Hi Jen,
I had 3-D conformal radiation rather than IMRT and the reason the oncologist gave was that since I had already had my tumor removed surgically and had a positive margin remaining, the broader 'sweep' of the 3-D radiation was more appropriate for my treatment. It's not possible for anybody but a trained radiation oncologist to make these type of decision.
As it was explained to me, IMRT (intensity modulated raiation therapy) shoots multiple beams and the strength of each beam can be adjusted so that the area that is being radiated can be precisely controlled. This allows one to use IMRT as a 'scalpel' in the sense that it can burn out a tumor very accurately. 3-D radiation uses multiple beams but doesn't adjust the intensity of each beam, so that it broadcasts over a wider area. My treatment ended with 3 days of 'boost' radiation which, as I understand it, is IMRT and was targeted on the tumor bed.
So, this could be very appropriate treatment for you. While time is of the essence, it may be worth getting a second opinion if there is another comprehensive cancer center nearby. They should be able to turn this around in a week or so.
Best wishes for a successful treatment. - Sheldon