During CNS irradiation, the patient is typically positioned:

The correct positioning during CNS irradiation is typically prone with the chin slightly extended. This position allows for optimal alignment of the patient's anatomy relative to the treatment area, which is crucial for effective dosing while minimizing exposure to surrounding healthy tissues.

When the patient is placed in the prone position, gravity can help to stabilize the head and neck, thereby reducing the risk of movement during treatment. Extending the chin slightly aids in aligning the cranial structures more effectively within the radiation field, ensuring that the beam is targeted accurately at the tumor area while sparing critical structures such as the spinal cord.

While the other positioning options may have some application in different contexts, they do not provide the same level of optimal alignment and stability crucial for CNS irradiation. Proper positioning is vital in radiation therapy, as it directly affects the precision of the treatment and the overall success of the care provided.