The incidence of nonampullary duodenal cancer in Japan 2016-2020: analysis of a national cancer registry.

Background and aim: Epidemiological research is scarce in Asian countries including Japan. We aimed to clarify the epidemiological trend of nonampullary duodenal cancer (NADC) in Japan using a national database.

Methods: All patients with NADC diagnosed between 2016 and 2020 were identified from the Japanese national cancer registry. The crude and age-adjusted incidences were determined and temporal trends including diagnostic mode, tumor stage, and treatments were analyzed.

Results: A total of 14 484 patients were included. The crude and age-adjusted incidences were 21.8-23.5 and 10.7-12.1 per 1 000 000 person-years. Male-to-female ratio was 1.7:1, and the incidence increased with age. The number of patients was highest among 70-79 year age group in both sexes. In total, 55% of patients were diagnosed during screening or surveillance for other diseases. In 54% of patients, the tumor stage was localized. The major treatment was surgery (32%), followed by endoscopic resection (27%). The trends for detection mode, tumor stage, and treatment were consistent during 2016-2020.

Conclusions: This study clarified and confirmed the high incidence of duodenal cancer in Japan. Characteristically, many NADC cases are diagnosed asymptomatically and treated by endoscopic or surgical treatment.