IMRT Low Fiber Diets and Normal Bowel Movements

This coming Tuesday I'm beginning 8 weeks of IMRT radiation treatments. They want me to start eating a low fiber diet to minimize gas in the bowels. Evidently gas can cause the prostate and other organs to move around a bit which could affect the accuracy of the beam radiation. Also they want me to empty my bowels and have a full bladder daily before receiving treatment. I currently eat a high fiber diet which has kept me regular for years, but just recently I had to go on a temporary low fiber diet for a few days prior to a radiation planning visit where they did CT and MRI imaging then tattooed 4 small dots in the groin area for alignment purposes which will be used during treatment. Anyhow, just those few days of low fiber really messed with my bowel movements. I was not regular and was constipated. Ended up aggravating a hemorrhoid. I'm worried about how to prepare and address these issues. Has anyone any advice on how they dealt with maintaining regularity and a healthy anus during these weeks of radiation treatments while eating a low fiber diet?