Worth chatting with a pharmacist to see if there's something you could take for a few weeks (eg charcoal!). I'd speak to a pharmacist for expert advice, especially if you are on any other meds, which may be causing the problem and/or may affect what you can take to alleviate it.
But the dietary/eating advice given above is probably key to moving forward. As I'm sure you understand, it's not wind per se that's the issue - just the fact that the presence of wind can move internal organs around, possibly making radiotherapy inconsistent.
But above all, don't let it worry you - these things happen - I was sent back to the toilet before my first session, and two other guys also had to sit on the naughty stair on the same day.