CTCAs were evaluated from 50 patients (30 males and 20 females). The mean age was 61 ± 12 (34–82) years. Nine patients who were status-post coronary artery bypass surgery, and two additional patients, had a scan including the heart, ascending aorta, and aortic arch. The low heart rate protocol was used in two patients. The average heart rate during the CTCA, after beta blockade, was 62.9 ± 8.6 (49–88) bpm.
The average maximum effective mAs for CTCA was 500 ± 37 (400–590) mAs. For the 33 subjects in which ECTCM was used, the average mean effective mAs was 387 ± 92 (250–550) mAs. For these patients, the average reduction in effective mAs, which approximately parallels the reduction in dose, was 34.5 ± 4.8% (20.4–44.4), and there was a significant inverse correlation between heart rate and this reduction (r = −0.77, P < 0.001).
Calcium scoring was performed according to protocol in 39 patients. ECTCM was used in all of these scans, with an average effective mAs reduction of 33 ± 17% (−3–64). Again, there was a significant inverse correlation between heart rate and the reduction (r = −0.33, P = 0.04).
Scanner-derived DLPs for bolus tracking were not available for six patients. For the remaining 44 patients, Es for the various components of a standard scan are summarized in . Using ImpactDose to estimate E, the mean E was 8.8 ± 2.9 (3.4–15.9) mSv for CTCA and 2.7 ± 0.8 (1.4–4.2) mSv for calcium scores performed. Mean E for a complete study was 11.7 ± 2.7 (7.2–17.1) mSv. The E from the premonitoring scan(s) was small, with a maximum dose of 0.26 mSv. The E from the monitoring scan, however, varied widely, averaging 0.58 mSv but ranging up to 1.3 mSv, depending on the number of images acquired until the 100 Hounsfield Unit threshold was reached.
Effective doses of scan components
Mean Es were similar when estimated from scanner-derived DLPs: 8.7 ± 2.9 (3.9–16.6) mSv for CTCA and 2.7 ± 0.7 (1.5–4.5) mSv for calcium scores performed. There was close correlation between the two methods of determining E, with correlation coefficients of 0.84 for CTCA and 0.86 for calcium scoring both significant with P < 0.001. Nevertheless, mean E varied between male and female patients, depending on the method employed. While using scanner-derived estimates the mean E for CTCA was greater in males than in females (9.2 vs. 7.9 mSv, P = 0.07), reflecting the higher tube currents (mean effective mAs after ECTCM of 393 vs. 373 mAs) employed for male patients who typically have larger habitus, using ImpactDose the mean E for CTCA was greater in females (9.6 vs. 8.2 mSv, P = 0.06), reflecting also the higher organ dose to the breast. Mean Es using ImpactDose for a complete CTCA and complete study, respectively, were 9.2 and 11.0 mSy in males and 10.1 and 12.7 mSv females.
The mean E was significantly greater in patients receiving a study including the ascending aorta and aortic arch, e.g. a bypass graft evaluation, than in a study in which only the heart was scanned. The mean ImpactDose estimates for a complete CTCA in these subjects were 12.5 mSv and 8.7 mSv, respectively (P = 0.001). This difference in E was due primarily to a significant difference in scan length (20.6 vs. 14.4 cm, P < 0.001), rather than a difference in CTDIvol (33.2 vs. 29.3 mGy, P = 0.053).
Equivalent doses and weighted equivalent doses to individual organs from CTCA are illustrated in . The highest weighted equivalent doses, reflecting the organ contributions to E, were those to the lungs (mean weighted equivalent dose 4.2 ± 1.3 (1.5–7.2) mSv) and, in women, breast (mean 1.9 ± 0.4 (1.3–2.8) mSv). These were followed by the esophagus (1.0 ± 0.3 (0.3–1.9) mSv), bone marrow (1.0 ± 0.4 (0.3–2.0) mSv), and stomach (0.7 ± 0.3 (0.3–1.2) mSv). In women, though equivalent dose to the breast was greater than that to the lung (mean 38 vs. 34 mSv, P < 0.001), the higher ICRP 60 tissue weighting factor for lung than breast led to a greater weighted equivalent dose for the lung. For the calcium scan, the greatest weighted equivalent doses were those to the lungs (1.2 ± 0.3 (0.7–1.7) mSv), female breast (0.5 ± 0.14 (0.3–0.7) mSv), esophagus (0.3 ± 0.08 (0.2–0.5) mSv), and bone marrow (0.3 ± 0.08 (0.2–0.5) mSv).
The range of estimated Equivalent Doses and Weighted Equivalent Doses from CTCA
Estimates of Attributable Risk of Cancer
Estimates of LARs of cancer incidence and mortality from CTCA are summarized in . The average risk of developing cancer from a single CTCA was approximately 1 in 1600, and the average risk of dying from cancer from the CTCA was approximately 1 in 1900. These estimates varied widely from patient to patient, with maximum risks of about 1 in 500 and 1 in 700, respectively, in the sample studied. Risks for female patients were roughly twice those from male patients, particular for younger women, as is illustrated in . There was a significant inverse correlation between LAR of cancer incidence and age for female patients (r = −0.79, P < 0.001), corresponding to lower cancer risk for older patients, and the same trend was observed for male patients, although without reaching statistical signficance (r = −0.32, P = 0.086). The primary contributor to cancer incidence and mortality risk from the CTCA scans was lung cancer, which accounted for two thirds of all cases and three quarters of all deaths attributable to the CTCA. For all patients, the greatest risk of cancer was from lung cancer. There was a strong correlation between the E of the CTCA and the LAR of cancer incidence, as shown in . LARs of cancer incidence and mortality from calcium scans were very small, averaging 1 in 5364 (range 1 in 1926 to 37092) and 1 in 6407 (range 1 in 2712 to 31620), respectively.
Lifetime attributable risks (LARs) and odds of cancer incidence and mortality from CTCA
BEIR VII estimate of lifetime attributable risk of cancer incidence as a function of age
BEIR VII estimate of lifetime attributable risk (LAR) of cancer incidence versus effective dose (E) of CTCA determined with ImpactDose