Why should a patient's posterior surface be as flat as possible during treatment of the entire spinal axis?

A patient's posterior surface should be as flat as possible during treatment of the entire spinal axis primarily to make the dose more uniform. When the patient's surface is flat, it reduces variations in the distance between the radiation source and the target tissue throughout the treatment area. This uniformity is critical because it helps to ensure that the radiation dose delivered to the tumor or target volume is consistent across the entire treatment field.

When the surface is irregular or contoured, it can lead to uneven dose distribution, which can result in some areas receiving too much radiation while others receive too little. This not only affects the efficacy of the treatment but also increases the risk of damage to surrounding healthy tissues. By ensuring a flat surface, radiation beams can be delivered more accurately, leading to optimal treatment outcomes.