A simple analytic formula for the central axis percentage depth-dose for high energy electrons in the 5-30 MeV range, penetrating homogeneous, low-Z materials (Z ≤ 20), is derived. Both the intensity due to the primary beam as well as the additional dose due to the ionization electrons are taken into account. We do not consider field size variations at this stage, therefore, our present form only applies on the central axis. Empirical expressions for the position of the maximum dose and the range are used. The final expression is compared with available experimental results and excellent agreement, generally better than 5%, is found.

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