Incidence, Risk and Trends of Multiple Primary Cancers in Patients With Colorectal Cancer: Evidence From the South Australian Cancer Registry.

Background: Colorectal cancer (CRC) is the fourth most diagnosed cancer in Australia. With advancements in treatment and an increase in survival rates, CRC survivors face an elevated risk of developing multiple primary cancers (MPCs), presenting a clinical challenge. Therefore, this study aimed to estimate the incidence, trend and risk of MPCs after a diagnosis of CRC in the South Australian population.

Methods: This study analysed South Australian Cancer Registry data on individuals diagnosed with CRC as their first cancer from 1982 to 2017. The incidence of MPCs was assessed using cumulative incidence functions, and age-standardised rates were estimated. Poisson regression was used to determine the risk, and standardised incidence ratios (SIR) and absolute excess risks (AER) were estimated. Trends over time were analysed using Joinpoint regression.

Results: The study included 26,729 CRC survivors. Of the cohort, 15% (3917) developed 4453 MPCs, with 96% diagnosed six or more months after index CRC. The cumulative incidence of MPCs was 22.5% (95% CI: 21.6-23.4). The median follow-up time until MPC diagnosis was 6.4 years. Common MPCs included prostate (18.9%), subsequent CRC (13.1%), lung (10.8%), haematological (10.2%) and breast (8.0%) cancers. The overall risk of MPCs was higher in CRC survivors (SIR: 1.12, 95% CI: 1.09-1.16; AER: 22.6 per 10,000) compared to the incidence in the general South Australian population. The incidence of MPCs has increased over time (annual percentage change = 1.95, 95% CI: 1.33-2.51).

Conclusions: CRC survivors are at increased risk of subsequent cancers, highlighting the need for targeted surveillance, particularly for prostate, lung, breast and blood cancers, for early detection and treatment.