Hi Locksley,
I can answer this one. I've just finished my first 5 sessions.
You will see the frame that they use on your planning session for R/T
You are on your back, but elevated slightly. Your arms are both above your head, well supported in a frame with padding.
Your arms are not straight but gently curved.
From the time you start to lay down, it would be 15 min. All of that time you need to stay still.
The first 5 min the staff are with you getting you into your precise position.
A few milimetres up, down and sideways etc.
If you are able to put your arms above your head with fingers touching, perhaps while on a recliner chair.
You would be ok.
If that is really uncomfortable or difficult for you, perhaps a physiotherapist could assist with some stretches.
Definitely discuss this with your Radiation Oncologist at your planning session. This seems to be 3-4 weeks prior to starting.
That is when I got my 3 tiny tattoos, and was put into the frame for planning.
I had a couple of consults with Radiotherapy a month or 2 prior. These were only phone consults, due to COVID.
But you could also mention your concerns then too.
Good luck.