Just to follow up Gary's very informative post. I had to return to Boston for my simulation, mask, etc. I asked the radiation oncologist about the difference between IMRT and 3D-CRT.
Her response was that the basic difference is that IMRT enables one to modulate each specific beam to get maximum conformity while 3D does not. She was of the opinion that since they are not targeting a tumor but rather trying to clean a positive margin and any additional loose cancer cells, that they want to deliver a uniform beam but proscribe carefully the radiated area. She said I would get the same benfits as IMRT (essentially save the parotid gland and tissues on the left side of my mouth, throat and neck while they radiate the right side.) but that there was no real advantage to IMRT in my case.
Since Mass General has a variety of radiation technologies at their disposal including IMRT, I am assuming that this is the right treatment for me.