Hi Nellie,
Do you mean IMRT? On the subject of number of arcs I believe you may be confusing the so called Rapidarc with the typical method. I had what is known as IGRT/IMRT. This stands for "image guided radiation therapy/intensity modulated radiation theapy". Basically the IG part refers to the alignment of your prostate and the radiation machine beam. The IM part refers to the shaping of the beam itself. The machine that has the rapidarc is made by Varian. I think there are some other similar machines. In the older Varian the machine makes a 360 degree rotation around you while you lay on the table. The machine stops at "stations" determined by your oncologist and at each station delivers a dose of radiation shaped to your specific situation.
In the case of a single arc as you refer to it the Varian machine is the rapidarc. It does the same thing as the older machine with the exception being that it does not stop at each station but makes the treatment in one, continuous pass. Primary advantage as I see it is to the treatment facility because it is faster than the stop/start.
As far as dosage I recieved 78 grays over 45 sessions. There are some medical papers that advocate higher per session doses and report better response for the same total. Sounds like you may be getting the higher dose per session or lower overall since you have had surgery. Keep in mind that my dose is for radiation as primary treatment not salvage.
Just a heads up for you. For the first 20 or so you may think as I did that not much is happening. Then comes the diarrhea and uncomfortable urination. As soon as it starts get the Immodium for the first and Flowmax for the second. You will feel a lot better. I also took psyllium powder (Metamucil) during all this as I take it daily anyhow. Sure made a difference by keeping the poop in a more solid state and less acidic.
Nellie, good luck with your treatment and I hope that your radiation treatment was as easy for you as mine was for me.
Don