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Background: Breast cancer is the leading cause of cancer-related death for women in Belarus. The risk of breast cancer after an earlier primary cancer, as well as the risk of developing multiple primaries after an earlier breast cancer was studied. Methods: The retrospective cohort study used a cohort consisting of 643693 cancer cases diagnosed between 1990 and 2007. Cases were identified from records of the Belorussian National Cancer Registry and followed for breast cancer development through 2007. Proportions and Standardized Incidence Ratios (SIR) of synchronous (latency between diagnosis's less than a year) and metachronous primary multiple breast cancers (PMBC) were investigated. It was considered 3070 PMBC (898 synchronous and 2172 metachronous with first tumor in breast).
Results: More often synchronous breast cancer combines with tumors of breast, skin and corpus uteri. Significantly high difference between observed and hypnotically expected (on the base of population incidence level) numbers was noted for all synchronous PMBC and for secondary cancers of breast, kidney, thyroid, ovary, skin melanoma. The highest risk of synchronous PMBC was established for combination with salivary glands (SIR ¼ 5,19; 95%CI 1,07-15,16) and breast tumors (SIR ¼ 5,14; 95%CI 4,52-5,74). Metachronous PMBC with first breast cancer was more frequently noted with tumors of skin, breast and corpus uteri. The highest risk of metachronous PMBC was established for combination with esophagus (SIR ¼ 3,17; 95%CI 1,45-6,01). Conclusions: High risk of PMBC could be caused by common etiological factors. Low rates (as for cervix uteri and liver cancer) could give evidence that breast cancer treatment decreases the risk of secondary tumors.
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