Blocking

When a part of the open beam is blocked for reducing dose to an organ at risk,

Primary dose in the patient under the unblocked region remains almost the same.

Moderate blocking does not affect the effective primary reaching the phantom. Scatter dose under the unblocked region decreases since the shadow region woudl scatter very little radiation.

The dose under the block comprises the primary leakage radiaton and scatter going into the shadow region from unblocked regions.