Will my facial hair grow back after radiation therapy?

Depends on dose/type: Depending on what disease is being treated, radiation is given in between 2 weeks to 7 weeks in standard fractionations. Near 7 weeks of radiation we expect no further hair growth. About 2-4 weeks, we can expect recovery of hair growth. I am assuming you are talking about x-ray/therapeutic radiation for cancer. There are other types of radiation that the above answer may or may not hold true.

Possibly: Assuming you lost the hair from treatment for a cancer in the oral cavity or pharynx, it would depend on the technique. These days most radiation doctors use imrt for head and neck cancers. This distributes the radiation widely and therefore less dose directly to the skin versus older methods. Often in this case the hair will grow back but sometimes more sparsely.

Depends: Do you want it to? This is dose dependent and time dependent to some extent. If you had lymphoma treatment, it is likely. High doees head and neck treatment, probably not.