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.
Best Answer
The dose that kills a tumor is deliberately aimed at that tumor. If, instead of using a collimated beam, you put a person in a wide beam for radio "therapy", you would be treating their entire body as a tumor and kill them.
The dose in RT is computed locally - "this" part of the body (these grams of tissue) absorbed (were exposed to) "this many" Joules of ionizing radiation .
You can't simply average local dose over the entire body - although blood perfusing the irradiated area will carry some damaged cells to other parts of the body, the majority of the tissue / cells in the body get a dose below the damage threshold - and the body is pretty good at repairing itself.
Note however that some organs are much more sensitive than others - there are tables of acceptable maximum dose (or 'dose sensitivity') that show that organs involved in reproduction (gonads) and blood production (bone marrow), and organs with a low ability to self-heal (spinal cord, brain, eyes) or that are acutely vital (heart), should be spared as much as possible during RT.
Having said that, RT does carry a risk - but when a patient is already diagnosed with cancer it is better to take away as much of the "known bad" tissue and take the risk of creating the potential of other mutated cells, rather than leave the cancer alone. At least that is the judgment that oncologists make on a per case basis when they refer for RT.
I will look up some references...