Is it possible to alter DNA after birth

This is for a book I'm writing. I want it to be medically accurate. Doctors are planting a growth in a girl's brain. I need it to be hereditary, so she can pass it on through reproduction.

The DNA would change this, right? It doesn't need to be completely accurate, just partly at least. What medical basis can you give? Thanks!

For starters, you would want the change to take place in the ovaries (female subject). Secondly, if a specially designed virus (complete with reverse-transcriptase) could (conceivably) inject a sequence of genes that could contain the genetic modification. The part that keeps this from being used in real life is finding a virus with the correct reverse-transcriptase to target specific areas that code for genetic diseases we wish to eliminate, and finding the specific start codes for some of them. Side note: this is similar to how some of the more nasty viruses, like HIV, work.

Edit: If you are doing this for a science-fiction book, you could make it two separate procedures: one to change the DNA in the ovaries, and one to get the change in the original subject.

Yes, you can mutate it. However, the benefits are far more limited the later that you mutate it. In addition to the concerns expressed by other answerers (e.g. rejection by the host) you will find that one of the most important parts of DNA is that it controls the development of cells and various bodily structures. Unfortunately, that means that after these structures have developed, those particular parts of the DNA are not normally read again. Especially for some of the more fundamental parts of anatomy, that means that not only do you have to modify the coding DNA but you also have to stimulate the body to attempt to develop the structure again, which is not normally possible.

Everything you (directly) do to a brain is NOT hereditary. The DNA of the ovaries that produce the eggs is the the only DNA that is transferred to the next generation.