Although published reports of cancer in twins are not numerous, there are case reports of concordance in a variety of tumors in twins. Cancer of the breast is most frequent; concordance has also been noted in uterus, gonads, eyes, stomach and rectum. In general, these reports tend to support the theory that genetic factors operate either in the concordance of cancer or in the site of the specific tumors. The significance of concordance remains in doubt, however. Reported study groups are not comparable because the materials and methods are disparate. In many reviews the methods of zygosity determination are questionable. The fact that not all cases of cancer in twins are reported introduces a constant source of bias. The following review illustrates this well.

In 1940, Madge T. Macklin reviewed the publications of tumors in MZ and DZ twins and concluded that tumors affected MZ twins far more than both members of a DZ pair. She observed concordance in tumor type, site, and age of onset, as more frequent in MZ than DZ twins. She later (1947) reexamined the same material and some additional twin pairs and concluded that MZ twins have identical tumors far more frequently than DZ twins. She emphasized that rare types of tumors were genetically determined.

In 1948, Busk et al reported on a series of 185 twin study pairs from the Danish Cancer Registry; they concluded that (1) there was a tendency toward a higher incidence of cancer in partners of MZ than of DZ cancerous twins; (2) these deviations from expected values were not considered statistically significant; and (3) there was a tendency for tumors in MZ twins to affect corresponding organs in both partners, this being not the case in DZ twins.