Age and time as factors in the left-to-right shift of the subsite of colorectal adenocarcinoma: a study of 213,383 cases from the California Cancer Registry.

GOALS: Using a data set of more than 200,000 cases, we can measure the effects of age, time, sex, and race/ethnicity on the shift of the site of origin of colorectal adenocarcinoma from the left to the right side. BACKGROUND: As people become older, there is a shift of the site of origin of adenocarcinoma of the colorectum from the left to the right side. Although some studies do show some relationship of this shift, in addition to age, to race/ethnicity and to sex, there are no large, total population-based data studying the effects of these factors and time trends in this shift. STUDY: 213,383 cases of adenocarcinoma of the colorectum for the years 1988 to 2003 from the California Cancer Registry have been studied. RESULTS: The left-to-right shift increases significantly with increasing age and year of diagnosis, and is greater in women than in men and is greater in whites than in other racial/ethnic groups. The time-related shift is a reflection of a lesser decrease in the incidence of colorectal adenocarcinoma on the right side than on the left. CONCLUSIONS: The most attractive hypothesis is that a greater likelihood of prior polypectomy, and thus prevention of more cancers, occurs on the left side than on the right.