Improving Radiation Dose from Diagnostic CT Examinations in Saskatchewan
Dumaine CS*, Leswick DA**, Fladeland DA**, Hyun JL***, Toews LJ****
*Medical Student, **Department of Radiology, ***Department of Community Health and Epidemiology, ****Chief CT technologist
College of Medicine, University of Saskatchewan, Royal University Hospital, University of Saskatchewan, College of Medicine, University of Saskatchewan, Royal University Hospital, Saskatoon Health Region,
Saskatoon, Saskatchewan, Canada Saskatoon, Saskatchewan, Canada Saskatoon, Saskatchewan, Canada Saskatoon, Saskatchewan, Canada
BACKGROUND RESULTS DISCUSSION
The potential risk associated with radiation from computed tomography (CT) scans is often When considering all scanners, mean effective doses for CT head, chest, and abdomen-pelvis were 3.4 mSv, 9.6 mSv, and 16.1 mSv respectively (Table 1). The significant differences from 2006 were a Comparison with previous studies outside SK is helpful. Data collection for the BC, UK, and German
underestimated by both physicians and patients.1 However, CT scans in Canada represent approximately 12% of 26% increase and 15% decrease in doses for scans of the head and chest, respectively. data was from 2004, 2003 and 2001, respectively.8-10 Although percentage of MDR scanners was similar to SK in
medical imaging procedures and account for 60-70% of medically produced radiation.2-4 It is also estimated that Since both of Saskatchewan’s SDR scanners have either been replaced or scheduled for replacement since completion of data collection, it is appropriate to analyze dose for MDR scanners as a group BC (85% vs 89%), the UK study contained only 37% MDR scanners.9 While the German study was comprised of
1.5 to 2% of cancers in the USA may now be attributable to CT.5 Given this current situation, an such an (Table 2). Significant changes were seen with increased dose from CT head exams (29% overall and 19% single phase), decreased dose for chest exams (26% overall and 31% for single phase) and decreased dose for solely MDR scanners, they were all 8DR or less.10 Overall CT head doses in Saskatchewan are now above those
underestimation of radiation dose and its effects has the potential for severe consequences.2,3,4 single phase AP scans (17%). The decrease in dose for single phase AP scans was offset by an increase for the multiphase scans, resulting in no significant overall difference. from other studies. Overall chest dose is now significantly less than that seen in 2006, and closer to the BC values.
Long term studies of 25,000 atomic bomb survivors (dose range 5 to 150 mSv, mean dose 40 mSv) and Variation in mean site specific chest and AP exam dose was decreased, illustrated for single phase studies (Figure 1). Note that mean both the dose differences between sites and standard deviations within 7-8 Overall AP dose remains similar to SK 2006 and BC 2004 values.7-8 Overall chest dose remains above those
400 000 nuclear industry workers (dose range 5 to 150 mSv, mean dose 20 mSv) showed significantly increased sites are smaller for the 2008 study (Figure 1 and Table 2). This decrease in variation is also shown by the change in the provincial dose histograms (Figure 2). from UK and Germany while AP doses are above those from UK and similar to Germany.9-10 However, caution
risk of cancer mortality in both groups.5 The 2006 BEIR VII report on Health Risks from Exposure to Low Levels 295 of patients received greater than one scan type during their CT visit (Highlights in Table 3). Highest combination doses were for patients receiving head, C-spine, chest, and abdomen-pelvis CT scans, must be used when comparing with the UK and German data because of differing scanner technology.
of Ionizing Radiation stated that the relationship between low dose ionizing radiation and the incidence of solid who were exposed to a mean dose of 33.8 ± 10.1 mSv (range 24.3 to 61.8 mSv). The greatest dose reductions seen in our study were for single phase exams of the chest and AP when
cancers and leukemia are the linear no-threshold and linear quadratic models, respectively.6 The BEIR VII excluding the SDR scanners from analysis. It is easier to see significant differences when analyzing single phase
models, atomic bomb survivor, and nuclear industry worker doses are all in the range of diagnostic CT scans. scans alone as the multiphase exams are a more heterogeneous collection of techniques. Differences in single
As of 2007, there were 419 scanners installed in Canada, 13 of which are used for diagnostic scans in TABLE 1 TABLE 2 TABLE 3 phase exams may also reflect changes to generalized scan protocols at institutions, and not the self-reference card.
