The association between oral contraceptive use and lobular and ductal breast cancer in young women

Abstract

Recent reports indicate that the incidence of lobular breast cancer is increasing at a faster rate than ductal breast cancer, which may be due to the differential effects of exogenous hormones by histology. To address this issue, we examined whether the relationship between oral contraceptive use and incident breast cancer differs between lobular and ductal subtypes in young women. A population-based sample of in situ and invasive breast cancer cases between ages 20 and 44 were recruited from Atlanta, GA; Seattle-Puget Sound, WA and central New Jersey. Controls were sampled from the same areas by random-digit dialing, and were frequency matched to the expected case age distribution. Odds ratios (ORs) and 95% confidence intervals (95% CIs) were calculated using polytomous logistic regression. Among the 100 lobular cancers, 1,164 ductal cancers, and 1,501 controls, the odds ratios for oral contraceptive ever use were 1.10 (95% CI = 0.68–1.78) for lobular cancers and 1.21 (95% CI = 1.01–1.45) for ductal cancers, adjusted for study site, age at diagnosis, and pap screening history. Our results suggest that the magnitude of the association between ever use of oral contraceptives and breast cancer in young women does not vary strongly by histologic subtype. These results are similar to previous studies that report little difference in the effect of oral contraceptive use on breast cancer by histology. © 2007 Wiley-Liss, Inc.

Recent reports indicate that the incidence rate of invasive lobular breast cancer is increasing faster than invasive ductal breast cancer,1, 2, 3, 4 but few epidemiologic studies have focused on identifying breast cancer risk factors by histologic subtype. Among the studies that have, the association between combination hormone replacement therapy and risk of invasive breast cancer differed for ductal and lobular cancers in most studies,5, 6, 7, 8, 9, 10 but not all.11 These findings suggest that the effects of other exogenous hormonal treatments, such as oral contraceptives, may also differ by histologic type.

It has been consistently shown that oral contraceptive use is associated with an increased risk of breast cancer in young women,12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25 but not older women.21, 23, 24, 25 Recently, Newcomer et al.26 reported that current oral contraceptive use, recent use, and older age at first use are associated with an increased risk of invasive lobular breast cancer, but not invasive ductal breast cancer in a population of middle-aged women, suggesting that oral contraceptives may have effects related to specific histologic subtypes. It is unknown whether the associations between oral contraceptive use and breast cancer differs by histologic subtype among younger women—the age group most likely to experience increased risk of breast cancer due to oral contraceptive use.25 To determine this, we explored the relationship of oral contraceptive use and lobular and ductal breast cancer subtypes within a population of young women. Previously, we reported an odds ratio (OR) of 1.2 for the association between oral contraceptive use and all breast cancers in this population,22 which is consistent with other results in the literature for women less than 45 years old.24

Material and methods

Study data were originally collected as part of the Women's Interview Study of Health (WISH), a population-based case–control study designed to investigate the effects of oral contraceptives, alcohol consumption and dietary intake during adolescence on breast cancer risk in young women.22 Cases were recruited from metropolitan Atlanta, GA, Seattle-Puget Sound, WA, and a 5-county area in central New Jersey and were defined as any woman between 20 and 44 years of age who was diagnosed with incident in situ or invasive breast cancer between May 1, 1990 and December 31, 1992. Eligible cases were identified using rapid reporting systems established in each of the 3 geographic areas and checks were made against population-based cancer registries in these same areas. Controls were randomly selected from the same geographic regions as cases using 13 cycles of random-digit dialing, and were matched to the expected age distribution of the cases.

Cases and controls were asked to give written informed consent, and then complete an in-person interview. During the interview, information was collected by a trained interviewer about demographic characteristics, lifestyle, medical history, contraceptive use, diet, reproductive and menstrual histories, physical activity, smoking habits, occupation, exogenous hormone use, alcohol consumption and family history of cancer. Anthropometric measures were also obtained. A book of color photographs and descriptions of oral contraceptive pills organized by brand, name and dose, corresponding packaging, and years the pill was available in the United States were shown to study subjects to help identify specific formulations they used. Additionally, a monthly reproductive history calendar marked with pregnancies, other reproductive events, and contraceptive use since menarche was used to aid in the recall of dates of oral contraceptive use. Interviews were completed for 86.4% of eligible cases and 78.1% of eligible controls.

