The size of a custom block is determined primarily by which factor?

The size of a custom block is primarily influenced by the distance from the source, which in most clinical settings refers to the radiation source used in treatment. This distance plays a crucial role because it affects the dose distribution to the target area during treatment. Custom blocks are designed to shield certain areas while allowing radiation to reach the tumor effectively.

When determining the size of the block, it's essential to consider how the distance from the treatment source affects the intensity of the radiation beam and how the block will shape that beam for optimal treatment delivery. The further away the block is from the source, the larger it needs to be to adequately block radiation from unintended areas while still directing the required dose to the target.

While the patient's anatomical dimensions do factor into the overall treatment plan and how blocks are used, the direct relationship between block size and distance from the source to ensure effective dose distribution makes this the primary consideration in sizing a custom block.