What is the positioning of the beam when treating a brain through opposed lateral fields to minimize divergence?

When utilizing opposed lateral fields for brain treatment, centering the beam just behind the eye is crucial for effective dose delivery while minimizing radiation exposure to surrounding critical structures. This positioning helps ensure that the central axis of the radiation beam adequately covers the target area, which is typically the tumor or area requiring treatment.

Placing the beam just behind the eye optimally aligns with the brain's anatomy and allows for an efficient entry to the tumor while considering the divergence of the beam. This is particularly important in brain treatments, as the goal is to maximize the radiation dose to the tumor while sparing sensitive tissues such as the eyes and other critical structures nearby.

In contrast, centering at the forehead would not adequately address depth with respect to the tumor location, while centering just in front of or at the base of the skull may increase the risk of unnecessary radiation to the eyes or lead to insufficient coverage of the treatment area. Correct beam positioning is essential for maximizing treatment efficacy and minimizing adverse effects on surrounding healthy tissue.