M.D. Family analyses are always interesting in the study of cancer. Most cancers are not commonly felt to be familial. There are certain exceptions of course like familial colon cancer. This is not however related to the more common colon cancers seen. Patients commonly ask about the role of the environment versus genetics in the development of malignancy.
Studies of twins are an interesting starting point.
There have been family studies of breast, prostate, ovarian and uterine cancers. It has been difficult to distinguish genetic affects versus non-genetic such as environmental or infectious. A study of twins allows physicians to better examine hereditary factors but the relative rarity of twins makes cancer analysis difficult. For non-twins, this is a chance to learn. Certainly, we don't share all our siblings' diseases.