Does Breast Cancer Increase Colorectal Cancer Risk?

Every year, over 264,000 women and 2,400 men in the United States are diagnosed with breast cancer. Given that the disease primarily impacts women, breast cancer is often discussed in the context of other predominantly women–centric cancers such as uterine and ovarian cancer. For one, there are many common risk factors, including changing estrogen and progesterone levels as well as mutations in BRCA1 and BRCA2 genes.

However, there are other cancers that are also frequently associated with breast cancer, one of which is colorectal cancer. Rather than looking at it from a metastatic point of view, breast cancer patients who have received treatment and since recovered are often at risk of developing colorectal cancer as a second primary malignancy (SPM).

Many may wonder how this is possible, given that the two organs are not in relative proximity to each other. This article draws connections between breast and colorectal cancer, and how breast cancer increases the risk of developing a second cancer even after treatment is successfully administered.

Breast cancer is a disease where tumors form in the breast when the cells grow uncontrollably. The parts of the breast where cancer can form are in the lobules, ducts or connective tissue. The lobules are the glands that produce breast milk, while the ducts are the tubes through which milk is secreted. Connective tissue in the breast comprises mainly fibrous and fatty tissue that holds everything together.

Breast cancer mainly occurs in two forms — invasive ductal carcinoma that forms in the ducts and spreads to other breast tissue; and invasive lobular carcinoma that originates from the lobules.

Patients who have been treated for breast cancer should be wary of the possibility of developing colorectal cancer as a second cancer due to a number of risk factors the cancers have in common.

For instance, obesity is a key exacerbator of carcinogenic growth in both the breast and colon, and high estrogen levels are also known to promote tumor growth.

Statistically, studies have found that women who have a previous breast cancer diagnosis are more likely to develop colorectal carcinoma compared to the general population. This is further supported by the higher rate of colorectal adenoma (or precancerous polyps) in former breast cancer patients, with the detection rate increasing from 21% in women below 40 to 54% in women between 51 and 60 years.

Obesity, estrogen and tumorigenesis

Like the risk factors of other cancers, the mechanisms of tumorigenesis of obesity and estrogen levels are not unrelated. While in colorectal cancer the primary mechanism through which high body fat is thought to promote cancerous growth revolves around high insulin/insulin-like growth factor 1 (IGF-1) and inflammatory cytokine levels, fat tissue is also known to produce excess amounts of estrogen.

Estrogen is a naturally occurring hormone that is crucial to sexual and reproductive health and development, but it can also become carcinogenic at elevated levels. Tumorigenesis largely depends on the presence or absence of two types of estrogen receptors (ER), ERα and ERβ. High levels of ERα promote cancer growth and progression due to its role as an oncogene, and conversely the lack of expression of tumor suppressor ERβ is also associated with uncontrolled cellular growth.

In breast cancer, tumors can be classified as ER-positive or negative, and ERα positive tumors make up about 75% of breast cancer cancers. Meanwhile, advanced colon cancer is associated with a loss of ERβ, the predominant ER in colon tissue. This closely associated relationship between estrogen and estrogen receptors with breast and colorectal cancer is the prime suspect in the development of colorectal cancer as an SPM after breast cancer.

Even though the mechanisms through which breast cancer increases colorectal cancer risk are probable and require further corroborative research, the close association nonetheless has implications on existing and recovering breast cancer patients.

As colorectal cancer treatment success has higher success rates the earlier it is detected, screening colonoscopy is recommended within five years of the initial breast cancer diagnosis or by age 40, whichever is sooner, especially for patients who have been diagnosed with breast cancer before age 40. For older patients, a screening colonoscopy is recommended within the first year of breast cancer diagnosis, as the risk of colorectal cancer also increases with age.

Above all, breast cancer patients should also be well-informed of the risks that follow their breast cancer diagnosis, so that cancer surveillance plans can be put in place to ensure that adenomas or tumors are detected and treated at the earliest possible interval.