The unit Gray has the dimensions $\text{J/kg}$. I've read, that a dose of about 3-5 Gy's could kill a person within a few weeks - or at least that's usually the case.
But I'm not really understanding the concept I think. If the lethal dose is 5 Gy, that would mean about 400 J for a 80 kg person. But how is that dose then distributed ? Does the entire body take an average radiation, summing up to 400 J, or...?
Because in radiotherapy, some fractions can be around 2.5-3 Gy's (Of course in smaller areas) - and in the long run, some patients get over 70 Gy's. Does that mean, that they actually calculate the tumour mass, and then figure out what amount of Joule has to be distributed into the tumour ? And if so, why doesn't that kill people, when they get way over 5 Gy's.
Hope you understand what I mean.