantly better than plain film for detecting small nodules in the bronchial lumen or limited bronchial wall thickening, distinguishing tumors from distal obstructive changes, and detecting metastatic lesions. The post-processing functions such as thin-layer reconstruction and MPR make CT even more superior.
2.Peripheral type lung cancer
Lung cancer that occurs far from the segmental bronchus is called peripheral type. The size, internal structure and tumor-lung interface of the lesion are very important for the differential diagnosis, for example, there are often small vacuoles inside the adenocarcinoma, with umbilical-like cut marks or deep lobulation, burr and pleural traction at the edges. PET or PET-CT can directly respond to the metabolic status of the tumor and is very accurate in the differential diagnosis of benign and malignant lung nodules larger than 1 cm, but there are still some false positives (e.g. infectious lung lesions) and false negatives of adenocarcinoma nodules. ) and false-negative adenocarcinoma nodules (e.g., alveolar carcinoma, carcinoid tumor, etc.). For lung nodules smaller than 1 cm, CT scans can be reviewed after 1-3 months and the doubling time of the nodule can be calculated using volumetric measurement and analysis software. The doubling time is the time required to double the volume of the lesion, and when the diameter of the mass doubles, the volume will increase 8 times, so the change in volume is more sensitive to the tumor growth than the diameter. When the tumor is combined with bleeding, infection or necrosis, the volume may increase dramatically and the doubling time may be less than 30 days, while the more common exceptions are some slow-growing alveolar or adenocarcinoma, which may remain unchanged for several years and the doubling time is more than 490 days or up to more than 10 years. or follow up closely on a regular basis.
3.Apical lung cancer
In the early stage, apical lung cancer only appears as thickening of the apical cap on one side of the lung on chest X-ray, which is easy to be missed or misdiagnosed. For patients with complaints of shoulder pain, chest and back pain, brachial plexus nerve damage or Horner’s syndrome, attention should be paid to observing the apical lung cap and local ribs, and even if the chest X-ray is negative, CT or MRI should be performed for further examination.
MRI can observe the anatomical details of the thoracic inlet and brachial plexus well, and is better than CT in determining the extent of tumor invasion and intraspinal invasion, and CT is better than MRI in determining the invasion of bone cortex.
4.Metastasis signs
The most common signs of metastasis include hilar, mediastinal and supraclavicular lymph nodes, intrapulmonary metastasis, pleural metastasis, pleural fluid, pericardial effusion, bone metastasis and adrenal metastasis.
The lymph node enlargement is usually in the area where the primary site drains, mostly in the ipsilateral hilum and mediastinum, but also in the contralateral mediastinum or hilum lymph nodes via the inferior and anterior mediastinum. Sometimes subserosal or posterior mediastinal lymph node metastasis can compress the esophagus and cause dysphagia, which is mostly seen in small cell lung cancer.
Intrapulmonary metastasis in the same lobe as the primary lobe is T4, which does not affect the resectability of surgery; intrapulmonary metastasis in different lobes is M1, and in principle, no further surgery should be performed. Generally, intrapulmonary metastases are mostly distributed in the area near the pleura, and the lower lung fields are more than the upper lung fields, which are mostly sphere-like with clear borders and no calcification or fatty density. Not all intrapulmonary nodules are pulmonary metastases, especially when the number is small and the morphology is strange, local HRCT should be performed for careful observation or PET for further examination, and if it is still uncertain, image-guided biopsy is feasible to facilitate the development of the correct treatment plan.
The direct signs of pleural metastasis are pleural nodules, irregular pleural thickening and enhancement, and the indirect signs are pleural effusion. Pleural metastasis is not necessarily present when pleural fluid alone appears, and it depends on repeated pleural cytology or pleural biopsy to determine whether it is malignant pleural fluid, and a small amount of reactive pleural fluid may appear in central lung cancer with obstructive pneumonia, cardiogenic pleural fluid may appear in patients with cardiac insufficiency, and a small amount of pleural fluid may appear in patients with history of dust inhalation. Even if a small amount of pleural fluid is combined with lung cancer, patients should be highly alert to the presence of pleural metastasis, and if the presence of pleural metastasis is confirmed, surgery is not recommended.
Pericardial effusion is rarely seen in the initial diagnosis of lung cancer, and it cannot be characterized in small amounts. A small amount of pericardial effusion is often seen after radiotherapy.
For each patient with a suspected lung cancer diagnosis, attention should be paid to the bony thorax during chest radiographs, and CT should always observe each image with a bone window as a rule to avoid missing bone metastases.
The scope of regular chest CT scan often requires including the posterior rib diaphragm angle area, and the bilateral adrenal glands are usually within the scan scope, and for small cell carcinoma, it is even more important to include the adrenal glands.