The following sections offer some specific clinical applications. The ability to control each collimator independently means that the isocenter can be positioned at places other than the center of the field. It may be placed at a mid-line, or mid plane point that will facilitate patient set up, beam junction plane, or be coplanar with a previous field's beam edge. This freedom allows for the possibility of matching adjacent fields with non-diverging beam edges with excellent dose distribution through the junction region. If the isocenter is placed in an ideal location anticipating possible future treatments or re-treatment, then the subsequent treatments can be simplified and the chance of injury eliminated.
Some examples follow.
The isocenter is placed inferior to the temporal lobe. The gantry is angled 5 degrees posteriorly to eliminate anterior diverging x-rays from exiting through the opposite eye. Eye and ear canal blocks may be inserted to protect these structures. TLD measurements reveal that the eyelid surface dose from scatter or transmitted x-rays averages about 7cGy when the center of the field receives a dose of 200cGy. A representative port film of the whole brain technique is included, showing the blocking.