Comparing coronary artery calcium and thoracic aorta calcium for prediction of all-cause mortality and cardiovascular events on low-dose non-gated computed tomography in a high-risk population of heavy smokers
Introduction
In the past, several studies using both ultrasound and plain radiography have shown an association between calcified plaques in the thoracic aorta (TAC) and cardiovascular and cerebrovascular events [1], [2], [3], [4]. CAC, measured by computed tomography (CT), has proven to be a strong and independent predictor of coronary events and all-cause mortality [5], [6]. Non-enhanced computed tomography (CT) can simultaneously detect both CAC and TAC. Several studies using non-enhanced CT have demonstrated a close association of TAC and CAC [7], [8], [9], [10], [11]. Based on these results, TAC has recently been suggested as an independent predictor of cardiovascular disease, but only two follow-up studies have been reported on this topic [12], [13]. The intuitive advantage of using TAC instead of CAC lies primarily in the fact that CAC measurement may be hampered by motion artifacts of the beating heart while this will hardly affect TAC measurements. As has recently been suggested [9], TAC could prove to be a substitute of CAC for prediction of cardiovascular events in non-gated CT.
Since the heart and thoracic aorta are both depicted on low-dose CT scans for lung cancer screening, and lung cancer and cardiovascular disease share an increased risk with the prolonged use of tobacco, CAC and TAC measurements could be employed to expand the scope of the screening effort and include estimation of cardiovascular risk of screening subjects as well. Adding these measurements at baseline could lead to improved detection of high-risk individuals and, consequently, improved primary prevention of CVD events through optimized preventive treatment of cardiovascular risk factors. So far, no large follow-up studies have investigated the relationship between TAC and CAC in a cohort of asymptomatic smokers. In this study, we investigated whether TAC, as measured on low-dose, non-gated CT, can be used as an independent predictor of all-cause mortality and cardiovascular events compared with CAC. As a secondary analysis, we compared the role of CAC and TAC for the prediction of coronary and non-cardiac events separately.
Section snippets
Study population
The NELSON study is a randomized controlled population-based trial comprising 15,822 men and women aged 50–75 years. Its overall aim is to investigate the beneficial effects of screening for lung cancer with low-dose CT. In 2003–2004, in three regions in the Netherlands and one region in Belgium all men born between 1928 and 1953 living in 101 distinct municipalities, and all women born between 1930 and 1955 living in the remaining 46 municipalities were invited by mail to participate in this
Statistical methods
Baseline characteristics were summarized for the subcohort and the three different case-groups separately. Categorical variables were compared with a χ2 statistic; continuous variables with the Mann–Whitney U-test. Unadjusted annualized event rates for all-cause mortality and CVD events were calculated per CAC and TAC quartile in the subcohort.
In the subcohort, the association between continuous measures of TAC and CAC was investigated with Spearman's rank correlation. Prevalence of CAC and TAC
Results
Table 1 shows the baseline characteristics of a representative baseline sample (subcohort) and all four event-groups. The subcohort included 808 subjects (671 men, 137 women; mean age, 60 ± 6). Compared with the subcohort, subjects in all four event-groups were more often men (p < 0.0001) and more were classified as having diabetes (7% versus 11–18%; p = 0.001). In subjects from the composite CVD, CHD and non-cardiac event-groups, hypertension was more frequent compared with the subcohort (p < 0.0001).
Discussion
CAC has previously been found to be independently associated with all-cause mortality and cardiovascular events [5], [6]. In the present study, TAC was closely associated with CAC. Although both CAC and TAC were associated with all-cause mortality and CVD events in this population of heavy smokers, risk factor-adjusted hazard ratios were consistently higher for CAC compared with TAC. Furthermore, only CAC was associated with coronary events, whereas TAC – and not CAC – was found to be
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