Table 2 Summary characteristics of study population by breast cancer case status

Median

   Percentage density (%)

16.9

11.0

<0.001

   Absolute dense area (cm²)

24.3

16.9

<0.001

Mean (SD)

   Age at diagnosis or reference date (y)

61.2 (7.1)

62.8 (6.9)

<0.001

   Age at mammogram (y)

60.6 (7.2)

62.8 (6.7)

<0.001

   Age at menarche (y)

13.5 (1.4)

13.6 (1.4)

0.084

   Age at menopause (y)

50.6 (3.8)

50.0 (3.9)

0.001

   BMI at diagnosis or reference date (kg/m²)

25.1 (3.6)

25.2 (3.9)

0.801

   Alcohol consumption (g/day)

2.8 (5.0)

2.3 (4.2)

0.008

   Percentage density (%)

20.7 (16.1)

15.7 (14.5)

<0.001

   Absolute dense area (cm²)

30.2 (23.7)

23.3 (21.5)

<0.001

Frequency, number (%)

   Categoric percentage density (%)

<0.001

   <10

477 (31.8)

680 (45.5)

   10-24

538 (35.9)

506 (33.8)

   25-49

384 (25.6)

250 (16.7)

   50-74

96 (6.4)

57 (3.8)

   ≥75

3 (0.2)

2 (0.1)

   Categoric absolute dense area (cm²)

<0.001

   <10

288 (19.2)

475 (31.8)

   10-24

471 (31.4)

509 (34.0)

   25-49

460 (30.7)

347 (23.2)

   50-74

211 (14.1)

114 (7.6)

   75-99

47 (3.1)

39 (2.6)

   ≥100

21 (1.4)

11(0.7)

   Parity and age at first birth

<0.001

   Nulliparous

190 (12.7)

148 (9.9)

   1-3 children, age at first birth <25 y

556 (37.1)

555 (37.1)

   1-3 children, age at first birth 25-29 y

420 (28.0)

399 (26.7)

   1-3 children, age at first birth ≥30 y

223 (14.9)

212 (14.2)

   ≥4 children, age at first birth <25 y

29 (1.9)

31 (2.1)

   ≥4 children, age at first birth ≥25 y

78 (5.2)

150 (10.0)

   Hormone replacement therapy

<0.001

   Never used hormones

722 (48.2)

844 (56.5)

   Ever used hormones

774 (51.7)

649 (43.4)

   Unknown status of hormone use

2 (0.1)

2 (0.1)

   Family history of breast cancer (ever)

207 (13.8)

119 (8.0)

<0.001

   Benign breast disease (ever)

351 (23.4)

141 (9.4)

<0.001