build-up region
The shallow depth region where radiation dose gradually increases from the surface to reach maximum value in tissue for megavoltage beams.
Full Definition
The build-up region describes the characteristic depth-dose behavior of megavoltage photon beams where the absorbed dose increases gradually from the surface entry point to reach its maximum value at a specific depth within the tissue. This skin-sparing effect occurs because of the forward-directed scatter of secondary electrons created by photon interactions. The depth of maximum dose depends on beam energy, with higher energy beams having deeper build-up regions. This phenomenon is advantageous for most treatments as it reduces surface dose and associated skin reactions.
Usage
Usage note: Advantageous for most treatments but may require bolus for superficial targets requiring surface dose.
In Context
- "The 18 MV beam provided adequate build-up region for skin sparing in the chest wall treatment." — Treatment planning report
- "Build-up region characteristics were considered when selecting the optimal beam energy." — Treatment technique justification