Table 2. HRs (95% CIs) for Indoor Tanning and Risk of Cutaneous Squamous Cell Carcinoma, Norwegian Women and Cancer Study, 1991-2015.
| Variable | No. of Participants (N = 113 290) | No. of Cases (n = 366) | HR (95% CI) | ||
|---|---|---|---|---|---|
| Complete-Case Analyses | Multiple-Imputation Analysisc | ||||
| Age Adjusteda | Multivariable Modelb | ||||
| Ever use of indoor tanning device | |||||
| Never | 33 721 (29.8) | 101 | 1 [Reference] | 1 [Reference] | 1 [Reference] |
| Ever | 79 569 (70.2) | 265 | 1.50 (1.19-1.90) | 1.45 (1.14-1.85) | 1.43 (1.17-1.74) |
| Current use of indoor tanning device | |||||
| No | 44 971 (39.7) | 146 | 1 [Reference] | 1 [Reference] | 1 [Reference] |
| Yes | 68 319 (60.3) | 220 | 1.27 (1.02-1.57) | 1.22 (0.98-1.52) | 1.27 (1.06-1.53) |
| Cumulative No. of sessions | |||||
| Never use | 33 721 (29.8) | 101 | 1 [Reference] | 1 [Reference] | 1 [Reference] |
| Lowest use | 53 458 (47.2) | 167 | 1.41 (1.10-1.82) | 1.38 (1.07-1.78) | 1.29 (1.04-1.60) |
| Medium use | 12 809 (11.3) | 47 | 1.57 (1.11-2.23) | 1.54 (1.08-2.19) | 1.60 (1.20-2.15) |
| Highest use | 13 302 (11.7) | 51 | 1.78 (1.27-2.51) | 1.68 (1.19-2.38) | 1.83 (1.38-2.42) |
| P value for trend | NA | NA | <.001 | .002 | <.001 |
| Duration of use | |||||
| Never | 33 721 (29.8) | 101 | 1 [Reference] | 1 [Reference] | 1 [Reference] |
| ≤10 y | 15 682 (13.8) | 49 | 1.56 (1.10-2.22) | 1.54 (1.08-2.20) | 1.41 (1.08-1.85) |
| >10 y | 63 887 (56.4) | 216 | 1.49 (1.17-1.89) | 1.43 (1.12-1.84) | 1.43 (1.16-1.76) |
| P value for trend | NA | NA | .002 | .007 | .001 |
| Age at initiation, y | |||||
| Never | 33 652 (29.7) | 101 | 1 [Reference] | 1 [Reference] | 1 [Reference] |
| ≥30 | 52 910 (46.7) | 188 | 1.50 (1.17-1.91) | 1.45 (1.13-1.86) | 1.36 (1.11-1.67) |
| <30 | 26 728 (23.6) | 77 | 1.41 (1.04-1.91) | 1.36 (1.00-1.86) | 1.51 (1.18-1.92) |
| P value for trend | NA | NA | .011 | .028 | <.001 |
| Duration of Use and Cumulative No. of Tanning Sessions | |||||
| Duration ≤10 y of use | |||||
| Never use | 33 721 (29.8) | 101 | 1 [Reference] | 1 [Reference] | 1 [Reference] |
| Lowest use | 13 362 (11.8) | 37 | 1.39 (0.95-2.05) | 1.38 (0.93-2.03) | 1.27 (0.94-1.71) |
| Medium/highest use | 2320 (2.0) | 12 | 2.49 (1.36-4.56) | 2.45 (1.33-4.49) | 1.99 (1.28-3.08) |
| Duration >10 y of use | |||||
| Never use | 33 721 (29.8) | 101 | 1 [Reference] | 1 [Reference] | 1 [Reference] |
| Lowest use | 40 096 (35.4) | 130 | 1.42 (1.09-1.85) | 1.38 (1.05-1.80) | 1.30 (1.03-1.63) |
| Medium/highest use | 23 791 (21.0) | 86 | 1.60 (1.20-2.15) | 1.53 (1.14-2.07) | 1.66 (1.29-2.13) |
| P value for interactiond | NA | NA | .204 | .194 | NA |
| Cumulative No. of indoor and outdoor tanning sessionse,f | |||||
| 1 (Lowest) | 14 897 (13.1) | 44 | 1 [Reference] | 1 [Reference] | 1 [Reference] |
| 2 | 30 122 (26.6) | 95 | 1.47 (1.03-2.11) | 1.39 (0.97-1.99) | 1.42 (1.02-1.98) |
| 3 | 37 127 (32.8) | 106 | 1.43 (1.00-2.04) | 1.34 (0.94-1.92) | 1.48 (1.08-2.02) |
| 4 | 14 970 (13.2) | 57 | 2.02 (1.36-3.00) | 1.89 (1.27-2.82) | 1.79 (1.24-2.59) |
| 5 (Highest) | 16 174 (14.3) | 64 | 2.21 (1.50-3.27) | 2.03 (1.38-3.01) | 2.43 (1.74-3.39) |
| P value for trend | NA | NA | <.001 | <.001 | <.001 |
Abbreviations: HR, hazard ratio; NA, not applicable; UVR, UV radiation.
Cox proportional hazards regression with age as the time scale and stratified by birth cohort.
Additional adjustments for residential ambient UVR exposure, hair color, and cumulative number of sunburns and sunbathing vacations.
Analysis with multiple imputation of missing data conducted using chained equations and a total of 40 imputed data sets, with the same adjustments as the multivariable model (n = 159 419; 597 cases).
Testing interaction between number of sessions and duration of use.
On the basis of the sum of quartiles of both variables (resulting in a score from 0 to 8), categorized as 0 to 1, 2 to 3, 4 to 5, 6, or 7 to 8.
For this variable, the multivariable model included residential ambient UVR exposure, hair color, and cumulative number of sunburns.