What is the reference depth for monitor unit calculations in electron beam therapy?

In electron beam therapy, the reference depth for monitor unit calculations is specified at D-Max, which is the depth where electron dose reaches its maximum value. This depth varies depending on the energy of the electron beam used, and it is crucial for accurate dosimetry because it represents the point where the dose distribution is most stable and uniform.

Using D-Max for monitor unit calculations is important because the maximum dose region is where the effectiveness of the treatment can be most accurately quantified, ensuring that the prescribed dose is delivered effectively to the target tissues while minimizing exposure to surrounding healthy tissues.

The other choices relate to different aspects of electron therapy. R50,D refers to the depth at which the dose falls to 50% of its value at the surface, which is more relevant for understanding dose fall-off rather than monitor unit calculations. The mean energy level signifies an average energy of the photon or electron beam but does not provide a specific point for dose delivery assessments. Surface level pertains to the very outer layer of tissue receiving the beam, which is significant in terms of initial dose distribution, but not the reference for monitor units.