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Active and Passive Smoking and Risk of Colorectal Cancer

Institution: Cancer Prevention Institute of California
Investigator(s): Peggy Reynolds, Ph.D.
Award Cycle: 2009 (Cycle 18) Grant #: 18XT-0108 Award: $323,850
Subject Area: Epidemiology
Award Type: Exploratory/Developmental Award

Initial Award Abstract
Colorectal cancer is the second leading cause of cancer death in the United States. While less common than either prostate or breast cancer, colorectal cancer survival rates are substantially worse, primarily due to late stage at diagnosis. Given that the majority of patients with colorectal cancer do not have a family history of the disease, it has been suggested that creating more targeted screening strategies that take into account lifestyle-related risk factors could increase early diagnoses and ultimately improve survival rates. The contribution of tobacco exposures in the etiology of colorectal cancer is controversial. Early studies of active smoking generally reported no association. Over the last decade, however, evidence has emerged suggesting elevated risks of colorectal cancer associated with active smoking only after a very long induction period. Furthermore, there is evidence suggesting that the effects of active smoking may differ by anatomic site (rectal vs. colon) and subsite within the colon, as well as by sex. Meanwhile, the relationship of passive smoking exposures to colorectal cancer risk remains virtually unexplored. Elucidating the potential role of tobacco exposures in the development of colorectal cancer offers the opportunity to identify a potentially modifiable risk factor, provide new insights into the causes of this disease, and may help inform colorectal cancer screening guidelines to reduce colorectal cancer mortality.

This study takes advantage of the enormous data resources available for the California Teachers Study (CTS) cohort. Comprised of over 130,000 women residing throughout the state of California for whom extensive lifetime histories of active and passive smoking exposures have been collected, the CTS offers an unparalleled opportunity to evaluate the relationship between passive and active smoking and colorectal cancer. We propose to use the highly-detailed information on active and passive smoking collected for the CTS cohort, in conjunction with approximately 900 cases of colorectal cancer diagnosed in the 12 years since its inception, to examine the risk of colorectal cancer associated with active and passive smoking exposures. We will use statistical models to estimate these risks after adjusting for other factors known to cause colorectal cancer. Our evaluation of active and passive smoking will include measures of dose, duration, age at initiation, years since cessation (among former smokers), and summary measures that incorporate timing, duration and dose into a single metric. Risks will be estimated for all colorectal cancers, and separately for cancers of the rectum, colon, and sub-sites within the colon. These analyses will address some of the critical gaps in our understanding of the role of active smoking in colorectal cancer etiology and will be the largest, most comprehensive analysis of passive smoking exposures and colorectal cancer conducted to date.

It is estimated that in 2008 approximately 50,000 people will die from colorectal cancer. If caught early, colorectal cancer has an excellent prognosis with five-year survival rates approaching 90%. Unfortunately, less than half of colorectal cancers are diagnosed at an early stage. Since the symptoms of colorectal cancer often do not present until late in the disease process, screening is critical to early detection and better survival. There has been growing interest in developing more tailored colorectal screening guidelines to take into account factors other than family history and some have argued for incorporating smoking habits into these guidelines. Until smoking becomes a formally-recognized risk factor for colorectal cancer, such changes will not occur. The proposed study, by addressing a number of the limitations to our current knowledge in how active and passive smoking may affect colorectal cancer risk, offers an important step towards both a greater understanding of the etiology of this disease and the development of prevention strategies to reduce the morbidity and mortality associated with it.