Table 3.

Year-to-Year Breast-Cancer Incidence and Means of Detection in the Observational Study According to Whether the Participants Were Using Hormone Therapy.^*

Variable	2000		2001		2002		2003		2004		2005†
	Taking Hormones at Entry	Not Taking Hormones at Entry	Taking Hormones at Entry	Not Taking Hormones at Entry	Taking Hormones at Entry	Not Taking Hormones at Entry	Taking Hormones at Entry	Not Taking Hormones at Entry	Taking Hormones at Entry	Not Taking Hormones at Entry	Taking Hormones at Entry	Not Taking Hormones at Entry
Breast cancer
No. at risk	15,544	24,353	15,344	23,925	15,121	23,515	14,856	23,027	14,582	23,027	12,885	18,773
Mean follow-up (mo)	11.9	11.9	11.9	11.9	11.9	11.9	11.9	11.8	11.6	11.4	11.2	10.7
No. of cases	118	99	106	100	122	117	68	99	60	80	52	64
Annualized incidence (%)‡	0.76	0.41	0.70	0.42	0.81	0.50	0.46	0.44	0.43	0.38	0.43	0.38
Hazard ratio (95% CI)§	1.89 (1.40–2.54)		1.79 (1.32–2.42)		1.65 (1.25–2.19)		1.18 (0.84–1.67)		1.28 (0.87, 1.88)		1.23 (0.82, 1.83)
Hazard ratio (95% CI) with adjustment for prior mammograms¶	1.91 (1.41–2.58)		1.75 (1.29–2.38)		1.62 (1.22–2.14)		1.19 (0.84–1.69)		1.25 (0.85, 1.84)		1.23 (0.82, 1.84)
Means of detection — %
Mammogram‖	86	79	85	77	85	76	83	75	83	75	75	67
Breast biopsy**	4.6	2.4	4.5	2.5	4.5	2.4	3.9	2.4	3.2	2.3	2.6	1.8
Use of estrogen and progestin during follow-up with adjustment for age (%)††	82	16	75	14	63	11	41	6	17	2	12	2

^*Participants were categorized according to whether they were taking estrogen and progestin at the time of study entry; those who reported that they were not taking estrogen and progestin may or may not have done so before entering the study. All participants had an intact uterus, had no history of breast cancer, and had undergone mammography within 2 years before enrollment. At the time of enrollment, a total of 25,328 participants reported that they were using estrogen and progestin, and 16,121 reported that they were not using hormone therapy.

^†Data were included through December 31, 2005.

^‡Annualized incidence was computed according to time period, where the total number of events for patients at risk for invasive breast cancer during a particular time period was divided by the total follow-up time. Follow-up time is defined as the time elapsed between the start of each period and the first occurrence of invasive breast cancer or censoring (death, loss to follow-up, or the end of each time period).

^§These hazard ratios were calculated with the use of a Cox proportional-hazards model for the effect on the risk of breast cancer among participants using hormone therapy as compared with those not using hormone therapy, stratified according to age and adjusted for age, race or ethnic group, body-mass index, education, smoking status, use or nonuse of alcohol, self-reported health, presence or absence of a family history of breast cancer, bilateral-oophorectomy status, level of physical activity, and Gail-model estimate of breast-cancer risk.

^¶These hazard ratios were calculated with the use of a Cox proportional-hazards model stratified according to age and adjusted for age, race or ethnic group, body-mass index, education, smoking status, alcohol use, self-reported health, family history of breast cancer, bilateral oophorectomy status, level of physical activity, and Gail risk as well as for the number of annual mammograms (none, one, or two) reported by the participants during the previous 2 years.

^‖The data on mammograms were self-reported.

^**The data on breast biopsies were self-reported.

^††Women in the WHI observational study initially consented to participation through March 31, 2005. To allow for further follow-up of this same cohort in an extension study, additional consent was required and was obtained from 71% of eligible participants. Data from the WHI extension study were not used in Figures 2 and 3.