Clinical and histopathologic information for cases was obtained from hospital and medical records and classified according to the International Classification of Diseases.27 Cases with an ICD-O behavior code of 2 were classified as in situ, and cases with a behavior code of 3 were classified as invasive. To facilitate comparisons with a previously published study of the effect of oral contraceptive use on lobular and ductal breast cancer incidence,26 we limited our analysis to cases of ductal carcinoma, NOS (ICD-O-2 code 8500) and lobular carcinoma (ICD-O-2 code 8520). Table I shows the histologic classification of all cases in the study population. There were 1,164 ductal cases with a mean age at diagnosis of 39 years and 100 lobular cases with a mean age at diagnosis of 41 years. The 1,501 controls had a mean age of 38 years old at the time of the telephone screening interview.

Table I. Frequency of Histologic Subtypes Among Breast Cancer Cases

Code	Classification1	Frequency
1 Histology classifications are based on the International Classification of Diseases, 2nd Edition27. 2 Histology code was missing or did not correspond to any code in the International Classification of Diseases, 2nd Edition.27
8000	Neoplasm, malignant	42
8001	Tumor cells, malignant	1
8010	Carcinoma, NOS	108
8020	Carcinoma, undifferentiated type, NOS	1
8021	Carcinoma, anaplastic, NOS	2
8033	Pseudosarcomatous carcinoma	1
8050	Papillary carcinoma, NOS	1
8060	Papillomatosis, NOS	1
8140	Adenocarcinoma, NOS	60
8141	Scirrhous adenocarcinoma	1
8201	Cribriform carcinoma	3
8211	Tubular adenocarcinoma	13
8230	Solid carcinoma NOS	1
8480	Mucinous adenocarcinoma	15
8481	Mucin-producing adenocarcinoma	3
8500	Duct carcinoma, NOS	1,164
8501	Comedocarcinoma, NOS	34
8503	Intraductal papillary adenocarcinoma	4
8504	Intracystic carcinoma	1
8505	Intraductal papillomatosis, NOS	1
8510	Medullary carcinoma, NOS	29
8511	Medullary carcinoma with amyloid stroma	1
8512	Medullary carcinoma with lymph. stroma	2
8520	Lobular carcinoma, NOS	100
8522	Duct and lobular carcinoma	37
8530	Inflammatory carcinoma	2
8540	Paget disease, mammary	2
8541	Paget disease and infiltrating duct carcinoma	2
8543	Paget disease and intraductal carcinoma	2
8570	Adenocarcinoma with squamous metaplasia	2
8800	Sarcoma, NOS	1
9680	ML, large cell, diffuse	2
	Unknown or not classified2	6

The association between oral contraceptive use and lobular and ductal breast cancers was estimated using polytomous logistic regression.28 All information about oral contraceptive use and other covariates refers to exposures prior to the reference date, which was defined as the age at diagnosis for cases or age at telephone screening interview for controls. Oral contraceptive use variables were created based on categories used in previously published studies.22, 24, 26, 29, 30 Lifetime oral contraceptive use of 6 months or longer was treated as ‘ever use’. Oral contraceptive use of less than 6 months or no use at all was treated as ‘never use’. Among ever users, oral contraceptive use within 24 months of the reference date was classified as recent use. Last use more than 24 months before the reference date was categorized as former use. In addition, we also examined age at first use of oral contraceptives, number of years since last use, duration of use, and the effect of oral contraceptive ever use where cases were further stratified by in situ (LCIS, N = 43; DCIS, N = 152) and invasive (lobular, N = 57; ductal, N = 1012) stage.

A binary variable describing menopausal status was evaluated as an effect measure modifier of oral contraceptive ever use on the multiplicative scale. A woman was considered post-menopausal if she reported of having a hysterectomy, bilateral oopherectomy, or natural cessation of menses; otherwise she was considered premenopausal. The interaction between oral contraceptive ever use and menopausal status failed to improve model fit and was left out of the final model.

Several breast cancer risk factors were assessed for confounding by backwards elimination, using the approach described by Rothman and Greenland.31 A covariate was considered a confounder if elimination of the covariate changed the regression coefficient for oral contraceptive use by more than 0.10 on the natural log scale compared to the fully adjusted regression model. Self-identified race, noncontraceptive exogenous hormone use, history of breast cancer in at least one first degree relative, previous breast biopsy, and menopausal status were coded as binary variables. Age at menarche, total pretax household income for the calendar year prior to interview, highest education level completed, lifetime average alcohol consumption, pap screening history in the past 5 years, and mammographic screening history in past the 5 years were coded as nominal categorical variables. Age at first birth and number of children were coded as categorical variables and examined jointly with a binary variable indicating parity.32 The elimination of pap screening history changed the effect estimate for oral contraceptive use by more than 0.10 on the natural log scale compared to the full model that was adjusted for all potential confounders; thus, pap screening history was retained in the final regression model and adjusted for as a confounder. When we repeated the backwards elimination without including pap screening history in the fully adjusted model, no additional covariates were confounders. All estimates were also adjusted for the frequency matching factors, study site and age at the reference date.

