Rare cancers imply cancers with which the number of patients is small in a population. However, the proportion of the rare cancers to total is not small. The RARECAREnet defined the threshold for rare cancer as an incidence rate (IR) of less than 6/100,000 per year on the basis of the population level in Europe [1]. Rare cancers were observed 24 % of the total cancer incidence rate (IR) in Europe[2]; 19–25 %, depending on race, in the USA [3], [4]; 20.4 % in Brazil [5]; and 15 % in Japan [6]. Using the updated RARECAREnet list [1], a collaborative study conducted by RARECAREnet Asia measured and compared the rare cancer burdens in Japan (16.3 %), Korea (23.7 %), Taiwan (24.0 %), and the EU (22.2 %) [7]. Furthermore, the study showed that cancers that are rare in Europe are also rare in Asia [7].

In Japan, the third-term Basic Plan to Promote Cancer Control Program approved in 2018 has aimed for all the people, including patients with cancer, to learn about and overcome cancer, including rare cancers. However, epidemiological knowledge of the characteristics of each rare cancer is limited. The age distribution and time trends of rare cancers in Asian countries have not yet been provided. A sufficient number of pathologically confirmed and coded cases with detailed morphology is essential to identify and study their detailed characteristics [2], [8]. The high-quality data of the Hiroshima Prefecture Cancer Registry (HPCR), enhanced by the Hiroshima Tumor Tissue Registry (HTTR), a population-based pathological tissue registry, enable the description of rare cancer characteristics. This study aimed to measure the IR of rare cancers according to subtype, as defined by the RARECAREnet list, to determine the age distribution and incidence time trends in the Hiroshima Prefecture.

Hiroshima City, located in the west of Hiroshima Prefecture, experienced atomic bombing in 1945. However, the atomic bomb radiation was high-dose rate radiation, and the radiation-affected geographic area was limited. The proportion of survivors exposed to atomic bomb radiation greater than 5 mGy was estimated to be 0.45 % among the atomic bomb survivors’ cohort of the Life Span Study (LSS) compared to the total population of Hiroshima Prefecture in 2009 [9]. Therefore, the cancer incidence related to atomic bomb radiation in Hiroshima Prefecture is low.