Is delivery critical or just get it close enough to tissue in ROI? Are we aiming to modify specific cells/layers? And what happens to adjacent tissue that is '

Is delivery critical or just get it close enough to tissue in ROI? Are we aiming to modify specific cells/layers? And what happens to adjacent tissue that is 'accidentally' modified? Do you have any literature recommendations on the state of the art in Gene Therapy 'delivery' mechanisms/apparatus?