In the case of a blocked field, what primarily affects the dose in the patient's shadow region?

When considering a blocked field in radiation therapy, the primary factor that affects the dose in the patient's shadow region is the thickness of the block. As radiation interacts with a blocking material, the degree to which it is attenuated is largely dependent on the material's density and thickness. A thicker block inevitably leads to a greater reduction in radiation transmission, thereby decreasing the dose in the shadow region behind the block.

The type of radiation can influence how effectively the block attenuates the dose, but it is the thickness that directly determines the extent of the shadow effect, as it dictates how much of the beam's intensity is absorbed or scattered. The duration of treatment and the distance from the radiation source serve other purposes in treatment planning but do not primarily dictate the dose reduction in the shadow region created by a block. Thus, the thickness of the block is the key determinant in how much dose the area behind it will receive.