Results

The distribution of breast cancer risk factors that were evaluated as confounders is shown in Table II. Although estimates are imprecise, the associations between lobular cancer and late age at first birth, high household income, postgraduate education, frequency of pap screening, and ever having mammographic screening are stronger than the associations between those same risk factors and ductal cancer (Table II).

Table II. Demographic and Reproductive Characteristics of Cases and Controls

	Controls (N = 1,501)		Lobular cases (N = 100)			Ductal cases (N = 1,164)
	N	%	N	%	OR1 (95% CI)2	N	%	OR1 (95% CI)2
1 OR, odds ratio, adjusted for study site and age at reference date. 2 95% CI, 95% confidence interval. 3 Parous women only. 4 Breast cancer in one or more first degree relatives. 5 Among women who reported ever having a pap smear. 6 Among women who reported ever having a mammogram.
Age at diagnosis (years)
20–24	9	1	0	0		4	0
25–29	57	4	2	2		34	3
30–34	225	15	6	6		141	12
35–39	474	32	19	19		362	31
40–44	736	49	73	73		623	54
Age at menarche (years)
11 or younger	348	23	23	23	1.31 (0.67–2.56)	278	24	1.16 (0.92–1.47)
12	402	27	32	32	1.52 (0.81–2.87)	364	31	1.31 (1.05–1.65)
13	444	30	30	30	1.35 (0.71–2.57)	313	27	1.04 (0.82–1.30)
14 or older	305	20	15	15	Referent	207	18	Referent
Missing	2
Parity
Nulliparous	335	22	20	20	Referent	297	26	Referent
Parous	1,165	78	80	80	0.83 (0.50–1.40)	866	74	0.77 (0.64–0.93)
Missing	1					1
Number of children3
1	297	25	19	24	Referent	242	28	Referent
2–3	742	64	58	73	1.11 (0.65–1.92)	563	65	0.92 (0.75–1.13)
4 or more	126	11	3	4	0.33 (0.10–1.16)	61	7	0.58 (0.41–0.82)
Age at first birth (years)3
<20	256	22	12	15	Referent	153	18	Referent
20–24.9	370	32	18	23	0.89 (0.42–1.91)	260	30	1.17 (0.90–1.51)
25–29.9	324	28	24	30	1.52 (0.73–3.13)	246	28	1.30 (1.00–1.69)
≥30	215	18	26	33	2.47 (1.21–5.07)	207	24	1.60 (1.21–2.12)
Non-contraceptive hormone use
No	1,358	91	91	91	Referent	1,094	94	Referent
Yes	139	9	9	9	0.83 (0.40–1.71)	68	6	0.56 (0.41–0.75)
Missing	4					2
Menopausal status
Pre-menopausal	1,310	88	88	89	Referent	1,044	90	Referent
Post-menopausal	186	12	11	11	0.69 (0.35–1.34)	119	10	0.72 (0.56–0.93)
Missing	5		1			1
Household income
≤$24K	267	18	8	8	Referent	212	19	Referent
$25K–$49K	522	36	28	29	1.61 (0.72–3.59)	359	31	0.85 (0.68–1.06)
$50K–$89K	473	32	42	43	2.45 (1.13–5.33)	402	35	1.04 (0.83–1.30)
≥$90K	203	14	20	20	2.42 (1.03–5.66)	171	15	1.00 (0.76–1.32)
Missing	36		2			20
Race
White	1,180	79	85	85	Referent	922	80	Referent
Non-white	319	21	15	15	0.71 (0.40–1.26)	236	20	0.92 (0.75–1.12)
Missing	2					6
Education level
High School or less	402	27	24	24	Referent	315	27	Referent
Technical School-College grad	898	60	53	53	1.10 (0.67–1.81)	675	58	0.98 (0.82–1.17)
Postgraduate work	201	13	23	23	1.87 (1.02–3.41)	174	15	1.09 (0.85–1.40)
Family history of breast cancer4
None	1,406	94	85	85	Referent	1,000	86	Referent
1 or more relatives	95	6	15	15	2.41 (1.33–4.36)	164	14	2.42 (1.86–3.16)
Ever had a pap smear
No	34	2	0	0		11	1	Referent
Yes	1,465	98	100	100		1,153	99	2.38 (1.20–4.73)
Missing	2
Number of pap smears in the past 5 years5
0–3	541	37	22	22	Referent	372	32	Referent
4–5	755	52	71	71	2.53 (1.54–4.15)	682	59	1.33 (1.13–1.58)
≥6	169	12	7	7	1.36 (0.56–3.27)	97	8	0.87 (0.66–1.17)
Ever had a mammogram
No	771	51	26	26	Referent	518	45	Referent
Yes	729	49	74	74	2.17 (1.34–3.53)	646	55	1.23 (1.04–1.45)
Missing	1
Number of mammograms in the past 5 years6
0–1	424	58	23	31	0.23 (0.12–0.45)	290	45	0.37 (0.27–0.52)
2–3	231	32	31	42	0.50 (0.27–0.94)	225	35	0.55 (0.39–0.77)
≥4	72	10	20	27	Referent	129	20	Referent
Missing	2					2
Alcohol use (average drinks per week)
0	576	39	30	30	Referent	400	34	Referent
<1–6.9	754	50	63	63	1.67 (1.06–2.63)	587	51	1.14 (0.96–1.35)
≥7.0	167	11	7	7	0.94 (0.40–2.19)	175	15	1.54 (1.20–1.98)
Missing	4					2
Previous breast biopsy
No	1,406	94	85	85	Referent	1,056	91	Referent
Yes	95	6	15	15	2.27 (1.25–4.12)	108	9	1.44 (1.08–1.92)

