Article

Impact of Reduced Patient Life Expectancy on Potential Cancer Risks from Radiologic Imaging

Center for Radiological Research and Department of Medicine and Radiology, College of Physicians and Surgeons, Columbia University Medical Center, New York, NY 10032, USA.
Radiology (Impact Factor: 6.87). 07/2011; 261(1):193-8. DOI: 10.1148/radiol.11102452
Source: PubMed

ABSTRACT

To quantify the effect of reduced life expectancy on cancer risk by comparing estimated lifetime risks of lung cancer attributable to radiation from commonly used computed tomographic (CT) examinations in patients with and those without cancer or cardiac disease.
With the use of clinically determined life tables, reductions in radiation-attributable lung cancer risks were estimated for coronary CT angiographic examinations in patients with multivessel coronary artery disease who underwent coronary artery bypass graft (CABG) surgery and for surveillance CT examinations in patients treated for colon cancer. Statistical uncertainties were estimated for the risk ratios in patients who underwent CABG surgery and patients with colon cancer versus the general population.
Patients with decreased life expectancy had decreased radiation-associated cancer risks. For example, for a 70-year-old patient with colon cancer, the estimated reduction in lifetime radiation-associated lung cancer risk was approximately 92% for stage IV disease, versus 8% for stage 0 or I disease. For a patient who had been treated with CABG surgery, the estimated reduction in lifetime radiation-associated lung cancer risk was approximately 57% for a 55-year-old patient, versus 12% for a 75-year-old patient.
The importance of radiation exposure in determining optimal imaging usage is much reduced for patients with markedly reduced life expectancies: Imaging justification and optimization criteria for patients with substantially reduced life expectancies should not necessarily be the same as for those with normal life expectancies.

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    • "Computed tomography (CT) is an important imaging procedure, but the radiation dose required is relatively high compared to conventional radiographic procedures. Therefore radiation protection should be optimised to minimise risks of adverse health effects (Brenner and Hall 2007, Smith-Bindman et al 2009, Brenner et al 2011, Pearce et al 2012). It is essential that performance and output of CT scanners are tested routinely. "
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    ABSTRACT: Background: Studies have shown that the computed tomography dose index (CTDI) within a phantom is an inappropriate dose metric for cone beam computed tomography (CBCT). A number of practical approaches have been proposed to overcome the limitations of CTDI100. The IEC have proposed the use of sequential measurements with a 100 mm chamber to cover the extent of the beam, and this has been recommended by the IAEA and IPEM for application in UK hospitals for beams of width >40 mm. The method involves the application of a correction factor to the standard CTDI100, which is equal to the ratio of two CTDI measurements free in air, for the beam width of interest and a reference beam width. The efficiency that is estimated as the ratio of CTDI to CTDI is considered to be a good indicator of the ability of the CTDI to record a value representative of the radiation exposure. The aim of this project was to evaluate the efficiency of this approach for different arrangements dosimetry using Monte Carlo simulation. Methods: Monte Carlo (EGSnrc/BEAMnrc) and (EGSnrc/DOSXYZnrc) user codes were used to model the kV imaging system of Varian Truebeam linac. The Monte Carlo model was benchmarked against experimental measurements in standard PMMA head and body phantoms of diameters 16 and 32 cm. Three phantom lengths (150, 600, 900 mm) were used, where the later two lengths were considered to represent the infinitely long head and body phantoms, respectively. The efficiency was estimated as the ratio of CTDIIEC calculated based on the IEC approach to CTDI calculated within the infinitely long phantoms. Beam widths studied ranged from 20 mm to 300 mm. Four scanning protocols using two acquisition modes were simulated. Two reference beams of width 20 and 40 mm were compared. The efficiency values for the CTDIIEC,w were compared with those of the standard CTDIw. Results: The efficiency values for the standard CTDIw for the scanning protocols used in this project were approximately stable over the beam widths (20 – 80) mm, where the efficiency was 79±1.2% and 74.2±0.9% of the CTDI,w for the head and body phantoms, respectively. When the beam width increased beyond 80 mm, the efficiency fell steadily, reaching ~30% at a beam width 300 mm for both phantoms as the beams extended beyond the edges of the phantoms. However, the efficiency values for the CTDIIEC,w were approximately constant over all the beam widths, where the values for the head and body phantoms were 82.2±0.9% and 75.7±0.7% of the CTDI,w, respectively. The difference between using 20 mm and 40 mm as the reference beam width was insignificant. The efficiency values for CTDIIEC,w calculated with 20 mm for the head phantom was within 0.39% of those calculated with 40 mm, and within 0.06% for the body phantom. Discussion: The IEC approach has the advantage that the beam width plays no role in determining the efficiency. The efficiency values of the CTDIIEC,w for the head and body phantoms were approximately equal to the efficiency of the standard CTDIw used for the conventional CT scans with narrow beams reported by different investigators. Thus, the CTDIIEC successfully extends application of the CTDI approach to CBCT scans. However, as with the standard CTDI, the CTDIIEC,w efficiency is still underestimated the CTDI,w by ~18% to ~24%. The disadvantage of the approach for routine quality assurance is that the number of the scans that are required to assess the CTDIIEC,w for a scanning protocol using a 100 mm pencil ion chamber is relatively large. Conclusion: The efficiency of the CTDIIEC,w approach was found to be similar to that of the CTDIw for standard CT scanners and it was not affected significantly by the width of the reference beam. Although the results have shown that the difference between using 20 or 40 mm as the reference beam was insignificant, we recommend using 40 mm rather than 20 mm as using a narrow beam width with the kV-system of TrueBeam might lead to uncertainty due to accuracy of the blade collimation.
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    • "Fully diagnostic PET/CT studies may expose patients to radiation doses as high as 25 mSv. However, Brenner and Hall [170] have correctly pointed to the greatly reduced relevance of this perceived risk for patients with limited life expectancy [171]. Furthermore, a recent analysis concluded that “risks of medical imaging at effective doses below 50 mSv for single procedures or 100 mSv for multiple procedures over short time periods are too low to be detectable and may be nonexistent” and that “predictions of hypothetical cancer incidence and deaths in patient populations exposed to such low doses are highly speculative and should be discouraged” because they “are harmful … and may cause some patients and parents to refuse medical imaging procedures” [172]. "
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    • "[30] [31] [32] [33] [34] "
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