A large-scale study on women with breast cancer found a 25 percent increase in the risk of developing a new non-breast cancer compared to women without cancer.
Breast cancer is the most common cancer among women in developed countries. Due to the high five-year survival rate ( 77 percent ), a considerable number of women have a long-term risk of developing a second cancer. Previous studies have shown a 20-30 percent increased risk for a second cancer in various sites, including the endometrium, ovary, thyroid, lung, soft tissue, blood, skin, stomach and colon, with higher risks among younger patients.
Researchers led by Lene Mellemkjær of the Danish Cancer Society in Copenhagen, Denmark conducted a huge study involving 525,527 patients with breast cancer in 13 cancer registries in Europe, Canada, Australia and Singapore.
Records were analyzed for second primary cancers during the period 1943 to 2000, with 133,414 women followed for more than 10 years after the initial diagnosis of breast cancer.
The increased risk of a second cancer was seen in many different sites, as shown in earlier studies.
" The excess of cancer after a breast cancer diagnosis is likely to be explained by treatment for breast cancer and by shared genetic or environmental risk factors although the general excess of cancer suggests that there may be additional explanations such as increased surveillance and general cancer susceptibility," the authors note.
The study found an almost 6-fold increase in the risk of cancer in connective tissue of the thorax and upper limbs, which suggests that radiation therapy, which has been used to treat breast cancer since the beginning of the 20th century, may play a role in developing a second cancer in organs close to the breast.
An increased risk of myeloid leukemia was also shown, possibly as a result of chemotherapy.
In addition, while previous studies had shown an increased risk of endometrial cancer with the use of Tamoxifen, the current study suggests that this may not be entirely due to the drug, since the increased risk was already shown within one year of breast cancer diagnosis, it was shown before 1975 when Tamoxifen was rarely used, and an increased risk of breast cancer was also seen after endometrial cancer.
Colorectal, kidney and postmenopausal breast cancer appear to share obesity as a risk factor, while ovarian cancer and breast cancer seem to have a genetic predisposition in common.
The study found an excess of ovarian cancer already within one year of breast cancer diagnosis, along with an increased risk of breast cancer after ovarian cancer.
" The overall impression from this very large study is that a breast cancer diagnosis has an effect on subsequent cancer risk in general, since so many cancer sites were seen to occur in excess of what was expected," the authors conclude. " The known effects of treatment and common risk factors do not seem to fully explain the excesses."
Source: International Journal of Cancer, 2005