@Vangirl apologies that you feel this way. My suggestion is to write a letter to the department telling them of your experience so they can take that feedback on board for future patients.
Sometimes things go wrong that are out of the staffs control, they may have gotten you changed expecting you to go in within 5 minutes but then had a machine issue which meant you were left waiting. Of course this should have been communicated to you. The department I work for does ring patients if we know we will definitely be unable to make your time, and reception tells people on arrival if we are more than 30mins behind so they can go away if they like.
And you are right, the area where a patient is changed should not be accessible to non patients that is what the main waiting room is for and any overlap of changed patients should be minimised.
Changing of staff mid patient is sometimes unavoidable as we are legally not allowed to be at the machine for more than 3 hours without a break, it is hard around lunchtimes.
It sounds like the staff at your centre need a kick up the bum and a reminder of why they became radiation therapists!