That's what the CT scan is about--the mapping--before you start. They need to assess your body shape and get an idea about depth and access. Different areas get different depths--if you are having your axillary lymph nodes zapped, for example, its a deep process as the area is thick. Boobs less so. There is always a bit of collateral damage. For example, I've got a high chance of burns top the top of my lung while they are targeting my sub clav nodes. There's also a bit more involvement with my spine and throat than is ideal, but it is not possible to avoid it because of the way I'm put together and the areas that need treating.
If you are concerned get your rad onc to show you a picture of what is actually happening (available after your first treatment) and ask them to explain the peripheral damage--which is measured against known damaging doses.
Marg