Washington - A major international study into the link between cell phone use and two types of brain cancer has proved inconclusive, according to a report due to be published Tuesday in the International Journal of Epidemiology. A 10-year survey of almost 13,000 participants found most cell phone use didn't increase the risk of developing meningioma -- a common and frequently benign tumor - or glioma - a rarer but deadlier form of cancer. There were suggestions that using cell phones for more than 30 minutes each day could increase the risk of glioma, the study by the World Health Organization's International Agency for Research on Cancer said. But the authors added that "biases and error prevent a causal interpretation" that would directly blame radiation for the tumour.