Question about positioning for radiation therapy

I went yesterday for the simulation in preparation to receiving  radiation therapy.  We did this earlier, in May, with me in a prone position in prep for neoadjuvant chemoradiation to treat a low rectal tumor, but  then found out via PET scan that I was stage 4 with a 1.5 cm lesion in the liver, and we went with 8 rounds of  chemo, Folfox plus Avastin, instead.  The treatment  shrank the rectal tumor so that the  last CT scan reported,"Previously identified rectal mass is not well seen on this exam and may reflect favorable treatment changes." The liver met had also shrunk from 1.5 to 1 cm.  However, when I met with the colorectal surgeon  a week or so ago, he did a digital exam and could still feel remnants of the rectal tumor.  He and the medical and radiation oncologists met and suggested that we delay surgery and try to shrink the tumor futher via  chemoradiation. So I'll be getting the radiation while wearing a portable 5FU pump 24/5 for 5 1/2 weeks. 

The radiation oncologist asked yesterday if I had trouble  lying  in a prone position. Because of a rather pronounced case of scoliosis, the result of childhood polio, being in a prone position for more than a few seconds is doable, but extremely uncomfortable and the port doesn't help the situation. So we did the simulation in a supine position and will do the radiation in that position.  From several  medical journal reports I've read,  the supine position, with a full bladder, is being used more frequently now, with no  major problems of   bladder or small bowel involvement/damage than happens in the  prone position and is actually  proving to  have some benefit in that, the   face -down position is hard for many patients to maintain for any length of time, and so is  subject to  being not consistently reproducible and prone to randon set-up errors.

I'm assuming that  positoining will not have any effect on most of the  possbile side effects resulting from damage to the bowel lining, etc..  However, I was all psyched up for having  beams aimed  at my rear and suffering  "sunburn" in my seating area. Now I'm wondering  just what effects having the beams aimed at  my belly will have. Have any of you experienced this face-up positioning?

Grace