Risk of second primary cancer among patients with a first primary appendiceal adenocarcinoma.

819 Background: Incidence rates of rare appendiceal cancers are increasing across the United States. Although this is suggestive of a growing population of appendiceal cancer survivors over time, the risk of second primary cancers specific to patients with a first primary appendiceal adenocarcinoma remains unknown. Methods: We conducted a population-based study of adults (age ≥18 years) diagnosed with a first primary appendiceal adenocarcinoma from 1992 to 2021, using data from the Surveillance, Epidemiology, and End Results (SEER) Program. We estimated cumulative incidence of second primary cancer, defined as any primary cancer diagnosed at least 3 months after appendiceal adenocarcinoma, with Fine and Gray methods to account for the competing risk of death. For comparison to the general population, we also estimated standardized incidence ratios [SIRs], or the observed number of second cancers relative to the expected number based on age-, sex-, and race-specific incidence rates. Results: A total of 5,827 adults diagnosed with a first primary appendiceal adenocarcinoma (mean age 58.0 years; 51.8% female; 68.5% non-Hispanic White) were identified during the study period, of whom 418 developed a second primary cancer. The most common types of second primary cancer were prostate (17.7%), colorectal (14.4%), female breast (10.5%), lung (8.4%), and melanoma (6.0%). At 10 and 20 years after appendiceal adenocarcinoma diagnosis, the cumulative incidence of second primary cancer was 7.5% (95%CI 6.8%-8.4%) and 11.7% (95%CI 10.5%-12.9%), respectively. Cumulative incidence significantly differed by sex (p<0.01) and age at appendiceal adenocarcinoma diagnosis (early-onset [age<50] vs late-onset; p<0.01). For example, at 10 years post-diagnosis, the cumulative incidence of second primary cancer was 6.4% (95%CI 5.4%-7.4%) for females and 8.8% (95%CI 7.6%-10.1%) for males. The overall SIR for any second primary cancer was 1.12 (95%CI, 1.02-1.23)—corresponding to 14.8 excess cancers per 10,000 person-years. SIRs for common types of second primary cancers were: 1.07 (95%CI 0.84-1.35) for prostate, 2.12 (95%CI 1.63-2.70) for colorectal, 0.91 (95%CI 0.68-1.19) for female breast, 0.81 (95%CI 0.58-1.09) for lung, and 1.20 (95%CI 0.79-1.73) for melanoma. Conclusions: Approximately one in every 13 adults diagnosed with a first primary appendiceal adenocarcinoma in the United States will be diagnosed with a second primary cancer within 10 years. After diagnosis of an appendiceal adenocarcinoma, patients have a 12% excess incidence in second primary cancers and double the excess incidence in second primary colorectal cancers compared to the general population. Implementation of cancer prevention and early detection strategies (e.g., colonoscopy screening, genetic testing) for patients with an appendiceal adenocarcinoma is a clinical priority.