Cancer of unknown primary: Incidence rates, risk factors and survival among adolescents and young adults.

Cancer of unknown primary (CUP) is a clinical challenge especially when it occurs in adolescents and young adults (AYA), aged 15-39 years, due to the sparse data in this population. The available data has not described the population-based epidemiological features of CUP among AYA. Therefore, we collected patient information from the Surveillance, Epidemiology and End Results (SEER) registry, 1990-2015. Age, gender, ethnic, five pathological classification groups were assessed along with an aggregate level socioeconomic status (SES) index and population density at the county level. Incidence rates, modeled relative risks and survival of AYA patients with CUP were assessed. Among 2,480 AYA patients, 907 met the definition of standard pathology classifications. The majority of AYA patients with CUP had a neuroendocrine, squamous cell and poorly differentiated carcinomas with 0.4 cases per 1,000,000 population. AYA living in areas with the highest SES level had the highest risks of CUP; adjusted relative risks (ARR) of 1.17 (95% CI 1.0-1.4) and 1.99 (95% CI 1.5-2.6), respectively. AYA living in nonmetropolitan areas had a lower risk of CUP (ARR = 0.16; 95% CI 0.1-0.2). The incidence of differentiated neoplasms has been decreasing slower than undifferentiated neoplasms since the early 1990s. The median overall survival (OS) was 11 months (95% CI 9-13 months) with squamous CUP having the longest median OS 16 years (95% CI 3-24 years). In conclusion, this analysis answers several gaps in the knowledge of CUP among AYA and provides a platform to better understand this disease and its management within this group.