Table 1.

Characteristics of cases (women with asynchronous contralateral breast cancer) and controls (women with unilateral breast cancer only) from the WECARE Study population

Variable	Median (range)	Cases (CBC) median (range)	Controls (UBC) median (range)
Age at first diagnosis (years)	45 (23–55)	44 (24–55)	45 (23–55)
Age at reference date (years)	49 (27–71)	49 (27–71)	49 (27–69)
Length of at-risk perioda (years)	4.2 (1.0–15.6)	4.2 (1.0–15.6)	4.3 (1.0–15.6)

Variable	Level	Cases (CBC)		Controls (UBC)
		N	%	N	%
Center	Iowa	83	16.2	162	16.2
	UC Irvine	86	16.8	148	14.8
	Los Angeles	145	28.4	277	27.7
	Seattle	65	12.7	133	13.3
	Denmark	132	25.8	279	27.9
Year at first diagnosis	1985–1988	183	35.8	345	34.5
	1989–1992	176	34.4	355	35.5
	1993–1996	122	23.9	237	23.7
	1997+	30	5.9	62	6.2
Chemotherapy	No	253	49.5	416	41.6
	Yes	258	50.5	583	58.4
Hormone treatmentb	No	382	74.8	686	68.7
	Yes	129	25.2	312	31.2
	Unknown	0	0	1	0.1
Radiation treatment	Never	266	52.1	187	18.7
	Ever	245	47.9	812	81.3
Histology of first breast cancer	Lobular	61	11.9	92	9.2
	Other	450	88.1	907	90.8
Stage of first breast cancer	Localized	355	69.5	631	63.2
	Regional	156	30.5	368	36.8
ER Status of first breast cancerc	Positive	244	47.7	509	51
	Negative	139	27.2	252	25.2
	Other	128	25	238	23.8
PR Status of first breast cancerc	Positive	211	41.3	423	42.3
	Negative	120	23.5	232	23.2
	Other	180	35.2	344	34.4
Age at menarche (years)	8–13	239	46.8	435	43.5
	13–19	272	53.2	564	56.5
Number of full-term pregnancies	None	102	20	170	17
	1–3	375	73.4	729	73
	4–14	34	6.7	100	10
Menopausal status/age at menopause at start of at-risk periodd	Premenopausal	247	48.3	411	41.1
	Postmenopausal age <45	109	21.3	249	24.9
	Postmenopausal age 45+	155	30.3	339	33.9
	Unknown	0	0	0	0
Family history of breast cancer	None	342	66.9	781	78.2
	1+	161	31.5	201	20.1
	Adopted	8	1.6	17	1.7

Excludes women with missing height and/or weight (n = 21), those who were outliers for BMI (n = 44) or weight (n = 4) at either age 18, first diagnosis or reference date, women who reported use of postmenopausal hormone therapy at any time prior to reference date (n = 525) and women with missing information on menopausal status (n = 3). This left 511 CBC cases and 999 UBC controls for this analysis

CBC asynchronous contralateral breast cancer, UBC unilateral breast cancer, ER estrogen receptor, PR progesterone receptor

^aBeginning 1 year after first diagnosis extending to the reference date (date of second diagnosis in cases)

^bHormone therapy includes all hormonal breast cancer treatments including: tamoxifen, raloxifene, toremifene citrate, anastrozole, letrozole, exemestane, aminoglutethimide, goserelin acetate, leuprorelin, fulvestrant and megestrol acetate

^cRefers to receptor status of the first primary breast cancer. The ‘Other’ category consists of women where no lab test was given, the test was given and the results are unknown or the test was given and the results were borderline

^dDefined as menopausal status as of 1 year after first diagnosis (the start of the at-risk period). If a woman reported that she was still menstruating up to 1 year after first diagnosis or was pregnant, she was classified as premenopausal