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Written By: Editorial Team | Updated : August 1, 2022 1:22 PM IST
How Smoking Increases Lung Cancer Risk?
Lung cancer is one of the most common and aggressive forms of malignancy with varying clinical manifestations. Globally, lung cancer contributes to 13 per cent of all new cancer cases and 19 per cent of all cancer-related deaths, whilst in India, it accounts for approximately 6.9 per cent of all new cancer cases and 9.3 per cent of all cancer-related deaths. Though 10 to 15 per cent of lung cancers occur in nonsmokers, the majority of them occur in smokers and the incidence and severity of the disease increase with the increase in the burden of smoking.
Current data estimate that almost 70 per cent of lung cancer patients present with either locally advanced or metastatic disease. This results in poorer quality of life and decreases overall survival. The 5-year survival rates of such advanced cancers are only 15 per cent. However if detected early, the 5-year survival can be improved to as high as 57 per cent.
Since early recognition and appropriate treatment of lung cancers are associated with improved quality of life and better survival, screening of high-risk populations like chronic smokers or those with high-risk occupational exposures or family history of cancers with low-dose CT scan was attempted in clinical trials and has been proven to reduce mortality from lung cancer in such high-risk populations. The radiation dose with Low dose CT (LDCT) was only 1.5 mSv in clinical studies in contrast to the conventional diagnostic chest CT, where it is estimated to be approximately 8 mSv.
The famous National Lung Screening Trial (NLST) enrolled 53,454 current or former heavy smokers aged between 55 to 74 years. Participants were required to have a smoking history of at least 30 pack-years and were either current or former smokers without signs, symptoms, or history of lung cancer. The participants were randomly assigned to receive three annual screens with either low-dose helical CT or standard chest X-ray. The study observed that participants who received low-dose helical CT scans had a 15 to 20 per cent lower risk of dying from lung cancer than participants who received standard chest X-rays. This also translated to approximately three fewer deaths per 1,000 people screened in the CT group compared to the chest X-ray group over a period of about 7 years of observation.
Earlier studies done on the use of LDCT in TB endemic regions suggested limitations on the use of this technology due to false-positive results and lower specificity for cancer detection. This was due to the artifacts which mimic lung cancer amongst patients with post tuberculosis sequelae. In addition, other benign lung nodules also resulted in higher false positive rates in these TB endemic regions, discouraging the use of the technology in such high TB endemic regions. On the contrary, in recent times, few studies done on the Indian population with high-risk factors favored their utility in detecting early cancers despite these limitations. These results are encouraging as they may lead to earlier diagnosis and appropriate management of lung cancer nodules.
Another utility of LDCT is that it can incidentally pick up other smoking-related lung diseases like chronic obstructive lung disease, interstitial lung disease, coronary artery disease, infections, and other disorders related to the chest which can then be adequately treated to prevent morbidity and improve quality of life.
Despite the evidence, a few limitations, which are associated with large-scale implementation of this screening include radiation and cost. Though radiation is a potential risk, the use of such a low-dose protocol can mitigate this risk and can be safely adopted. With more strong evidence emerging, in the future we may start expanding screening with LDCT to facilitate early detection and appropriate management.
(The article is contributed by Dr. Viswesvaran Balasubramanian, Consultant Interventional Pulmonology and Sleep Medicine, Yashoda Hospitals Hyderabad)
