During external beam radiation therapy, patients are exposed to secondary radiation sources, contributing to out-of-field doses with potential long-term adverse effects.Understanding photon and electron energy spectra is essential to evaluate the secondary effects of modern radiotherapy. This study aimed to evaluate the photon and electron fluence spectra and mean energy beyond the field edge as well as in-field regions for several small radiotherapy fields. The study used International Atomic Energy Agency (IAEA) phase-space files for the 6 MV photon beams produced by three commonly used linear accelerators to generate small and standard fields. The mean photon and electron energies were calculated at multiple depths and off-axis distances for the three linear accelerators and a predefined 6 MV spectrum. The study found that the photon fluence spectra strongly depend on spatial positions and vary significantly as a function of depth, off-axis distance, field size, and linac model. Furthermore, the behavior of electrons is depth-dependent beyond the field edge, where the mean electron energy near the surface is greater than in-field regions, especially in small fields, leading to surface dose enhancement.

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