Unless your iPhone is from a parallel universe with different laws of physics, no, it's not going to give you cancer or nuke your brain.
The wavelengths cellphones output are too large and their total energy is too low to do DNA damage (see ionizing vs non-ionizing radiation). At worst they'll heat up the surrounding tissue, but we're talking tenths of a degree here. Nothing your body doesn't deal with day to day. Cell phone transmitters generally top out at about 0.5 to 1 watts.
You have a significantly greater cancer risk just going outside than you do using your phone.