It can certainly happen where your toilet bowl can give off a very unpleasant, offensive odor despite your best cleaning efforts.
This typically means the odor problem is coming from another place. Take a look at the base of the toilet and make sure there is no leaking, if there is this is the cause. Another potential source of the odor could be from a burst sewage pipe below the toilet.
If you suspect this call your licensed plumber. Now there is a wax ring between the waste pipe, below your toilet, and the toilet. If this ring is defective it will also cause the odor problem.
If all else fails, call your licensed plumber for help.