Lung cancer is a very malignant tumor, but early stage lung cancer can be cured by surgery or advanced radiotherapy techniques. However, recent advances in treatment have failed to bring down the mortality rate of lung cancer significantly, which may be related to the fact that most people are already at an advanced stage when lung cancer is detected. So, what means can be used to detect lung cancer earlier?
Chest radiographs do not detect early lung cancer
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A routine physical examination of the lungs is an x-ray (chest x-ray). Can a chest x-ray detect early lung cancer? The answer is no.
In 2011, the National Lung Cancer Screening Study Group (NLST) conducted the National Lung Cancer Screening Trial, which included 53,454 high-risk individuals at 33 large medical centers across the United States with positive screening results for lung nodules or other suspicious findings, comparing chest radiographs with low-dose spiral CT (LDCT).
The results showed that the chest radiograph had a positive rate of only 6.9% and a false positive rate of 94.5%. However, LDCT had a positive rate of 24.4% and a false positive rate of 96.4% (false positive rate is defined as the percentage of patients who are not actually sick but are found to be sick). The analysis found that chest radiography did not reduce mortality from lung cancer.
In 2013, the same study group again included 53,439 high-risk individuals from 33 large medical centers across the United States and defined high-risk as asymptomatic individuals aged 55 to 74 years who smoked at least 30 pack-years (packs per day x years of smoking). The panel still identified lung nodules or other suspicious findings as positive.
The positive chest radiograph rate was found to be only 9.2%, and further analysis of the positive results showed that the detection rate of lung cancer on chest radiograph was only 0.7% (190 lung cancers were diagnosed, including 70 in stage I and 112 in stages IIB-IV). The sensitivity and specificity of chest radiographs were 73.5% and 91.3%, respectively.
The NLST echoes the results of another large study, both confirming that chest radiograph screening does not reduce lung cancer deaths. In contrast, LDCT reduced lung cancer deaths by 20.0%, with a sensitivity of 93.8% and a specificity of 73.4%.
It is on the basis of this finding that the U.S. Preventive Services Task Force (USPSTF) officially recommends LDCT, rather than chest radiographs, for screening people at high risk for lung cancer.
Why doesn’t a chest radiograph detect early lung cancer?
The reasons why chest radiographs do not detect early lung cancer
There are several reasons why chest radiographs have difficulty detecting early-stage lung cancer.
1. Size of the tumor
Unlike LDCT, which can detect tumor lesions of 1 mm or more, chest radiographs have difficulty detecting masses of less than 1.5 cm.
2. Location of the tumor
Anatomically, certain locations of lung cancer are difficult to detect on chest radiographs. For example, a retrospective study found that 72% of lung cancers missed by chest radiographs were located in the upper lobe of the lung, 22% of which were obscured by the clavicle. The study also found that tumors in the periphery of the lung were more easily detected, while tumors near the heart, spine, and hilum were more difficult to detect.
3. Difficult to distinguish from pneumonia, tuberculosis, etc.
The reason why chest films allow the human body to be imaged on screen or film is based on the penetrating, fluorescent and photographic effects of X-rays on the one hand, and the difference in density and thickness of human tissue on the other. As a black-and-white image, a chest radiograph will look like a substance of similar density. Therefore, it is difficult to clearly distinguish between infection, inflammation, scarring, and new tumors from a chest radiograph.
What are the possible risks of using chest radiographs for screening?
The biggest problem with using a chest radiograph for lung cancer screening is the potential delay in treatment. The average time to double the number of cancer cells in the early stages of growth is 125 days (perhaps as little as 7 days), so if a chest radiograph is used, lung cancer is easily missed in its early stages, and by the time the tumor is large enough to be detected by X-ray, the patient is likely to have lost the chance of a surgical cure.
In conclusion, screening for lung cancer by chest X-ray is not reliable and LDCT should be used as the appropriate method.
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Co-reviewed by: Guangdong Provincial People’s Hospital Guangdong Provincial Lung Cancer Institute Dr. Wang Zhen, Associate Chief Physician Dr. Linlin Lai