Objectives: Familial cancer risks were studied in offspring whose parents had a similar (concordant) or a different (discordant) cancer in order to assess the modification of cancer risks from one generation to another.
Methods: We used the nation-wide Swedish Family-Cancer Database to calculate familial rate ratios (FRRs) to the offspring when their parents had concordant and discordant cancers. Cancer sites were included if there was at least one pair of parents with the same cancer. In situ cancers were included in a separate analysis in order to increase the numbers of cases.
Results: The risk of colon, all bowel, lung and breast cancer and melanoma increased 1.1-1.2 times when one parent and 1.3-1.6 times when two parents had any discordant cancer, suggesting involvement of environmental and hereditary effects shared by many forms of cancer. When both parents had colon cancer or melanoma, the respective risks in the offspring were 3.0 and 9.3 but only based on single triplets. For all bowel cancer the risk was 3.4, approximately multiplicative from the familial one parent-offspring risk. For concordant lung and breast cancer triplets the risk in offspring was 11.8 and 29.4, respectively.
Conclusions: Even discordant cancer in parents increased cancer risk in offspring. This may be due to environmental and hereditary causes, and deserves consideration in epidemiological studies. The high risks in families where both parents had the same cancer suggest interactions of hereditary and environmental factors.