The Swedish Family-Cancer Database was used to analyze relationships between parents and offspring with in situ cancers and between in situ cancers in one generation and invasive cancer in the other generation. A total of 130,000 in situ cancers and close to 400,000 invasive cancers were included from 1959 to 1994. The data on family relationships and cancers came from registered sources and should be free from bias. The offsprings' familial relative risks (FRRs) were calculated for concordant and discordant parental cancer sites. The most common male in situ site was skin (both melanoma and precancerous epithelial lesion), whereas cervix, breast, and skin were common female sites. Increased FRRs were observed for concordant sites: colon, breast, cervix, skin (melanoma), and, in males, precancerous epithelial lesions. The findings were consistent when in situ cancer-in situ cancer and in situ cancer-invasive cancer relationships were explored. FRRs were higher for in situ colon cancer and melanoma than the respective estimates in invasive cancers, and for the remaining sites, they were equal or somewhat lower. At discordant sites, increased FRRs of in situ cancers were observed for female breast and melanoma and, at many sites, implicated in tobacco and human papilloma virus carcinogenesis, together with cervix. Family histories of in situ cancers deserve clinical attention.

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