Rationale and Objectives
Radiation dose is an important drawback of computed tomography (CT) colonography, especially for its use as a screening tool for colorectal cancer. It is therefore important to know the present radiation dose. Our objective is to assess the effective radiation doses used for CT colonography and its trend over time.
Materials and Methods
Institutions performing CT colonography research were asked to provide their CT colonography protocols. Median effective doses were calculated and compared with a 2007 inventory. Separate analyses were performed for protocols using intravenous contrast medium and for academic versus nonacademic institutions. Differences in effective dose were tested for significance, using Wilcoxon rank-sum or Wilcoxon signed-rank test.
Results
Sixty-two of 109 (57%) institutions responded, providing protocols for 58 institutions. Median effective dose for daily practice protocols was 7.6 mSv (4.3 mSv and 2.0 mSv for supine and prone, respectively) and for screening 4.4 mSv (2.6 mSv and 2.0 mSv, respectively; P = .01). For daily practice with and without contrast medium, the median effective doses were 10.5 mSv and 4.0 mSv ( P < .001), respectively. Academic and nonacademic institutions used similar doses (all comparisons P > .05). For institutions also participating in the 2007 inventory, effective dose for both daily practice and screening protocols were similar ( P > .05).
Conclusion
In 2011 the median effective radiation dose for daily practice protocols was 7.6 mSv and for screening 4.4 mSv. Median effective doses have not decreased as compared to 2007. Academic and nonacademic institutions use similar radiation dose.
Computed tomography (CT) colonography is an accurate diagnostic test for the detection of colorectal polyps and cancer . In clinical practice this structural radiological examination is used as an alternative for colonoscopy and when colonoscopy is incomplete. CT colonography is also an accepted method for colorectal cancer screening . However, the US Preventive Services Task Force did not recommend CT colonography for colorectal cancer screening , among other reasons, because of the radiation exposure and its concurrent risks. Although the benefits of CT colonography in screening greatly outweigh the possible risks of CT colonography, there are important reasons to have the lowest possible dose . A reduced dose and subsequent improved benefit-risk ratio will improve the acceptance of the technique amongst professionals and screening participants.
In 2007 a questionnaire was performed to determine the median effective radiation dose for screening and daily practice CT colonography protocols among research institutions. The median effective radiation dose was 5.7 mSv and 9.1 mSv, respectively . Several studies have shown that the radiation dose for CT colonography can be reduced to 1 mSv per acquisition (ie, 2 mSv for an examination consisting of a prone and supine scan, with excellent diagnostic performance for polyp detection) . The effective radiation dose for screening as reported by the participating institutions could therefore approximately be lowered by a factor of three. This might substantially improve the benefit-risk ratio of this technique as the dose is linear related to the risk of cancer induction, which is especially important for CT colonography for screening.
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Material and methods
Selection Procedure
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Table 1
The Questionnaire Sent to CT Colonography Institutions
A. CT Scanner Manufacturer and Type Number of detector rows (1, 2, 4, 8, 16, 40, 48, 64, or 128) Slice width (mm) Tube voltage (kV) Rotation time (s) Pitch (table feed per rotation/total collimation)
B. Only for Scans without Automatic Current Selection or Tube Current Modulation Tube current (mA) or Tube current × rotation time (mAs) or Tube current × rotation time/pitch (effective mAs)
C. Only for Scans with Automatic Current Selection and/or Tube Current Modulation (for a Male Patient of ±170 cm and ±70 kg) Actual length (cm) Actual weight (kg) Preset or reference mAs (if available) Realized CTDIvol (mGy) Realized DLP (mGy ∗ cm) Realized average mAs Length of scan or scans (cm) Use of X/Y modulation? (Y/N) Use of Z-modulation? (Y/N)
CT, computed tomography; CTDIvol, CT dose index; DLP, dose length product.
Institutions were asked to fill out this questionnaire for all available protocols (daily practice with or without intravenous contrast medium, and screening, whatever applicable. Parameters were requested for prone and supine position separately).
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Effective Dose Calculations
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Statistical Analysis
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Results
Protocols
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Current Effective Doses
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Table 2
Median Effective Dose Values and Computed Tomography Parameters per Protocol per Number of Detector Rows
A. Screening ProtocolsDetector rows128643216 Number of protocols 1 13 6 3 Tube voltage (kV) 80 120 120 120 Rotation time (s) 0.5 0.5 0.5 0.5 Collimation (mm) 0.625 0.6 0.6 1.25 mAs (prone/supine) 50/50 36/44 29/54 50/50 Dose modulation ∗ 0 7 2 0 Effective dose (mSv) 1.3 4.4 4.4 5.8 IQR (mSv) - 2.5–7.6 3.4–8.8 3.4–7.3
B. Daily Practice Protocols (with and without Intravenous Contrast Medium Combined)Detector rows128643220164 Number of protocols 7 27 13 2 12 1 Tube voltage (kV) 120 120 120 120 120 120 Rotation time (s) 0.5 0.5 0.5 0.5 0.5 0.5 Collimation (mm) 0.625 0.625 0.6 0.6 1.25 2.5 mAs (prone/supine) 50/50 37/55 44/88 38/76 30/64 20/165 Dose modulation ∗ 3 20 7 1 5 0 Effective dose (mSv) 3.2 7.0 7.8 6.2 7.8 11.4 IQR (mSv) 2.3–11.0 3.6–10.5 4.6–11.6 4.4–8.1 4.9–12.3 -
C. Daily Practice with Intravenous Contrast MediumDetector rows128643220164 Number of protocols 2 11 8 1 7 1 Tube voltage (kV) 120 120 120 120 120 120 Rotation time (s) 0.5 0.5 0.5 0.5 0.5 0.5 Collimation (mm) 0.625 0.625 0.6 0.6 1.25 2.5 mAs (prone/supine) 123/95 40/141 46/155 47/102 50/78 20/165 Dose modulation ∗ 2 10 5 1 4 0 Effective dose (mSv) 10.9 10.2 10.7 8.1 10.2 11.4 IQR (mSv) 10.9–11.0 8.0–17.0 8.0–12.5 - 8.3–14.3 -
D. Daily Practice without Intravenous Contrast MediumDetector rows12864322016 Number of protocols 5 16 5 1 5 Tube voltage (kV) 120 120 120 120 120 Rotation time (s) 0.5 0.5 0.5 0.5 0.5 Collimation (mm) 0.625 0.625 0.6 0.6 1.25 mAs (prone/supine) 30/30 37/49 44/50 30/50 26/55 Dose modulation ∗ 1 10 2 0 1 Effective dose (mSv) 2.6 3.7 4.3 4.4 4.9 IQR (mSv) 1.8–7.1 3.0–6.6 3.0–6.2 - 4.0–6.2
IQR, the interquartile range for the effective dose.
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Geographic Variation
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Academic vs. Nonacademic
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Comparison with 2007
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Other Parameters
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Discussion
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Acknowledgments
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