David, you asked....

We have to distinguish between dose (radation/kg) and total amount of radiation

One Gray is the radiation energy absorbed per kilogram of matter. So this is a dose.
Assume your irradiated volume is 1 liter (about 1kg of tissue).
If you get 70 gray this means that you get a total of 70 Joules. Also with IMRT this should be delivered mainly to the tumor sparing surrounding structures (incl bones if possible)
In the old style radiation you also wanted to get around 70 gray max dose for your tumor tissue, however since the radiated volume could not be a neatly selected you ended up getting more radiation (still 70 gray but twice the amount of tissue in example 140 Joule)

If you get 70 gray total body exposure and if you are 75kg then you get a total amount of radiation of 5250 Joules. Even a 6-20 Gray whole body exposure (500-1500 Joules) can be fatal.

The other issue is localization, for example I would be very careful about exposure of the neck to excessive sunlight.
UV radiation also causes damage and this is on top of the damage caused (and hopefully mostly repaired) by the x-rays.
I would be less concerned about tanning your legs after H&N IMRT. Still, it is probably a good idea not to bake in the sun anyway. If you had cancer that means that your DNA repair system already screwed up once. Also, H&N IMRT should not be a problem for subsequent prostate RT (as an example)

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