The ORs for ever use of oral contraceptives, adjusted for age and study site only (without adjustment for pap screening history), are 1.27 (95% CI = 0.79, 2.05) for lobular breast cancer and 1.27 (95% CI = 1.07, 1.52) for ductal breast cancer. The estimated effects of different patterns of oral contraceptive use on the relative odds of ductal and lobular cancer, adjusted for age, study site, and history of pap screening, are shown in Table III. Ever use of oral contraceptives is associated with a slightly elevated OR for ductal breast cancer (OR = 1.21, 95% CI = 1.01–1.45). Recent oral contraceptive use is more strongly associated with ductal cancer than former use (ductal, recent use: OR = 1.45, 95% CI = 1.08–1.96 vs. ductal, former use: OR = 1.17, 95% CI = 0.97–1.41). A shorter time interval since last use of oral contraceptives, extended use of oral contraceptives, and later age at first use were also associated with increased odds of ductal cancer (Table III). The association between oral contraceptive ever use and lobular cancer (OR = 1.10, 95% CI = 0.68–1.78) is similar to the association for ductal cancer but is not statistically significant. The association between lobular cancer and patterns of oral contraceptive use are shown in Table III, however, because of the small number of lobular cases in this analysis, many of the estimates are imprecise and no conclusions can be made about trends by recency, duration or age of first oral contraceptive use.

Table III. Odds Ratios for the Association Between Breast Cancer and Oral Contraceptive use, by Histological Type

Oral contraceptive use	Controls		Lobular cases			Ductal cases
	N	%	N	%	OR1 (95% CI)2	N	%	OR1 (95% CI)2
1 Odds ratio, adjusted for study site, age at reference date, and frequency of pap screening. 2 95% confidence interval. 3 Never use, <6 months of reported oral contraceptive use. 4 Former user, last use was >24 months ago. 5 Recent user, last use was ≤24 months ago.
Never3	425	28	25	25	Referent	276	24	Referent
Ever	1,076	72	75	75	1.10 (0.68, 1.78)	888	76	1.21 (1.01, 1.45)
Former4	911	85	73	97	1.16 (0.72, 1.89)	750	84	1.17 (0.97, 1.41)
Recent5	165	15	2	3	0.33 (0.08, 1.47)	138	16	1.45 (1.08, 1.96)
Age at first use (years)
<20	557	52	30	40	1.01 (0.57, 1.78)	433	49	1.17 (0.95, 1.44)
20–24	448	42	36	48	1.06 (0.61, 1.81)	380	43	1.19 (0.97, 1.48)
≥25	71	7	9	12	1.96 (0.87, 4.44)	75	8	1.56 (1.08, 2.24)
p-trend					0.31			0.02
Time since last use (years)
0–4	219	20	4	5	0.45 (0.15, 1.36)	194	22	1.55 (1.18, 2.03)
5–9	165	15	9	12	0.99 (0.45, 2.22)	134	15	1.27 (0.95, 1.68)
≥10	692	64	62	83	1.21 (0.74, 1.99)	560	63	1.11 (0.91, 1.36)
p-trend					0.29			0.63
Duration of use (years)
<1	91	8	9	12	1.63 (0.72, 3.65)	70	8	1.13 (0.80, 1.61)
1–3	395	37	31	41	1.23 (0.70, 2.14)	295	33	1.11 (0.89, 1.38)
≥4	590	55	35	47	0.92 (0.53, 1.59)	523	59	1.30 (1.06, 1.59)
p-trend					0.69			0.01

