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Table 1 Baseline characteristics of cases and controls, and odds ratios for total breast cancer in the NBCSP case-control study

From: Mammographic density and risk of breast cancer according to tumor characteristics and mode of detection: a Spanish population-based case-control study

Characteristic No. of controls (%) No. of breast cancer cases (%) Odds ratioa (95% CI) P valueb
Screening round (baseline exploration)     
   September 1996 to August 1998 3,948 (84.1) 987 (84.1)   
   September 1998 to August 2000 408 (8.7) 102 (8.7)   
   September 2000 to August 2002 224 (4.8) 56 (4.8)   
   September 2002 to August 2004 108 (2.3) 27 (2.3)   
Age at baseline exploration (years)     
   <50 1,733 (37.0) 433 (37.0)   
   50-54 986 (21.0) 245 (20.9)   
   55-59 897 (19.1) 228 (19.5)   
   60-64 870 (18.6) 217 (18.5)   
   ≥65 202 (4.3) 49 (4.2)   
Age at menarche (years)     0.357c
   ≥15 724 (15.5) 161 (13.7) 0.95 (0.77-1.18)  
   14 1,243 (26.5) 290 (24.7) 1.00 (reference)  
   13 1,281 (27.3) 363 (31.0) 1.22 (1.03-1.45)  
   12 867 (18.5) 225 (19.2) 1.11 (0.92-1.35)  
   <12 569 (12.1) 132 (11.3) 1.00 (0.79-1.25)  
   Unknown 4 (0.1) 1 (0.1)   
Age at first live birth (years)     <0.001
   <20 102 (2.2) 13 (1.1) 0.51 (0.28-0.91)  
   20-24 1,441 (30.7) 293 (25.0) 0.81 (0.68-0.95)  
   25-29 1,823 (38.9) 457 (39.0) 1.00 (reference)  
   30-34 522 (11.1) 134 (11.4) 1.03 (0.83-1.28)  
   ≥35 175 (3.7) 57 (4.9) 1.31 (0.95-1.80)  
   Nulliparous 625 (13.3) 218 (18.6) 1.41 (1.17-1.70)  
Age at menopause (years)     0.001
   ≤45 777 (16.6) 147 (12.5) 1.00 (reference)  
   46-50 1,244 (26.5) 298 (25.4) 1.25 (1.01-1.56)  
   >50 1,035 (22.1) 293 (25.0) 1.48 (1.17-1.87)  
   Premenopausal 1,631 (34.8) 434 (37.0) 1.44 (1.14-1.83)  
   Unknown 1 (0.0) 0 (0.0)   
Family history of breast cancer     <0.001
   None 4,075 (86.9) 945 (80.6) 1.00 (reference)  
   Second-degree relative 327 (7.0) 109 (9.3) 1.43 (1.14-1.80)  
   First-degree relative ≥50 years 172 (3.7) 64 (5.5) 1.61 (1.19-2.16)  
   First-degree relative <50 years 114 (2.4) 54 (4.6) 2.06 (1.48-2.88)  
Previous breast biopsy     <0.001
   No 4,281 (91.3) 1,001 (85.4) 1.00 (reference)  
   Yes 407 (8.7) 171 (14.6) 1.82 (1.50-2.22)  
Use of hormone replacement therapy     0.602
   No 4,208 (89.8) 1,046 (89.2) 1.00 (reference)  
   Yes 480 (10.2) 126 (10.8) 1.06 (0.86-1.31)  
Mammographic density (%)     <0.001c
   0-10 1,711 (36.5) 263 (22.4) 1.00 (reference)  
   11-25 1,136 (24.3) 244 (20.8) 1.52 (1.25-1.85)  
   26-50 1,158 (24.8) 375 (32.0) 2.47 (2.05-2.98)  
   51-75 529 (11.3) 215 (18.3) 3.27 (2.62-4.09)  
   >75 135 (2.9) 59 (5.0) 3.74 (2.63-5.32)  
   Unknown 19 (0.4) 16 (1.4)   
  1. aOdds ratios for total breast cancer and 95% confidence intervals (CIs) obtained from conditional logistic regression models that accounted for the matching factors (screening round, year of birth, and place of residence). bP values for homogeneity of breast cancer risk across categories, except in the case of age at menarche and mammographic density. cP values for linear trend using an ordinal variable