Unveiling the Characteristics and Survival Outcomes of Double Primary Cancers in Taiwan: A Nationwide Population-based Comparison Cohort Study.

[AIM] The incidence of second cancers has increased notably in recent decades, yet a comprehensive understanding of double primary cancers remains elusive. This nationwide retrospective cohort analysis aims to address this gap by providing actionable insights for establishing screening and evaluation protocols in Taiwan.

[METHODS] We integrated data from the Registry for Catastrophic Illness Patient Database and the Taiwan Cancer Registry, focusing on patients diagnosed with specified cancers between 2000 and 2017. Double primary cancers were identified on the basis of International Classification of Diseases for Oncology codes and categorized as synchronous (within 180 days) or metachronous (beyond 180 days). Three cohorts-single primary, synchronous, and metachronous cancers-were compared for their cancer site distributions, stage distributions, relative risks, and survival rates.

[RESULTS] Key findings revealed that among 1,080,450 patients with initial cancer diagnoses, 45,084 (4.17%) developed second cancers. Among males, those with oral cancer most frequently develop esophageal cancer. In females, breast, colorectal, and lung cancer commonly develop either synchronously or metachronously. Synchronous cancers were often diagnosed at stage IV (first synchronous cancer: 37.2%, second synchronous cancer: 36.0%). Mortality rates for common double cancers were significantly higher than those for single primary cancers.

[CONCLUSION] This study provides practical insights into the characteristics and outcomes of double primary cancers. This highlights the critical need for screening and managing second cancers in newly diagnosed cancer patients, especially for synchronous cancers. Visual aids such as pie charts and tables were developed to facilitate clinical recommendations for double cancer screening.