Northward shift in cancer mortality inequality across Japanese prefectures, 1995–2023, a nationwide ecological study

BACKGROUND: Cancer has been the leading cause of death in Japan since the 1980s, with known variation across prefectures, but evidence on how this pattern has changed over time is limited. METHODS: We analysed age-standardized cancer mortality across all 47 Japanese prefectures in 1995 and 2023 using vital statistics data. For each prefecture, excess mortality was calculated relative to a leave-one-out national reference and standardized to the 1995 national population. Excess mortality was decomposed by cancer site. We also compared prefectures' age-standardized mortality rates (ASMRs) between the two years. RESULTS: All-cancer ASMR declined in every prefecture. In 2023, excess mortality ranged from 32·6 per 100 000 (95% CI 26·8 to 38·4) in Aomori to -22·4 (-26·1 to -18·6) in Nagano, a gap of 55·0 deaths per 100 000, with 30 prefectures differing significantly from the national reference. The geographic pattern shifted between 1995 and 2023. In 1995, excess mortality was concentrated in western Japan, particularly Osaka and Fukuoka, and was largely driven by liver cancer. By 2023, excess mortality had shifted to northern prefectures, led by Aomori and Hokkaido, and was distributed across multiple cancer types, including colorectal, lung, and stomach cancer. The overall gap narrowed from 69·6 to 55·0 per 100 000. CONCLUSIONS: Geographic inequality in cancer mortality persisted in Japan, but its pattern changed substantially. The findings suggest that national cancer control should monitor subnational inequality directly and support regionally targeted strategies.