Primary dose in photon beams with lateral electron disequilibrium

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Abstract
It is shown that in narrow monoenergetic photon beams under conditions of lateral electron disequilibrium, primary absorbed dose P(r) is a simple function of beam radius r : P(r)=Pλ⋅(1−e−γ⋅r), where Pλ is the primary dose in broad beams for which complete lateral electron equilibrium exists, and γ depends on photon energy and absorbing medium. This formula was evaluated using Monte‐Carlo‐generated data for the primary dose in water from monoenergetic photons in the energy range from 2 to 8 MeV. The primary dose was studied in beams of radii 0.006 cm<rd≤24 cm. It was concluded that the saturation equation above provides an accurate description of the primary dose from monoenergetic photon beams, as judged by comparison with Monte Carlo results.

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