ESR News June 2015

Radiation doses and risks associated with lung cancer CT screening and colorectal cancer CT screening

John Damilakis, Professor of Medical Physics

Cancer has an enormous impact on mortality worldwide. Lung cancer is the most common cause of cancer death, followed by liver, stomach, colorectal and breast cancer (1). It is crucially important to develop cost-effective methods to identify individuals with abnormalities suggestive of a specific cancer before there are any symptoms. Many studies have examined the potential of CT for lung cancer screening and colorectal cancer screening. Are radiation doses and radiogenic risks associated with the above screening methods significant obstacles to their acceptance as mass screening tools?

The National Lung Screening Trial (NLST) is a US study comparing low-dose CT with chest radiography in the screening of smokers for early detection of lung cancer (2). This study has found that screening with low-dose CT could reduce lung cancer mortality by 20% in comparison with chest radiography. The NLST recommends CT screening for current or former smokers (former smokers having quit smoking within the past 15 years) aged 55 to 74 with a smoking history of at least 30 pack-years (1 pack/day for 30 years, 2 packs per day for 15 years etc) and no history of lung cancer. The decision to screen with CT should be based on careful consideration and discussion between patient and healthcare provider (3). It should be stressed that low dose CT screening for lung cancer has its risks. Individuals screened with low dose CT have a high rate of false positive results (3) that may lead to unnecessary follow-up invasive testing such as bronchoscopy or biopsy. Another point of concern is the radiation dose from low-dose CT. Patient dose from a low-dose chest CT examination is about 1.5 mSv. The International Committee on Radiation protection (ICRP) and the Committee on the Biological Effects of lonizing Radiation (BEIR) of the US National Research Council have provided estimates of cancer mortality risks per unit dose. I cannot give a conclusive answer to the question expressed in the first paragraph of this article, due to the absence of data in the literature regarding the risk of radiation-induced cancer following low-dose CT for lung cancer screening. Studies are needed to determine the doses absorbed by the primarily irradiated radiosensitive organs of female and male patients who have undergone low-dose CT screening for lung cancer on modern multi-slice CT scanners, in order to assess the potential risk of radiation-induced cancer for a typical patient cohort subjected to screening.

CT colonography (CTC) has been used as a method of screening for colorectal tumours, as well as for large colorectal polyps, mainly because of its accuracy, its low invasiveness in comparison with colonoscopy, and the potential for detecting extra-colonic abnormalities. The effective dose from CTC ranges from about 3 mSv to about 9 mSv. The radiation dose depends on the CT scanner technology. A recent study (4) shows that, in comparison with 64-slice CT technology, 256-slice CT systems deliver about 40% less radiation. A single low-dose CTC performed with a 256-slice CT scanner would result in a 0.01% lifetime cancer risk (i.e. 1 in 10,000 for a typical patient cohort). The risk of radiation induced cancer from CTC studies every five years between the ages of 50 and 80 is small, i.e. 0.074% and 0.071% in females and males respectively. Another study on radiation-related cancer risks from CTC screening (5) shows that the benefits from CTC screening every five years from 50 to 80 years outweigh the radiation risks.

In conclusion, CT is an important tool for lung cancer screening and colorectal cancer screening in high-risk patients. Further studies are needed, especially for lung cancer screening, to explore radiation risks and demonstrate conclusively that the benefits of CT outweigh the radiogenic risks.

 

References

  1. World Health Organization, Cancer, Fact sheet No 297, Updated February 2015, https://www.who.int/mediacentre/factsheets/fs297/en/
  2. Aberle DR, Adams AM, Berg CD, Black WC, Clapp JD, Fagerstrom RM et al. National Lung Screening Trial Research Team. Reduced lung-cancer mortality with low-dose computed tomographic screening. N Engl J Med 2011;365:395-409
  3. Albert J. Radiation risk from CT: implications for cancer screening. AJR 2013;201:W81-W87
  4. Perisinakis K, Seimenis I, Tzedakis A, Papadakis AE, Kourinou KM, Damilakis J. Screening computed tomography colonography with 256-slice scanning: should patient radiation burden and associated cancer risk constitute a major concern? Invest Radiol 2012:47:451-6
  5. Berrington de Gonzalez A, Kim KP, Knudsen AB, Lansdorp-Vogelaar I, Rutter CM, Smith-Bindman R et al., Radiation-related cancer risks from CT colonoscopy screening: a risk-benefit analysis. AJR 2011;196:816-23