What must the gantry of the accelerator be adjusted to for effective treatment?

For effective treatment in radiation therapy using an accelerator, the gantry must be adjusted to a position where the beam central axis (CAX) is perpendicular (normal) to the treatment couch top. This alignment is crucial because it ensures that the radiation is delivered accurately to the target area while minimizing dose to surrounding healthy tissues.

When the gantry is vertical with the beam CAX normal to the patient's couch position, it facilitates optimal irradiation of the tumor while maintaining proper geometry for the treatment plan. This position allows for better delivery of the prescribed dose as determined by the treatment plan and contributes to effective targeting of the cancerous tissues.

Other options do not support effective treatment delivery. For example, positioning the gantry horizontally may create challenges in accessing the desired treatment angles, potentially increasing discomfort without an adequate focus on the treatment area. Adjusting the gantry at an arbitrary angle that is not specifically configured for optimal beam penetration could lead to suboptimal treatment outcomes, as it may not adequately conform to the tumor shape or location. Finally, setting the gantry at a fixed angle without consideration of the individual patient’s anatomy would disregard the need for personalized treatment planning essential for achieving the best therapeutic effects.