Background: The incidence of second primary colorectal cancer in patients with a history of colon cancer, compared with patients with a history of adenomatous polyps, is unknown. It is unclear whether guidelines for colonoscopy screening in patients with polyps are appropriate for patients with previous colon cancer.
Objective: To determine the incidence of second primary colorectal cancer after treatment for localized colon cancer and to compare this incidence with that of first primary colorectal cancer in both the general population and high-risk patients.
Design: Historical cohort study.
Setting: An international, multi-institutional trial of adjuvant 5-fluorouracil-based chemotherapy for localized colon cancer.
Patients: 3278 patients with resected stage II and stage III colon cancer.
Measurements: Occurrence of endoscopic or radiologic colon surveillance and incidence of second primary colorectal cancer.
Results: Forty-two cases of second primary invasive colon cancer were found over 15 345 person-years of follow-up, yielding an incidence rate of 274 per 100 000 person-years (95% CI, 196 to 369 per 100 000 person-years) and a cumulative incidence of 1.5% (CI, 1.1% to 2.0%) at 5 years. This rate was compared with rates of first colon cancer in two reference groups: the general population and patients who had undergone frequent colonoscopy and polypectomy because of a history of adenomatous polyps; standardized incidence ratios were 1.6 (CI, 1.2 to 2.2) and 6.8 (CI, 2.7 to 22.0), respectively.
Conclusion: The incidence of second primary colorectal cancer remains high despite intensive surveillance strategies.