Quality of Japanese Cancer Websites by Type of Information Provider.

Online cancer information is a key health resource in Japan, particularly for older adults. However, the quality may vary depending on the source. This study aimed to evaluate Japanese-language cancer webpages across different sources in terms of understandability, actionability, readability, and credibility. We analyzed 100 webpages about five major cancers (breast, colorectal, lung, prostate, and gastric) retrieved via Google. Pages, classified as academic (n = 14), medical (n = 51), or corporate (n = 35), and assessed using PEMAT-P, jReadability, and JAMA Benchmark Criteria. Overall, 79 pages (79.0%) scored ≥ 70 in understandability, while only 5 pages (5.0%) reached this threshold in actionability. Corporate pages showed the highest proportion scoring ≥ 70 in understandability (n = 34, 97.1%), followed by academic (n = 14, 100.0%) and medical (n = 31, 60.8%) sources (p < 0.001). Readability was uniformly low, with 98 pages (98.0%) rated as somewhat to very difficult. Corporate sources also displayed significantly higher credibility scores than other sources (p < 0.001). Corporate websites demonstrated clearer structure and higher transparency of source attribution, potentially reflecting organizational standards. However, actionable content remained limited across all sources. These findings highlight a misalignment between user search needs and information design, underscoring the importance of structuring content not only for clarity and reliability, but also for supporting users in taking informed health actions. This has implications for digital cancer education targeting aging populations.