We explored whether the association between oral contraceptive ever use and breast cancer differed by stage among lobular and ductal subtypes, although our estimates are based on small numbers. Among the lobular cases, the effect of ever using oral contraceptives was higher for the in situ cases (LCIS, ever use: OR = 1.60, 95% CI = 0.72–3.54; invasive lobular, ever use: OR = 0.86, 95% CI = 0.47–1.56) (Table IV). Among the ductal cases, the effect of ever using oral contraceptives was higher for the invasive cases (DCIS, ever use: OR = 0.75, 95% CI = 0.51–1.10; invasive ductal, ever use: OR = 1.30, 95% CI = 1.07–1.58) (Table IV).

Table IV. Odds Ratios for the Association Between Breast Cancer and Ever use of Oral Contraceptives, by Histology and Stage

Oral contraceptive use	Controls		Lobular			Ductal
	N	%	N	%	OR1 (95% CI)2	N	%	OR1 (95% CI)2
1 Odds ratio, adjusted for study site, age at reference date, and frequency of pap screening. 2 95% confidence interval. 3 Never use, <6 months of reported oral contraceptive use.
In situ
Never3	425	28	8	19	Referent	44	29	Referent
Ever	1,076	72	35	81	1.60 (0.72, 3.54)	108	71	0.75 (0.51, 1.10)
Invasive
Never3	425	28	17	30	Referent	232	23	Referent
Ever	1,076	72	40	70	0.86 (0.47, 1.56)	780	77	1.30 (1.07, 1.58)

Discussion

The aim of this analysis was to investigate the effect of oral contraceptive use on specific breast cancer histologies in a population of young women. We first reported an association between oral contraceptive use and breast cancer in the WISH population in 1995.22 Our current results suggest that the effect of oral contraceptive ever use is similar for ductal and lobular breast cancer. However, the estimated effect for lobular cancer is imprecise because of the small number of lobular cases in this population. Among ductal cancers, we were also able to estimate increased odds of breast cancer with recent use, shorter time since last use, and long duration of oral contraceptive use. As expected, our estimated association between oral contraceptive use and ductal breast cancer is similar to previously reported positive associations between breast cancer and current use, more recent use, and long duration of use of oral contraceptives13, 14, 16, 17, 18, 20, 21, 23, 24, 33, 34, 35; this is likely because ductal cancers make up approximately 75% of all invasive breast tumors diagnosed in women under age 50.36

Previous studies have addressed the effect of oral contraceptive use by breast cancer histology, but among study subjects with a wide age range. Analyzing younger and older women together could mask heterogeneity of effects due to age, since previous age-specific analyses of the association between oral contraceptive use and breast cancer (regardless of histologic type) have observed variation by age.17, 21, 23, 24, 25 Romieu et al.18 reported that there was little difference in the effect of oral contraceptive use among different breast cancer histologies, but the authors did not report the effect estimates for the different histologies. Newcomer et al.26 reported ORs of 1.2 (95% CI = 0.9–1.6) for the association between oral contraceptive ever use and invasive lobular cancer and 1.0 (95% CI = 0.9–1.1) for the association between oral contraceptive ever use and invasive ductal cancer. Despite the younger age of our study subjects, our results are compatible with those of Romieu et al.18 and Newcomer et al.26, in that the association with oral contraceptive use did not differ strongly for ductal and lobular cancer.

