What is the impact of documenting collimation on dose calculation?

• A. It has no impact on dose calculations.
• B. It increases the exposure.
• C. It reduces the area exposed, thereby changing the dose calculation.
• D. It affects only the image contrast.

Answer: (C)

Collimation changes the amount of tissue that actually receives radiation, and dose calculations are based on both the beam output and the area irradiated. When you document the collimation, you’re specifying the true field size used. A smaller field means less tissue is exposed, so the total energy absorbed by the patient decreases even if the beam intensity remains the same. If the calculation uses that smaller field, the estimated patient dose will be lower. Conversely, assuming a larger field would overestimate the dose. So documenting collimation directly affects the dose calculation because it reflects the reduced irradiated area, not because the exposure is inherently higher or because it only changes image contrast.