My understanding is that metal fillings are a problem for imaging (and so are also a problem for planning radiation treatment). I do not know whether metal fillings are a problem for the radiation treatment itself.

For imaging, metal fillings can cause distracting "artifacts" which make the image difficult to interpret. With enough "artifacts", the image may become useless. There are alternative imaging strategies that can avoid the artifacts if the equipment is capable enough (I don't recall exactly, its something like using higher frequencies, or high energies, than usual. I think its pretty common these days in CT imaging.)

I have quite a number of metal fillings, as well as several crowns. Everyone who reviewed my full mouth X-rays agreed that my teeth were sufficiently healthy that no dental work was needed before my radiation treatment. I think the primary concern was whether I had any dental infections... fortunately, I didn't (and still don't). Such an infection might normally require a root canal procedure; however, if one is discovered prior to radiation treatment, I think they usually recommend that the tooth be pulled (as there usually isn't time for a root canal). I've heard of some people getting many teeth pulled.

Rob