Whatever needs to be done tooth wise, do it now, for several reasons being radiation damages the teeth directly, and by the dry mouth it causes to the terth by demineralization, and if any teeth need to be extracted after radiation Tx, you will need to do hyberbaric chamber treatment, 20 before extraction, 10 after, plus, if anything needs correcting in order to do treatment, insurance will usually pay for it, but not after, in case you have no or limited dental civerage, and afterwards may be considered cosmetic. You will need 2 weeks healing after an extraction to do radiation, and will probably need dental clearance.

Ask your dentist about dental trays, prescription fluoride. Some make vinyl dental guards to use during radiation to protect the mouth from radiation scatter from dental fillings if made from metal, and reduces damage. This may need to be coordinated with the radiation oncologist and or before the mask is fitting.

Good luck.