I think you are on the right track @JoeyLiz.
I am not fond of ceremony--it makes me feel embarrassed, unworthy, irritated, delayed, scrutinized or all of the above. Not everyone feels that way. I'll admit to having a good bawl in the rain in the carpark at the end of rads. There is something a bit prosaic about crying in the rain compared to getting recognition I'd finished a medical marathon, but it was enough for me. I can see some sort of closing or transition ritual being quite useful for those who choose to participate.
The chemo bell has been around for a while. What to do with someone finishing rads?
I always associated radiotherapy with burns so blowing out a candle would work on one level. The issue (apart from the fire sensors) is that blowing out a candle has different conotations for different cultures.
Maybe some sort of water ceremony? Pouring water from one vessel to another--difficult without introducing some quasi religious element. Pouring glitter into a dedicated fountain? Difficult without having a maintenance dude who nearly stokes out about cleaning filters.
Good plan though, keep mining for ideas. Mxx