Increased incidence of rare cancers and varied age distributions by cancer group: A population-based cancer registry study in Hiroshima Prefecture, Japan.

Background: Epidemiological characteristics of many types of rare cancers are limited especially in Asia. Therefore, this study aimed to describe the burden and changing time trends of rare cancers in Hiroshima, Japan.

Methods: The internationally agreed RARECAREnet list of rare cancers was used to identify patients diagnosed with cancers from 2005 to 2015 who were registered in the Hiroshima Prefecture Cancer Registry. Quality indicators specific to rare cancers were assessed by cancer grouping. Crude incidence rates (IRs) and age-standardized rates (ASRs) were calculated for 216 single cancers (rare and common) included in the list. A joinpoint regression was used to analyze age distribution and time trends in the ASRs for 12 internationally agreed rare cancer families. Quality indicators, ASRs, and IRs in Japan were identified to examine IR differences and the effects on data accuracy.

Results: The 231,328 cases were used to calculate the IRs of each cancer. Epithelial tumors in rare families increased with age, but nonepithelial tumors occurred at any age. The proportion of rare cancer families to total cancers was stable. The time trend for families of head and neck cancers (annual percent change and 95 % confidence interval: 2.4 %; 1.2-3.7 %), neuroendocrine tumors (6.6 %; 5.1-8.1 %), and hematological cancers (4.3 %; 3.2-5.5 %) markedly increased.

Conclusion: The ASRs of several rare cancers increased because of increased knowledge of these diseases, improved diagnostic techniques, and aggressive diagnoses.