Saskatchewan.2 A radiation dose survey of 1690 patient exams was carried out in Saskatchewan in 2006. This Unfortunately, head CT dose increased in 2008. In three higher volume sites, there had been a change in head CT
survey showed that provincial mean effective radiation doses for head, chest, and abdomen-pelvis (AP) CT scans Mean Dose (mSv) and % Difference from SK 2008 MDR Overall Mean Values MDR Single Phase Mean Values Combined Mean Dose ± protocol at the request of the reading radiologists at these sites for higher-quality images.
n Range (mSv)
were 2.7 mSv, 11.3 mSv, and 15.5 mSv, respectively.7 SK SK BC UK Germ. Europe %Change %Change Scans St. Dev. (mSv) The SK 2006 study revealed significant variability in average doses between sites.7 This was
While the Saskatchewan results were similar to other published studies and within the range of 2008 2006 2008 2006 highlighted considering dose to a theoretical patient receiving scans of the head, chest and AP. Although the
2008 20067 20068 20049 200310 200011 p-Value p-Value Chest & AP 136 22.1 ± 9.6 2.9 - 59.7
published guidelines, there was significant variation of effective doses amongst the 12 CT scanners involved in the provincial 2006 mean dose for this combination was 29.5 mSv, this ranged between 20.6 to 53.3 mSv at the
Head 3.4 2.7 2.8 1.5 2.8 2.4 Head 3.6 ± 1.6 2.8 ± 1.6 +29% 3.2 ± 1.2 2.7 ± 1.5 +19% Face-Neck,
study.7, 8-11 For our follow-up survey, we wished to see if Saskatchewan doses could be lowered and become better (-21%) (-18%) (-56%) (-18%) (-29%) 18 26.2 ± 12.8 7.8 - 55.6 various sites.7 For this theoretical patient, the 2008 mean dose would be 29.1 mSv, ranging between 22.1 and 41.0
(n=897) (n=534) (p<0.0001) (n=661) (n=406) (p<0.0001) Chest & AP
standardized across the province. mSv depending on the MDR scanner used, illustrating decreased dose variation between sites. Decreased
Chest 9.6 11.3 9 5.8 5.7 11.1 Chest 10.0 ± 4.7 13.6 ± 9.0 -26% 9.5 ± 3.9 13.7 ± 9.7 -31% Head, Chest & variation between and within sites is also shown by the decreasing standard deviation, especially within sites and
(+18%) (-6%) (-40%) (-41%) (+16%) (n=537) (n=284) (p<0.0001) (n=479) (n=200) (p<0.0001) 15 24.0 ± 8.3 13.4 - 45.1 for MDR provincial wide single phase chest and AP exams.
AP 16.1 15.5 16.5 7.1 14.4 no data There has been relatively little data published about doses for patients receiving multiple scans during
AP 16.7 ± 9.9 17.0 ± 10.3 -2% 13.9 ± 6.0 16.8 ± 10.6 -17% Head, Neck,
(-4%) (+2%) (-56%) (-11%) available (n=913) (n=484) (p=0.612) (n=749) (n=319) (p<0.0001)
20 33.8 ± 10.1 24.3 - 61.8 the same visit to the CT department. The 20 patients receiving scans of the head, neck, chest, and AP at a single
Chest & AP
visit received an average of 33.8 mSv effective dose. These multi-body part patients can be thought of as similar
METHODS to people undergoing full body CT screening. Evaluation of a rather conservative full body screening protocol
Before commencing this project, ethics approval was obtained from the University of Saskatchewan TABLE 1: Overall mean effective dose and percent difference from the SK TABLE 2: Overall and single phase mean effective dose ± SD and number of scans (n) for TABLE 3: Highlights of total doses for patients scanning from neck (C3) to the symphysis pubis revealed an effective dose of 12 mSv, equating to between 0.13%
Research Ethics Board. All of the Saskatchewan hospitals with CT scanners used for diagnostic exams 2008 Study. Dates for SK studies reflect data collection with other dates MDR scanners only. Note the 19% increase in single phase head dose as well as the 31% and receiving multiple scans at the same visit. Data from all and 0.03% (for 25 and 75 year old people respectively) increased risk cancer mortality from a single scan.13 Since
participated in this survey; this group of CT scanners represents a variety of CT scanner brands and detector row indicating publication year. 17% respective dose reduction for single phase cheat and AP scans. scan combinations not shown in the interest of brevity. these doses (and those from our survey) are within the range of the data from atomic bomb survivors, the
numbers. calculation of cancer risk is not an extrapolation of higher dose data.13
Materials sent to each site included data collection sheets and instructions, a summary of the 2006 There are limitations to our study. As with any dose survey, this remains a sample of current practice
