Trends in brain cancers (glioma) in New Zealand from 1995 to 2020, with reference to mobile phone use

1Introduction

Mobile phones, which emit radiofrequency electromagnetic energy, have become an almost universal exposure following their introduction in the 1980 s [1] . Radiofrequency exposures are classified as a possible human carcinogen by the International Agency for Research on Cancer (IARC) [1] , and many research studies into the possible link between mobile phone use and brain cancers have been done. The large international Interphone study organised by the IARC showed no overall association, but did show an increased risk of glioma, the most common brain cancer, in the highest exposure category [2] . The authors concluded that “biases and error prevent a causal interpretation” [2] . While several other studies have been consistent with this [1] , a group of studies by investigators in Sweden have reported increased risks, with one study showing an odds ratio of 1.3 (95% confidence interval, CI, 1.1–1.6) for mobile phone use overall, increasing to OR= 3.0, 95% CI= 1.7–5.2 with long latency [3] . Such odds ratios, applying to an exposure with high prevalence in the population, would produce an increase in overall rates of glioma after an appropriate latent period. Thus many studies of time trends in brain cancers have been done, all of which are limited by the time dimensions of the possible association [4] .

The incidence of cancers of the brain and central nervous system (CNS) combined, as truncated age standardised rates at ages > = 15 years, in New Zealand in 2003–07 was 7.9 per 100,000 in men and 5.4 in women; this was similar to rates in Australia, Canada, the U.S. white population, and most European countries [5] . International comparisons of rates of glioma specifically are more limited as diagnostic and coding practices vary in national registries; 18 more detailed clinical registry studies of glioma were identified in a systematic review [6] , but the incidence rates given were not consistently calculated and so are not easily comparable.

In New Zealand (NZ) analysis for the period 1995–2010 showed no consistent increases in brain cancers in the age range 10–69, although an increase in glioma incidence was seen at ages over 70 [7] . The current analysis extends this by a further 10 years, studying brain cancer incidence from 1995 to 2020. It is estimated that mobile phones were used by at least half the population from the year 2001 in Australia [4] , and New Zealand use was similar [8] so this study will be able to detect even a moderate increase in glioma incidence up to 20 years since widespread population usage.