The issue of whether cell phone use could cause cancer has been mired in confusion, with some studies failing to find an increased risk of brain tumors among cell phone users, while others suggest greater risk among the most frequent of users. 
Part of the problem is that studying people is less controlled than testing animals in labs. Researchers have asked individuals diagnosed with brain tumors to recall how much they used their cell phones and compared it with usage by healthy people, but it can be hard for people to accurately remember their use. Other studies have followed healthy people for years to see whether those who use their phones the most develop more cancers, and while they have not found that to be the case, heavy users could differ in other ways that affect their cancer risk. 
To get around these challenges, some researchers have turned to rodents. They expose mice or rats to known doses of radiation that are equivalent to -- or sometimes more than -- what people get from their cell phones. 
The researchers at the National Institute of Environmental Health Sciences gave rats high doses of radiation every day for two years and compared them with rats that did not receive radiation. The researchers looked at how many animals developed tumors in the brain and in nerve cells of the heart. 
The researchers found that 2% to 3% of the hundreds of male rats that were irradiated developed brain tumors, compared with none of the control rats. The number of female rats that developed these cancers was smaller, about 1% of the animals, and could have been due to chance. Similarly, between 2% and 7% of the irradiated male rats developed heart tumors, compared with only about 2% of the irradiated female rats and none of the control rats.

Source: QNA