Nearly one in two children with cancer are never diagnosed and may die untreated, according to a Lancet study which estimates that there are almost 400,000 new cases of childhood cancer annually, while current records count only around 200,000.
The model makes predictions for 200 countries and estimates that undiagnosed cases could account for more than half of the total in Africa, South Central Asia and the Pacific Islands.
In contrast, in North America and Europe, only three per cent of cases remain undiagnosed. If no improvements are made, researchers estimate that nearly three million further cases will be missed between 2015 and 2030.
"Our model suggests that nearly one in two children with cancer are never diagnosed and may die untreated," said Zachary Ward from the Harvard T H Chan School of Public Health in the US.
"Accurate estimates of childhood cancer incidence are critical for policymakers to help them set healthcare priorities and to plan for effective diagnosis and treatment of all children with cancer," Ward said in a statement.
"While under-diagnosis has been acknowledged as a problem, this model provides specific estimates that have been lacking," he said.
Previous estimates for the total incidence of global childhood cancer have been based on data from cancer registries, which identify cases in defined populations.
However, 60 per cent of countries worldwide do not have such registries and those that do only cover a small fraction of the overall population.
Many patients are not diagnosed and are therefore not recorded. This can occur due to lack of access to primary care, with patients dying undiagnosed at home, or due to misdiagnosis.
The model developed for this study, the Global Childhood Cancer microsimulation model, incorporates data from cancer registries in countries where they exist, combining it with data from the World Health Organisation's Global Health Observatory, demographic health surveys and household surveys developed by Unicef.
The model was calibrated to data from public registries and adjusts for under-diagnosis due to weaknesses in national health systems. The study, published in The Lancet Oncology journal, provide estimates of under-diagnosis for each of the 200 countries. They estimate that in 2015 there were 397,000 childhood cancer cases globally, compared to 224,000 that were recorded as diagnosed.
This suggests that 43 per cent (172,000 cases) of global childhood cancer cases were undiagnosed. There was substantial regional variation, ranging from three per cent in both Western Europe (120 undiagnosed cases out of 4,300 total new cases) and North America (300 of 10,900 cases) to 57 per cent (43,000 of 76,000 new cases) in Western Africa.
In most regions of the world, the number of new childhood cancer cases is declining or stable. However, the researchers estimate that 92 per cent of all new cases occur in low and middle-income countries, a higher proportion than previously thought. "Health systems in low-income and middle-income countries are clearly failing to meet the needs of children with cancer.
Universal health coverage, a target of United Nations Sustainable Development Goals, must include cancer in children as a priority to prevent needless deaths," said Rifat Atun, from Harvard University in the US.
Taking population growth into account, the authors estimate that between 2015 and 2030 there will be 6.7 million new cases of childhood cancer worldwide. Of these, 2.9 million cases will be missed if the performance of health systems does not improve.
The findings will help guide new policies in health systems to improve diagnosis and management of childhood cancers.