How does the dose build-up in the initial portion of the buildup region behave?

The behavior of dose build-up in the initial portion of the buildup region is characterized as non-linear and very steep due to the physics of radiation interactions with tissue. During this phase, the dose increases significantly as the depth of tissue increases, and this increase is not uniform.

As photons or electrons penetrate the tissue, they undergo interactions that lead to ionization, which produces secondary electrons. These secondary electrons then continue to create further ionizations, resulting in an efficient energy deposition as energy is transferred to the surrounding tissue. However, this energy deposition does not follow a simple linear pattern. Instead, at shallow depths—especially just below the surface—there is an exceedingly rapid rise in dose due to the initial interactions and buildup of secondary electron fluence.

This non-linear increase can create a steep dose gradient. Erroneously assuming it to be linear or uniform could lead to miscalculations in dosimetry, impacting treatment planning and patient safety. Understanding the nature of this dose build-up is essential for accurate treatment delivery, particularly in radiation therapy where precise dosimetry is required for effective cancer treatment.