Mike
thanks.... the gas method used 15-O and is used to study oxygen metabolism. This is more of a research tool. The half life of 15O is just a tad more than 2 minutes, which means that you need cyclotron next to the facility.
For cancer, FDG is generally used to assess glucose uptake.

I checked some more and it appears that if the masks are used at all then only for the initial PET/CT scan for IMRT planning.
And then the PET/CT and IMRT setup should be compatible.... which was why the RO was pretty adamant as to where to get the PET scan. The same RO also suggest to take the mask for the post TX PET. The idea is to minimise head movements which may lead to artifacts. This is most likely overkill.
However, I do think that you DO need a proper head/neck rest. Since there is a moderate uptake in the mouth/neck region anyway, everything should be done to preserve the integrity of the scan. (i.e. excessive talking, chewing etc before the scan) Note that this is not first hand knowledge. I am not quite crazy enough to provoke a positive scan by trying this.

Markus