results, a copy of “Computed Tomography: An Increasing Source of Radiation Exposure” by Brenner et al., and a and not optimal practice.7-8 Because of the promise of anonymity, we are again unable to link doses to scanner
site-specific reference card. This reference card displayed the provincial and site-specific mean Dose Length design, including number of detector rows. This promise of anonymity also prevents us from publicly
Product (DLP) from 2006. It also displayed the DLP value that was 25% greater than the site’s 2006 DLP, for congratulating the sites with greatest improvement (Site 8). We are also uncertain as to the reasons behind
head, chest, and AP scans. The DLP of a CT scan is an indicator of radiation dose and is displayed on the CT Saskatchewan’s CT dose changes. Lastly, although the differences were significant on a provincial basis, some
console. The intention of these reference cards was to allow CT technologists to conduct a pre-scan comparison sites contributed more than others to these changes.
between the estimated DLP of the scan they were about to perform and their mean DLP from 2006. In this way,
we hoped that technologists would recognize scans utilizing higher than usual amounts of radiation, and adjust the
parameters of the scan if it was appropriate to do so.
Data was collected for a two week period in July, 2008, by CT technologists at each location. The data
collected included the body part scanned, the number of phases for the scan, the DLP for each phase, and the total CONCLUSION
DLP. The technologists were asked to record this information as the scan was planned. Using the reference cards,
they were also asked to compare the DLP of the scan they were planning to their site’s 2006 DLP and record Significant dose and variation reduction was seen for single phase CT chest and AP exams between
whether the DLP was lower, similar to, or greater than 25% higher. If they recorded that their DLP value was 2006 and 2008 while CT head dose increased over the same interval. Although these chest and AP dose reductions
greater than 25% higher, they were asked to rationalize this high dose. A list of reasons for which CT scans are are important for patient safety, cautious optimism is encouraged as doses (particularly for multiple scan patients)
known to have high radiation dose was supplied, and included: tall patient, overweight patient, multiple phase remain at levels with a known increased risk of malignancy.
scan, cardiac scan, and vascular (angiographic) scan. Technologists could also fill in their own answer if this list
did not encompass their reason for utilizing a high radiation dose. FIGURE 1: Mean dose for the entire province and at each site for single phase scans of the Head (1a), Chest (1b) and AP (1c). X-axis legend as follows: Scanner #(n 2006, n 2008). Scanners 1 & 2 are SDR scanners while 3-12 are MDR (scanner # unrelated to #
of detector-rows). There was no single phase data for scanners 12 or 13 in 2006. Note an increase in CT head dose for 4 sites, and decrease in chest and AP dose several sites, most impressive at site 8. Variation within sites is smaller, as shown by decreasing error
To calculate effective radiation dose, the DLP of each scan was multiplied by a conversion factor based bars at several sites. Variation between sites is also less in 2008.
on body part as follows: 0.0023 mSv/mGy·cm for head and sinus CT scans, 0.015 mSv/mGy·cm for abdomen CT
scans, 0.019 mSv/mGy·cm for pelvis CT scans, 0.017 mSv/mGy·cm for chest and combined abdomen-pelvis (AP) ACKNOWLEDGEMENTS
CT scans, and 0.0054 mSv/mGy·cm for C-spine, neck, and facial bone CT scans.7-8, 12 We would like to sincerely thank all of the participating CT technologists across the province for their
Mean effective radiation doses and standard deviations were calculated for each exam type, both time, effort, and valuable input. Without their help and dedication, this project would not have been possible.
provincially and for each individual site. Provincial mean effective radiation doses and standard deviations were We would also like to extend our thanks to the University of Saskatchewan College of Medicine Dean’s
also calculated for both single-phase and multiple phase head, chest, and AP exams on single detector row (SDR) Project Summer Research Program and the UofS/SDH continuing research fund for financial support.
