Assessing the burden of UBC worldwide is reliant upon the availability of cancer registry data . The International Association of Cancer Registries (IACR) has made major steps towards increasing the proportion of the world population covered by national or regional cancer registries. To date, national cancer registries are mainly situated in European countries and Australia. In other, more developed countries, often regional cancer registries are present whereas in less developed countries there is a lack of national and/or regional population-based cancer registries [4, 5].
Several international rules and guidelines have been developed to enhance comparability across cancer registries. In general, cancer registries adhere to the International Classification of Diseases for Oncology (ICD-O) and other systems such as the TNM system and grading systems.
Despite the use of uniform coding systems like the ICD-O and TNM, UBC has some specific features that should be kept in mind when comparing data from different areas of the world or different time periods. Some cancer registries include non-invasive (Tis and Ta) UBC while other registries only include invasive (≥ T1) cancers. T1 tumors are invasive, but not muscle-invasive. Their clinical course and treatment are more like Ta tumors, which is reason to group Ta and T1 tumors as non-muscle-invasive tumors. Because Ta tumors make up about 50% of all new UBC diagnoses, inclusion or exclusion of these Ta tumors has an enormous impact. While non-muscle-invasive tumors are usually treated locally, treatment of muscle-invasive tumors often involves a radical cystectomy or alternatively, (chemo)radiation and have much worse survival rates. Naturally, such registration differences may have a huge effect on the comparison of incidence, prevalence and survival estimates.
In addition to registration differences, the term ‘invasive’ may be confusing in reports or publications if it is not well-defined because it may both be used to describe cancers that invade the lamina propria (≥ T1, as defined by the TNM system), or tumors that invade the bladder muscle (≥ T2, as usually referred to by clinicians).
In this paper, most data are derived from GLOBOCAN 2018 which includes non-invasive UBC, unless stated otherwise. GLOBOCAN (The Global Cancer Observatory by the International Agency for Research on Cancer [IARC]) is based on several key IACR projects, including Cancer Incidence in Five Continents (CI5), and offers an interactive web-based platform presenting global cancer statistics. It includes a module that provides data visualization tools that present current national estimates of the incidence, mortality and prevalence of 36 cancer types in 185 countries by sex and age group .