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Pediatric CT scans and childhood leukemia
After exclusions, eight cases (0.7%) and nine controls (0.3%) had undergone at least one CT scan. The median RBM dose was 10.1 mGy (IQR 4.79 – 13.6) for the exposed cases and 6.29 mGy (IQR 5.69 – 7.14) for the exposed controls (Figure 3). The corresponding literature- based values were 26.5 mGy and 17.6 mGy. The RBM doses calculated with NCICT from thoracic CT scans var- ied between 1.8 and 6.8 mGy (median 4.0 mGy) and sim- ilarly, the doses for head CT scans varied between 1.6 and 10.7 mGy (median 6.6 mGy).
The OR for any versus no CT was 2.82 (95% CI: 1.05 – 7.56). For two or more pediatric CT scans, the OR was 5.22 (95% CI: 0.89 – 69.9). For any head CT scans, an OR of 4.00 (95% CI: 1.39 – 11.5) was obtained.
The overall excess OR of childhood leukemia was 0.13 (95% CI: 0.02 – 0.26) per mGy of absorbed RBM dose cal- culated with the NCICT software (Table 3). Using the cumulative RBM dose estimates from the literature, an excess OR of 0.05 (95% CI: 0.01 – 0.10) per mGy was obtained. In an analysis by dose tertile calculated with
A
B
Figure 3. Histograms of (A) the ages of the subjects at the time of computed tomography scan and (B) the cumulative doses received by the subjects, calcu- lated with NCICT.
NCICT, the excess OR relative to zero dose were 1.26 (95% CI: -0.50 – 10.1) for the first group, 0.09 (95% CI: -0.89 – 10.5) for the second, and 5.00 (95% CI: 0.10 – 31.7) for the last (Figure 4).
For the most common subtype, precursor B-cell acute lymphoblastic leukemia, the excess OR per mGy was 0.14 (95% CI: 0.02 – 0.29) using estimates from NCICT and 0.06 (0.01 – 0.11) for literature-based estimates. The excess OR for any versus no CT scans was 2.25 (95% CI: 0.08 – 8.75) for acute lymphoblastic leukemia and 2.88 (95% CI: 0.22 – 11.4) for precursor B-cell acute lym- phoblastic leukemia. In the analysis by age at diagnosis/reference date, the excess OR for any versus no CT scans was 3.50 (95% CI: -0.25 – 25.9) for children aged 2 – <7 years and 1.27 (95% CI: -0.32 – 6.54) for those aged 7 – <15 years.
Covariate (confounder) adjustments (large for gesta- tional age, maternal smoking during pregnancy, parental education and parental socioeconomic status) did not alter the OR for CT exposure by more than 0.05 units, with the exception of maternal smoking, which increased the OR related to the number of pediatric CT scans (0 ver- sus 1 or more) (approximately 0.10 units). Nevertheless, we preferred the unadjusted model, as missing data on maternal smoking resulted in wider confidence intervals for the main variables.
The OR were higher when the subjects with Down syndrome were not excluded (for 1 or more CT scans OR=5.21, 95% CI: 2.19 – 12.4 and for cumulative RBM dose excess OR=0.19 per mGy, 95% CI: 0.07 – 0.32). No evidence of a different effect of the RBM doses on leukemia risk for subjects with or without Down syn-
Table 3. The frequencies of computed tomography scans for subjects >2 years old at the reference date and odds ratios calculated with exact condi- tional logistic regression.
Cases Controls
911 2730
CT scans
0 903 2721 1 4 7 2 or more 4 2
by type (1 or more)
ALL 7 7 pre-B-ALL 7 6
by age-group (1 or more)
2-<7years 3 2 7 - <15 years 5 7
by dose index (NCICT/literature)
low, 4.79a/11.6b mGy 3 4 medium, 6.72a/19. 8b mGy 1 3 high, 13.8a/33.2b mGy 4 2
per 1 mGy (NCICT)
TOTAL pre-B-ALL
per 1 mGy (literature)
TOTAL
pre-B-ALL
OR (95% CI)
1.85 (0.39, 7.36) 6.22 (0.89, 68.9)
3.25 (1.08, 9.75) 3.88 (1.22, 12.4)
4.50 (0.75, 26.9) 2.27 (0.68, 7.54)
2.26 (0.50, 10.1) 1.09 (0.11, 10.5) 6.00 (1.10, 32.8)
1.13 (1.02, 1.26) 1.14 (1.02, 1.29)
1.05 (1.01, 1.10)
1.06 (1.01, 1.11)
The reference group for all calculated odds ratio (OR) is zero CT scans for categorical vari- ables. Study subjects with Down syndrome or cancer diagnoses were excluded. All reported OR are from an unadjusted model.The median doses for dose-index classes calculated with NCICT are marked with a.The respective class medians based on literature are marked with b. ALL: acute lymphoblastic leukemia; pre-B-ALL: precursor B-cell acute lymphoblastic leukemia.
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