Table 2.
Multivariable Cox regression for RFS in 2284 ER+ breast cancer patients
| Characteristic | Events | Adjusted HR (95% CI) | P a |
|---|---|---|---|
| AI–endocrine treatment ratiob | |||
| AI < 25% | 185 | 1.00 (Reference) | |
| AI 25%-75% | 118 | 0.85 (0.65 to 1.12) | .27 |
| AI>75% | 74 | 0.63 (0.46 to 0.86) | .004 |
| Age | 1.05 (0.98 to 1.11) | .11 | |
| Age*(follow-up time − 5)c | 1.03 (1.00 to 1.05) | .01 | |
| Trastuzumab | |||
| No | 375 | 1.00 | |
| Yes | 2 | 0.56 (0.13 to 2.39) | .44 |
| Grade | |||
| I | 60 | 0.45 (0.29 to 0.69) | <.001 |
| II | 26 | 0.61 (0.48 to 0.77) | <.001 |
| III | 138 | 1.00 | |
| Unknown | 153 | 1.38 (1.00 to 1.90) | .05 |
| Positive lymph nodes | |||
| 0 | 70 | 1.00 | |
| 1-3 | 156 | 1.37 (1.02 to 1.83) | .03 |
| 4-9 | 89 | 2.29 (1.66 to 3.16) | <.001 |
| >10 | 62 | 4.55 (3.17 to 6.52) | <.001 |
| pT-stage | |||
| 1 | 119 | 1.00 | |
| 2 | 201 | 1.23 (0.98 to 1.55) | .07 |
| 3 | 39 | 1.50 (1.02 to 2.19) | .04 |
| 4 | 8 | 1.68 (0.79 to 3.57) | .18 |
| Unknown | 10 | 1.11 (0.56 to 2.18) | .75 |
| PR status | |||
| Negative | 77 | 1.00 | |
| Positive | 277 | 0.50 (0.38 to 0.64) | <.001 |
| Unknown | 23 | 0.75 (0.46 to 1.20) | .23 |
| HER2 status | |||
| Negative | 289 | 1.00 | |
| Positive | 51 | 1.17 (0.83 to 1.66) | .36 |
| Unknown | 37 | 0.76 (0.53 to 1.09) | .14 |
| Ovarian ablation | |||
| No | 308 | 1.00 | |
| Yes | 69 | 1.25 (0.95 to 1.64) | .10 |
P values are based on a 2-sided Wald test. AI = aromatase inhibitor; CI = confidence interval; ER+ = estrogen receptor positive; HR = hazard ratio; PR = progesterone receptor; pT-stage = pathologic T-stage; RFS = recurrence-free survival.
The AI–endocrine treatment ratio, included in the model as a time-dependent variable, is defined as the percentage of total endocrine treatment duration (AI+tamoxifen) spent on AI treatment.
Interaction between age at diagnosis and follow-up time centered at 5 years was included to accommodate nonproportional hazards. At 5 years of follow-up, 2 patients who differ 1 year in age have an adjusted hazard ratio of 1.05, meaning that the older patient has a 5% higher risk of a RFS event compared with the younger patient. The hazard ratio increases by 3% for each additional year of follow-up, so, for example, at 6 years of follow-up, the adjusted hazard ratio equals exp{ln(1.05)+(follow-up time-5)*ln(1.03)}= 1.08.