The dose to a patient’s skin and build-up region increases with which factor?

• A. Decreasing field size
• B. Decreasing energy
• C. Increasing field size
• D. Placement of bolus material

Answer: (D)

Photons have a skin-sparing buildup: the dose rises with depth just beneath the surface until it reaches a maximum at a shallow depth, then falls off. Placing bolus material on the skin adds tissue in the beam’s path, so the buildup region shifts toward the surface. The beam effectively deposits its maximum dose nearer or at the skin, increasing the dose to the skin and the superficial buildup region. The bolus therefore makes the surface dose—and the dose in the initial buildup layer—higher by providing the necessary electron equilibrium right at the skin’s surface.