don’t worry too much about the process involved in holding your breath during radiation - the staff will talk you through it and coach you beforehand . If you miss the chance to hold breath in at any time, the radiation beams don’t happen ( and they’ll rerun it/ get you to try again you can’t damage yourself so,don’t stress about that ).
‘I used mepitel film on,left breast - no burns ,no major redness , only got a little radiation rash on chest about size of apple ( I had gotten accidentally sunburnt about 5 months beforehand in that patch ) last few days easily treated over 4 days post radiation with a steroid cream.