Previous studies that have considered differences in the effect of oral contraceptive use on histologic subtypes of breast carcinoma in situ have found a negative association with LCIS among women 20–79,37 no association with DCIS among premenopausal women,38 and no association with any type of breast carcinoma in situ among recently screened women 35–64 years old.39 Although we determined that mammographic screening was not a confounder in our data, it is possible that the confounding effect of mammography differs in populations where screening is more prevalent. This is especially relevant to the study of in situ lesions, which are detected primarily as a consequence of mammographic screening. Given that many of the women in the WISH study population had not yet reached the recommended age for mammographic screening, our positive association for LCIS and negative association for DCIS may not be comparable to results from populations where more women are of screening age. Our findings by stage may be attributed to random variation due to small sample sizes, and thus additional studies of oral contraceptive use and DCIS and LCIS are needed. A more precise estimation of the effect of oral contraceptive use on in situ breast cancer may clarify whether there is heterogeneity of effect among in situ subtypes.

The effect of oral contraceptives on incident breast cancer is small (i.e., close to the null), and so it is possible that even small amounts of residual confounding or other biases could distort the true effect. We used backwards elimination to select confounders to identify joint confounding by 2 or more covariates. Using this method, only pap screening history was a confounder in this dataset. However, to be a confounder pap screening must be a risk factor for oral contraceptive use and not affected by use.40 Because it is possible that some women receive pap screening for the purpose of obtaining an oral contraceptive prescription, making pap screening the result of oral contraceptive use, pap screening may or may not meet the criteria of a confounder. To ensure that our results would not be affected by adjusting for a variable whose role as a confounder is unclear, we repeated the backwards elimination procedure without pap screening in the model. No other covariates met our statistical criteria for confounding, suggesting that the inclusion of pap screening in our initial model did not mask confounding by other covariates. Furthermore, there was no substantial difference in the effect of oral contraceptives on lobular and ductal breast cancer when we did not adjust for pap screening.

The small number of lobular cases in our study limits our power to detect effects associated with detailed patterns of use. Although the primary aim of the WISH study was to examine breast cancer risk in younger women in relation to patterns of oral contraceptive use,22 it was not designed specifically to evaluate hypotheses about lobular breast cancer. Lobular cancers are a small proportion of all breast cancer cases diagnosed, and the mean age at diagnosis for lobular breast cancer is older than the mean age at diagnosis for ductal cancer,36 and so it was expected that there would be a small number of lobular cases in the WISH population. Because of the small number of lobular cases, it is possible that some effect estimates do not reflect the true association of oral contraceptive use, and instead may be the result of random error. A further limitation was our inability to estimate the association separately in white and African American women, because of both the small number of lobular cases and small proportion of African American participants. Therefore, additional research in younger women is needed to confirm our results. In particular, future studies should be designed to have adequate numbers of women with lobular subtype so that the effect of different patterns of oral contraceptive use on in situ and invasive breast cancer can be analyzed in addition to ever use of oral contraceptives.

Despite the imprecision of the lobular results, by focusing specifically on young women our analysis provides information that has not yet been reported by previously published studies. Additionally, the WISH study design limits the chance that the results are biased due to selection bias or disease misclassification. Study subjects were accrued from areas with high quality, population-based cancer registries that operated with a standard protocol for case ascertainment and data reporting, so case identification procedures were consistent across the study centers. Furthermore, previous studies have shown that women are able to recall past oral contraceptive use with a high degree of accuracy,41, 42 and that recall is not differential when exposure information is collected after a breast cancer diagnosis.43 Misclassification of oral contraceptive use was further minimized by using memory aids that have been shown to enhance recall of oral contraceptive use.44 Oral contraceptive information was collected after informed consent, and the interviewer was blind to case status. Thus, any potential misclassification of oral contraceptive use should be independent of case status. Histology and stage data were abstracted directly from patient records, so there is potential for misclassification of histological data or inconsistency of diagnosis between study sites. However, studies of diagnostic inter-observer reproducibility have shown good agreement among pathologists in distinguishing breast lesions' stage and histologic subtype.45, 46, 47

In conclusion, our study adds to the available data characterizing breast cancer in young women. Although our estimates are imprecise, our data suggest that the relationship between oral contraceptive use and lobular and ductal cancers is similar in young women, as was reported previously for older women by Romieu et al.18 and Newcomer et al.26 Several approaches, including histological and molecular subtyping, have been used to define the etiologic heterogeneity of breast cancer. However, the knowledge that lobular cancer is increasing emphasizes the promise of continued research on the relationship between risk factors and specific breast cancer histologic subtypes. Future research could aid in determining whether different breast cancer histologic subtypes have different etiologies, and may identify factors that contribute to the increasing incidence rates of lobular cancer.