An examination of 3,450 cases found that current smokers were diagnosed with colon cancer approximately seven years earlier than people who never smoked. The study is also one of the first to link exposure to second-hand smoke, especially early in life, with a younger age for colon cancer onset.

The article appears online in the Journal of Cancer Research and Clinical Oncology.

The message for physicians and patients is clear: When making decisions about colon cancer screening you should take into account smoking history as well as family history of disease and age, said lead author Luke J. Peppone, Ph.D., research assistant professor of Radiation Oncology at the James P. Wilmot Cancer Center at the University of Rochester.

Peppone's group examined data from patients diagnosed with colorectal cancer between 1957 and 1997 at Roswell Park Cancer Institute in Buffalo. (Peppone joined the University of Rochester in 2007, coming from RPCI. Co-authors are from RPCI.)

Over the 40-year period smoking habits changed, with a decrease in the percentage of current or active smokers and an increase in the percentage of former smokers. Still, the age at colon cancer diagnosis was 6.8 years younger among current smokers and 4.3 years younger for former smokers who quit less than five years ago, the results showed. People who quit more than five years ago had no significant increased risk.

However, people who reported they began smoking as young teens (before age 17) or who smoked heavily (1 pack a day or more) were the most likely to be diagnosed with cancer much younger than their never-smoking counterparts. Past exposure to second-hand-smoke was an additional, significant risk factor, compared to never smoking. In fact, when active smokers and passive smoking were combined into one subgroup, the age at cancer diagnosis was nearly 10 years earlier, Peppone said.

Although smoking is a well-known risk factor for many cancers, only recent studies have suggested that cigarettes may cause colon cancer.

The biological reasons for the cigarette smoke-colon cancer risk are unclear. However, researchers believe that cigarette smoke reduces the body's resistance to malignancies, just as smoking can depress immune function in general, impairing the ability to fight off infections and viruses. Carcinogens from smoke reach the bowel through direct circulation or by swallowing smoke and passing it through the intestines.

Colorectal cancer is the third most commonly diagnosed cancer among men and women. Genetics account for about 10 percent of new cases, the study said, while more than 75 percent of the cases arise from sporadic mutations and/or environmental and lifestyle factors such as smoking, a poor diet, alcohol use, lack of exercise and obesity.

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