Most Cancers Are Not Under Our Control: Important Study

Some things can certainly help us to avoid cancer, if we are careful to avoid smoking, getting exposed to the sun or inhaling polluted air or food. Heredity too can be the reason for about 5% to 10% of cancer.

However, there is no accounting for some tumours, which just grow for no reason. How many of these tumours are malignant? And what is the reason behind that kind of cancer?

Christian Tomasetti and Bert Vogelstein at Johns Hopkins University published a new revelatory paper in Science, which shows that a tissue's stem cells are most of the time responsible for two-thirds of the difference in cancer risk among different tissues.

Most of the tissues in our body have stem cells, or feeder cells or factories manufacturing many more cells of the same type. It is a process that keeps the body's cells "fresh", even as it makes the blood and immune cells dynamic. It is a "replicative power" that drives the body's moves and allows tissues to replace the dying cells.

However, it is the way in which cancer is formed, making the cells pick up mutations in the DNA after they divide. Stem cells copy their DNA and divide to make more such cells. Only a small proportion of a tissue's cells are made up of stem cells, so Tomasetti and Vogelstein examined whether the number of stem cells in a specific tissue would impact its tendency to develop cancer.

While charting out the stem cell data for 31 types of tissues, they found that there is a dramatic link between the number of stem cells in a tissue and its incidence of cancer in a person's life.

"Think of cancer as the risk of having an accident if you are driving a car," says Tomasetti, a biostatistician in the department of oncology at Johns Hopkins Kimmel Cancer Center and the Johns Hopkins Bloomberg School of Public Health. "If you drive the car on a cross country trip, your risk of an accident is much higher than if you take a local trip to the grocery store. The risk correlates to the length of the trip. The trip to the grocery store might be thought of as bone cancer, which has few stem cell divisions. While the cross country trip might be more like colon cancer, which has many more cell divisions."

The link among cancers both common as well as rare was clear too. When it was more likely that the cells could divide and develop DNA errors or mutations, leading to uncontrolled growth, it was more likely that the tissue would develop tumors.

Tomasetti called it a surprising revelation, and pointed out that about 65% of cancer across tissue types could be explained by stem cell divisions. A detailed understanding of both how large a tissue's stem cell population is, as well as how active it is, could finalise whether it would develop cancer or not.

For instance, both the brain as well as the colon cells can cause glioblastoma and medulloblastoma. The colon contains about one hundred million cells. Yet the colon stem cells divide about 6,000 times on an average during lifetime, compared to nearly zero for the brain stem cells. Hence, the rate of colon cancer is 22 times higher than the rates of the brain tumors.

This leads to an explanation as to why people who don't smoke still end up with lung cancer, why rates of colon cancer outrun rates of cancer in the small intestine, though they are shorter. Tomasetti says that they could be due to the different stem cell activity in these tissues.

This discovery has changed the map for cancer. Recently, cancer rates declined as efforts were made to motivate people to avoid cancer by giving up smoking or keeping away from ultraviolet rays. However, it is also important to change the strategy to fight cancer. Tomasetti and Vogelstein have decided to take a look at cancers that could be due to genetic bad luck, or due to environmental or hereditary factors.

Hence, melanoma, ovarian cancer, brain cancers, lung cancer among non-smokers, the most common leukemias and bone cancers, for example, are simply not in your control. They could be caused by random mutations due to stem cell divisions in the bone, blood, ovaries, brain and skin. All these could make some mistakes that make them malignant. For such cancers, a change in the lifestyle or trying to prevent the cancer may not help. But being vigilant about screening, and understanding the first few signs of trouble quickly can help to save lives.

However, cancer due to heredity or exposure to smoking or exposure to radiation and carcinogens is still valid. Hence, it is important to prevent them by taking precautions and being careful about actions that could lead to cancer.

 "Everything we know about altering lifestyles to prevent cancer from the environmental point of view we absolutely need to continue doing," says Tomasetti. "If anything it puts more stress on the need to spend even more money on early detection. It may be the key tool for quite a few cancer types."

Two common cancers that are still not clear are breast cancer and prostate cancer, as the scientists are still not clear about their stem cell populations, and their renewal, although they are working on that.

Meanwhile, though we cannot prevent the tumors, we can still treat them if they are detected early and removed through chemotherapy or radiation. "My biggest fear is that people will say forget about it, and then do nothing. The opposite is true. We need to do everything we did before, but we want to do it even more than before," says Tomasetti.