Teeth cleaning or anything that may cause cuts, abrasions, to the gums should be avoided, right before, during, and maybe a few months after treatment. Check with your Radiologist for more specifics. Even flossing if causing gum bleeding, should be ceased and infections can enter the blood stream from these little cuts. Fillings can cause radiation scatter, if in the radiation field, causing more radiation effects, but your doctor knes best. Mouth guards are usually molded latex, made before treatment, mask, so it can be form fitting, but heard of someone's radiation oncologist making a quick fix with wax.

I'm not an expert with the dental trays for fluoride Tx. which is different than mouth guards, but believe they should be used from day one. Good luck.