BMC Cancer (2001) 1:8 http://www.biomedcentral.com/1471-2407/1/8 Background Many organochlorines have become ubiquitous in the environment and in the human body due to their resist- ance to degradation and their high solubility in lipids, which leads to accumulation in adipose tissue. Some of these compounds are suspected of disrupting the endo- crine system and thereby increasing the risk of hormone- dependent disorders and diseases such as breast cancer [1–3]. Epidemiological studies indicate that total lifetime exposure to estrogen predict the risk of breast cancer [4,5]. Estrogen also plays an important role for the prog- nosis of breast cancer as treatment with the anti-estro- genic drug tamoxifen improves the survival of postmenopausal women with estrogen receptor positive (ERP) breast cancer [6]. Furthermore, estrogen stimu- lates proliferation of ERP breast cell lines and may there- for be associated with ERP human breast cancers only [7]. Previous studies of the potential effect of estrogenic or- ganochlorines on breast cancer risk have yield inconsist- ent results [8–23] and only a few has taken into account estrogen receptor status (ER) [16,17,19]. In a small Cana- dian study women with ERP tumors, but not those with estrogen receptor negative (ERN) tumors, had a higher DDE and PCB body burden compared to women with be- nign breast disease [16]. An excess risk of developing an ERP breast cancer associated with exposure to certain PCB congener was also demonstrated in a study conduct- ed in Sweden [19]. In the so far only study on breast cancer survival, an ad- verse effect of exposure to dieldrin was identified, but ER was not obtained at that stage [24]. It has been postulated that ERP and ERN are two differ- ent entities of breast cancer having different biological and etiological mechanisms. Thus, distinguishing be- tween breast cancer which growth is dependent on estro- gen and those which is not i.e. ERP and ERN tumors, may have implications for studying potential estrogenic organochlorines possible influence on breast cancer risk and survival. Presented in this paper are results of anal- ysis on breast cancer risk and overall survival while ac- counting for ER of the primary tumors. To avoid problems with multiple comparisons, the anal- yses were restricted to organochlorines with estrogenic properties or compounds that are indicators of previous exposure to estrogenic ones. Materiel and methods The background material constitutes the Copenhagen City Heart Study (CCHS), which in 1976-78 enrolled a random sample of 10,317 women living within 10 wards of the National University Hospital of Copenhagen, Den- mark. This study was designed as a cohort-nested case- control study, where the case group consisted of 268 women between 25 and 80 years of age diagnosed with primary breast cancer between initiation of the CCHS and end of follow up in 1993. They were identified through computerized linkage to the Danish Cancer Reg- istry using the unique 10-digit ID-number issued to each resident in Denmark. As controls, a random sample of 536 women matched on age, and vital statistics was se- lected from the remainder of the CCHS cohort. Informa- tion on lifestyle factors, reproductive history, and socioeconomic conditions was obtained through stand- ardized questionnaires in connection with a physical ex- amination, which included body weight, and non-fasting blood samples. The serum was analyzed for cholesterol, triglycerides, and the remaining volume frozen. After the 17 years of follow up stored serum was retrieved from 240 (89.5%) cases and 477 (89.0%) of the matched con- trols. Information on vital statistics, date of death until July 31 1997, breast cancer characteristics, and adjuvant treat- ment, were achieved through linkage to the Civil Regis- tration System, the Causes of Death Registry, and the nation-wide clinical breast cancer trial conducted by the Danish Breast Cancer Co-operative Group. The latter was also able to provide information on where each woman diagnosed with breast cancer was treated. This made it possible to contact the pathology departments where the tumor tissue had been reviewed. Paraffin em- bedded tumor tissue was requested for estrogen receptor status determination immunohistochemically by micro- wave antigen retrieval, a mouse monoclonal anti-estro- gen receptor antibody ER1D5, and the streptavidin- biotin detection system [25]. The threshold for designa- tion of estrogen receptor positivity was staining of >= 10% of the cell nuclei in the specimen. Breast cancer characteristics included tumor size, degree of spread, and stage of disease. Patients who were de- fined as having received adjuvant therapy with tamoxifen include those treated solely with tamoxifen, and those who had a combination therapy including tamoxifen. The stored serum was analyzed for potential estrogenic organochlorine compounds and their metabolites by the U.S. Center of Disease Control and Prevention in Atlan- ta: Hexachlorobenzene (HCB), dieldrin, polychlorinated biphenyls (ΣPCB calculated as the sum of following con- geners: IUPAC numbers 28, 52, 56, 66, 74, 99, 101, 105, 110, 118, 138, 146, 153, 156, 170, 172, 177, 178, 180, 183, 187, 189, 193, 194, 195, 201, 203, and 206), and p,p'- DDE. The analytical technique involves a two-stage sol-