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Patient dose from CT: a literature review.

Article Excerpt
As the number of computed tomography (CT) procedures performed in the United States continues to increase, there is growing concern about patient protection issues. Currently, no system is in place to track a patient's lifetime cumulative dose from medical sources, and questions have arisen regarding the possible threat to public health from the widespread use of CT, especially in pediatric patients.

The authors reviewed the published literature to determine whether patients are receiving a higher absorbed dose of radiation and explored several proposed models to optimize the radiation dose delivered to patients and track cumulative lifetime dose. The literature review was performed using various key scholarly databases and search engines. In an effort to limit the search to the most recent literature, only articles that were peer reviewed and published from 2000 to 2006 were selected for review. References to secondary sources were avoided whenever possible. For comparison purposes and to estimate the relative risk increase for stochastic effects such as cancer, patient doses are reported as "effective doses," which are measured in sieverts (Sv) in the International System of Units. Table 1 summarizes the key findings of this review.

Trends in Patient Dose

Physicians have come to rely on sophisticated imaging techniques to render more accurate diagnoses, (1) and CT has become the first-choice modality for many diagnosticians. (2) Scan times have been reduced, resulting in increased patient throughput and enhanced image quality.

A recent study by Aldrich and Williams (1) quantified changes in numbers of radiology exams at Vancouver General Hospital from 1991 to 2002 and examined the correlation to the radiation dose received by the patient. In addition to a 4-fold increase in CT exams, they also found that the average annual effective dose per patient almost doubled during the study period, from 3.3 mSv in 1991 to 6.0 mSv in 2002.1 Other studies have described the average axial scanning effective dose for various regions of the body as 6.2 mSv. (3) Aldrich and Williams concluded that CT is the largest contributor to patient dose in radiology. This could be because more CT scanners are in use and their performance has been enhanced, along with increasing indications for CT exams) In 2003 it was estimated that up to 29% of all CT units in the United States were capable of performing multidetector spiral scans, and it is likely that this number is much higher now. (3)

CT is not the only modality that has experienced more use and has the potential to deliver higher patient radiation doses. Vano and Gonzalez studied radiation injuries from interventional fluoroscopic procedures. (4) Their study outlined possible root causes of radiation injuries from fluoroscopic procedures, including misuse, system faults and nonoptimized operational protocols. It drew attention to the fact that optimizing technique and standardizing practice could benefit the field of radiology and protect patients from overexposure to ionizing radiation. Although not pivotal to the discussion of correlating increased use of CT to an increased patient radiation dose, Vano and Gonzalez's study calls attention to the fact that dose to the patient can be reduced by careful attention to technique and optimization.

Yoshizumi and Nelson pointed out the need to balance optimization of image quality against radiation dose in developing clinical protocols. (3) Their study described fundamental concepts of radiation dose in detail, including the CT dose index and other technical factors such as pitch effect, dose profile in the penumbra and signal-to-noise ratio. Yoshizumi and Nelson concluded that multi-detector CT (MDCT) radiation dosimetry issues have not been addressed adequately and have lagged behind advances in the actual technology. (3)

Other researchers also are questioning the effect of newer imaging technologies on patient radiation dose. Berland and Smith proposed that the absorbed dose could be up to 40% higher using MDCT compared with older generation scanners. (5) Golding and Shrimpton suggested that "evidence indicates a strong trend of increasing population dose owing to rising use of CT and to increased dose per examination." (6) A significant body of literature focuses on discovering a causal link between increased use of the CT scanner and an increase in radiation absorbed dose to the patient population.

Numerous studies have suggested that, although CT is not the most commonly performed radiologic examination, it is the largest source of radiation dose. Nagel et al found that, although CT represents only about 4% of all radiologic examinations, it is responsible for up to 35% of the collective radiation dose to the population from radiologic examinations. (7) In a related National Cancer Institute report, data suggested that the use of CT in adults and children has increased approximately 7 fold in the past 10 years. (2) In large U.S. hospitals, CT represents 10% of diagnostic procedures and accounts for approximately 65% of the effective radiation dose for all medical examinations....