My RO "spanked" me big time when I arrived top down for the rads (initially). Then some thinking ensued.... UV light does a lot of damage that normally gets repaired and does not lead to mutations and cancer etc. However, when you are treated with x-rays this also damages the DNA. The point is that cancer cells are inefficient at repair and die.
The UV exposure is an additional and unnecessary burden for your skin. This is because now the repair system of normal cells must repair that additional damage too and that may have (really) bad effects. Why take any risks here?

ALSO, I would be extremely careful to put sun block on x-ray exposed tissue (talk with you RO about that).

M