Which of the following statements is true regarding field size and patient dose?

• A. Doubling the field size does not affect dose.
• B. Doubling the field size will double the DAP.
• C. Halving the field size will double the DAP.
• D. Field size changes have no relation to dose.

Answer: (B)

Increasing field size raises the total radiation energy delivered to the patient because it expands the area being irradiated, and DAP (dose-area product) is the product of the entrance dose and the irradiated area. If you keep the exposure technique the same and double the field size, the area doubles, so the DAP doubles. In other words, more tissue is exposed, and the total dose integrated over that tissue goes up proportionally with the area. If you halve the field size, the DAP would halve (assuming the same entrance dose). This is why field size changes are directly related to patient dose via DAP.