Multiple primary cancers among colorectal cancer survivors in Queensland, Australia, 1996-2007

To quantify the demographic and clinical factors associated with an increased risk of multiple primary cancers (MPCs) among colorectal cancer survivors. Standardized incidence ratios for MPCs were calculated for residents of Queensland, Australia, who were diagnosed with a first primary colorectal cancer between 1996 and 2005 and survived for at least 2 months. Relative risk ratios were calculated for all MPCs combined and selected individual sites using multivariate Poisson models. A total of 1,615 MPCs were observed among 15,755 study patients. The cohort had a significant excess risk of developing subsequent colorectal (SIR = 1.47, 95 % CI 1.30-1.66) or non-colorectal (SIR = 1.24, 95 % CI 1.18-1.31) cancers relative to the incidence of cancer in the general population. Age at initial diagnosis, follow-up time, initial colorectal subsite, and surgical treatment were independently associated (p < 0.01) with the overall risk of developing MPCs after adjustment. The relative risk ratio was 1.23 (95 % CI 1.07-1.41) for those aged 20-59 years compared with the 70-79 age group and 0.82 (95 % CI 0.72-0.92) for 1-5-year follow-up relative to the first year. The likelihood of being diagnosed with a MPC was 33 % higher (95 % CI 1.12-1.56) for surgically treated patients and 45 % higher (95 % CI 1.29-1.64) after proximal colon cancers relative to rectal cancer. While these population-based results do not incorporate all possible risk factors, they form an important foundation from which to further investigate the etiological causes that result in the development of MPCs among colorectal cancer survivors.