With regard to rare cancers, as 24% of all new cancer cases are rare, rare cancers belong to both cancers and rare diseases, however their specificities bring them closer to common cancers. Rare diseases are defined in the European Union as diseases with a prevalence of fewer than 5 cases out of a population of 10,000. Prevalence – the number of patients with the same diagnosis at a given time – is affected by mortality. However, it has been proposed by the Surveillance of Rare Cancers in Europe (RARECARE) project to consider a definition of rare cancers based on incidence, meaning the number of newly-diagnosed cases per year.
According to this definition, rare cancers are identified as those with an incidence of less than 6 per 100,000 persons per year. Using this definition would help minimise the risk of mistaking a rare cancer (such as testicular cancer), which is frequently cured and thus has a rather high prevalence, for a common cancer, or a frequent cancer (such as small-cell lung cancer), which has a low life expectancy and thus a low prevalence, for a rare cancer. Thus, RCE recommends this definition in the context of defining rare cancers.
In 2016, the Joint Action on Rare Cancers (JARC) re-examined the list of 198 rare cancers as developed by the RARECARE project which encompasses rare cancers into 12 families, and categorised them into 3 tiers based on the International Classification of Diseases for Oncology (ICD-O) classification, which incorporates topographical and histological labels:
- First tier: The first tier entities were intended to be major cancer entities in a clinical sense (e.g. “epithelial tumours of nasal cavity and sinuses”, “soft tissue sarcoma”, etc.) and to be of organizational importance: for example, they could underlie patient referral policies. Focusing on referral to patients, Tier-1 entities can be grouped into gross partitions which gives rise to families of rare cancers, diving them into major groups (e.g. “rare cancers pf head and neck”, “sarcoma”, etc)
- Second tier: The second tier of clinically distinct entities is based on morphologies and topographies (e.g. “squamous cell carcinoma of nasal cavity and sinuses”, “soft tissue 16 sarcoma of limb”, etc.). The Tier-2 entities had to be viewed as clinically relevant by clinicians. In general, these diagnoses had to correspond to consistent diagnostic and therapeutic approaches (for example, they could be used as eligibility criteria in a clinical trial). The Tier-2 entities were then assembled into a smaller number of Tier-1 entities.
- Third tier: The third tier corresponds to the morphological entities of the International Classification of Diseases for Oncology. Tier 3 corresponds to the morphological entities of the ICDO. Then, the experts were asked to group the ICD-O3 morphological entities, to give rise to a second tier of clinically distinct entities (Tier 2) based on morphologies and topographies (e.g. “squamous cell carcinoma of nasal cavity and sinuses”, “soft tissue 16 sarcoma of limb”, etc.).
More information on the epidemiology of rare cancers is available here: www.rarecarenet.eu
Find out more about the families of rare cancers here.