CT scanners and multi-detector row (MDR) CT scanners. Comparison of all values was made to their 2006
Analysis was also done to calculate the total effective radiation dose received by patients undergoing
CT scans of two or more different body parts during a single visit. For each combination of body parts, the mean,
range, and standard deviation was calculated. The body parts assessed in this analysis were head (including facial REFERENCES
bones, sinuses, and CTA COW), neck (including CTA carotid), chest, and abdomen/pelvis. 1. Lee Cl, Haims AH, Monico EP, et al. Diagnostic CT scans: assessment of patient, physician, and radiologist awareness of radiation dose and possible risks. Radiol. 2004; 231: 393-398.
2. Canadian Institute for Health Information. Medical Imaging in Canada, 2007. (Ottawa, Ont.: CIHI 2008.)
Upon completion of data analysis, radiation dose data was disseminated back to the participating sites, 3. Aldrich JE, Williams J. Change in patient doses from radiological examinations at the Vancouver General Hospital, 1991-2002. Can Assoc Radol J. 2005; 56(2): 94-99.
which were made aware of their own dose data while being blinded to the identities of the other hospitals. 4.
Mettler FA Jr, Wiest PW, Locken JA, Kelsey CA. CT scanning: patterns of use and dose. J Radiol Prot. 2000; 20: 353-359.
Brenner DJ, Hall EJ. Computed Tomography: An Increasing Source of Radiation Exposure. New England Journal of Medicine 2007; 357: 2277-84.
Any details linking specific detector row numbers and site names to radiation dose levels have been omitted from 6. National Research Council (U.S.). Committee to Assess Health Risks from Exposure to Low Level of Ionizing Radiation. Health risks from exposure to low levels of ionizing radiation : BEIR VII
Phase 2. Washington, D.C.: National Academies Press, 2006.
external data presentation and publication. Because of the small number of CT scanners involved in this study, 7. Leswick DA, Syed NS, Dumaine CS, Lim HJ, Fladeland DA. Radiation Dose from Diagnostic Computed Tomography in Saskatchewan. Can Assoc Radiol J; [in press].
8. Aldrich JE, Bilawich AM, Mayo JR. Radiation doses to patients receiving computed tomography examinations in British Columbia. Can Assoc Radiol J. 2006; 57(2): 79-85.
divulging this information makes it virtually impossible to uphold the promise of site anonymity, which was a key 9. Shrimpton PC, Hillier MC, Lewis MA, Dunn M. Doses from computed tomography examinations in the UK—2003 review. Report NRPB-W67. Chilton (UK): NRPB; 2004.
10. Brix G, Nagel HD, Stamm G, et al. Radiation exposure in multislice CT versus single slice spiral CT: results of a nationwide survey. European Radiol. 2003; 13(8):1979–1991.
factor in the recruitment process. 11. European Commission. European guidelines on quality criteria for computer tomography. EUR 16262 EN. Luxembourg: Office for Official Publication of the European Communities; 2000.
FIGURE 2: Distribution of effective doses for head (2a) chest (2b) and AP (2c) scans for all Saskatchewan patients. 2008 histogram shown in blue with 2006 inset in purple. The shape of the CT head histogram has slightly widened, reflecting increased dose at 12.
Huda W, Ogden KM, Khorasani MR. Converting dose-length product to effective dose at CT. Radiology 2008; 248:995-1003.
Brenner DJ, Elliston CD. Estimated Radiation Risks Potentially Associated with Full-Body CT Screening. Radiology. 2004: 232: 735-738
some sites. Although chest and AP range is wide, much of this is because of a few outliers. Tightening of the 2008 histogram for both chest and AP scans indicates decreased variation and a more normal distribution of doses.