Table 1.
Characteristics of the study subjects and night work exposure parameters.
| Characteristics | Controls (N=280) | Cases (N=278) | P |
|---|---|---|---|
| Age (years), mean ± SD (min-max) | 54.34 ± 7.69 (36-74) | 54.46 ± 7.56 (36-74) | 0.924b |
| No. of children (min-max) | 0-9 | 0-5 | 0.055b |
| Age at first birth (years), mean ± SD (min-max) | 26.78 ± 4.17 (19-41) | 26.69 ± 4.18 (18-43) | 0.758b |
| Breast cancer in first-degree family (yes/no) | 28/253 | 49/227 | 0.008c |
| Alcohol consumption ≥twice/week (yes/no) | 12/269 | 23/256 | 0.052c |
| Daily exposure to x-rays (yes/no) | 51/230 | 57/222 | 0.494c |
| Hormone therapy in the past 2 years (yes/no) | 59/216 | 70/204 | 0.526c |
| Night shift exposure metrics | |||
| Never night work | 71 | 70 | |
| Ever night work | 210 | 209 | |
| Never ≥ 3 consecutive night shifts (low exposure group) | 21 | 28 | |
| 3 or more consecutive night shifts < 5 years (medium exposure group) | 49 | 41 | |
| 3 or more consecutive night shifts ≥ 5 years (high exposure group) | 140 | 140 | |
| Receptor status | |||
| ER+ | 178/203 | ||
| PR+ | 138/200 |
aAge at diagnosis (cases) and age at selection of reference (controls). bDerived from Mann-Whitney U-test (two-sided). cDerived from Pearsons Chi-square test (two-sided). dBreast cancer in mother or sister. eHormone replacement therapy in postmenopausal women.