Risk factors and outcomes of second primary cancers in colorectal cancer patients by primary tumor site.

e15704 Background: Patients with colorectal cancer (CRC) have an increased risk of developing a second primary cancer (SPC) at any site. Understanding the risk of developing a SPC based on the site of the primary tumor and patient demographics is crucial for developing effective preventive strategies. Methods: CRC cases diagnosed between 2000 and 2015, with a follow-up period of 5 years, were identified using SEER.Stat 8.4.4. Tumor sites were categorized into group 1: right colon (cecum, ascending colon and hepatic flexure), group 2: transverse colon, group 3: left colon (splenic flexure, descending colon and sigmoid colon), and group 4: rectosigmoid and rectum. Fine and Gray competing risks regression was used to evaluate the subdistribution hazard ratio (SHR) of developing a SPC and their survival based on the primary tumor site, while adjusting for covariates like age, gender, ethnicity, and race. All analyses were conducted using STATA 18.0. Results: Out of 402,888 patients with CRC, 37,827 (9.39%) developed a SPC within the 5-year follow-up period. Among those who developed a SPC, 22,234 (58.73%) survived till the follow-up period. The most prevalent sites for developing SPC in males were prostate (17.79%), lung and bronchus (10.45%), and rectum (8.90%), and in females were breast (15.37%), lung and bronchus (10.76%) and rectum (7.37%). Compared to right colon, transverse colon had higher risk of developing SPC (aSHR = 1.16 (1.11,1.20)), while rectosigmoid/rectum had lower risk (aSHR = 0.81 (0.78,0.83)). Higher risk was observed in males (aSHR = 1.32 (1.29,1.35)), and in Whites, compared to Hispanics (SHR = 0.87 (0.83,0.90)) and Asian/Pacific Islanders (aSHR = 0.86 (0.83,0.89)); Stage I patients had higher risk than Stage II (SHR = 0.96 (0.93,0.98)), Stage III (aSHR = 0.84 (0.81,0.86)), and Stage IV (SHR = 0.42 (0.40,0.43)) patients. Among SPC cases, no significant difference in cancer-related mortality was observed between the colon groups. Males had higher risk of death due to cancer compared to females (aSHR = 1.06 (1.02,1.09)). Mortality increased with increasing age (aSHR 1.103 (1.01,1.10)) and advancing stage. Blacks had higher risk of cancer-related death compared to Whites (aSHR = 1.13 (1.07,1.19)). Conclusions: The risk of developing a SPC varies significantly by the site of the primary tumor, with transverse colon at the highest risk. Males, Whites, and Stage I patients had higher risk of developing SPCs, while cancer-related mortality was higher in males, Black patients, older individuals, and advanced stages. These findings emphasize the need for tailored surveillance and management strategies. Biological and genetic factors driving these disparities need to be explored.