To minimize beam divergence into the eye opposite the entrance port when treating the whole brain, what should be done?

Centering the axis near the lateral canthus is the optimal approach to minimize beam divergence into the eye opposite the entrance port during whole brain radiation therapy. By positioning the treatment axis at this point, the radiation beam can be directed more effectively towards the target area of the brain while reducing the likelihood of unnecessary exposure to the opposite eye.

This technique takes into account the anatomy of the skull and the orientation of the eyes, allowing for a better collimated beam that spares critical structures. Proper alignment in this manner ensures that the dose is concentrated within the intended target while also protecting sensitive areas like the eyes from excessive radiation, which can lead to complications.

Other methods, while they may have some impact, do not sufficiently address the goal of minimizing divergence into the opposite eye. For example, rotating the collimator can adjust the shape of the treatment field but doesn’t directly relate to the alignment in the context of eye protection. Similarly, turning the patient’s head slightly or rotating the couch may alter angles superficially but can also introduce other variables, potentially complicating dose distribution and alignment across the entire treatment field. Thus, centering the axis near the lateral canthus is the most effective method for achieving the desired result.