External beam radiation therapy can expose patients to unintended sources of radiation such as patientscattering, collimator scattering, and leakage from the machine head. These sources can contribute toout-of-field doses that have the potential to induce long-term adverse effects and secondary cancers, evenat relatively low doses. To better evaluate the secondary effects of modern radiotherapy treatments anddetermine the impact of spectral variations on energy-dependent dosimeters, it is first necessary tounderstand the photon and electron energy spectra in the beam-bordering and out-of-field regions. Thisstudy aimed to evaluate the photon and electron fluence spectra as well as mean energies beyond thefield edge and the corresponding spectra on in-field regions for several small radiotherapy fields. Phase-space files of a 6 MV photon beam produced by a Varian TrueBeam™ linac for eight small circularfields of 0.75 cm, 1.0 cm, 2 cm, 2.5 cm, 3 cm, 3.5 cm, 4 cm, and 5 cm and for the reference 10 cm fieldat SSD = 100 cm were generated using the BEAMnrc code. The photon and electron fluences in eachfield were calculated at 0.125, 1. 5, and 10 cm water depth and several off-axis distances usingFLURZnrc. The study found that the photon fluence spectra strongly depend on spatial positions andvary significantly as a function of depths, off-axis distances, and field size. Compared to the spectrumwithin the field, the spectrum outside of the field showed greater variations. Changes in depth caused amore significant difference in the shapes of photon spectra since beyond Dmax, the photon spectra weresignificantly weighted towards the low-energy photons. Furthermore, the behavior of electrons is depth-dependent beyond the field edge, where the mean electron energy at depth near the surface is greater thanin-field regions, especially in small fields. This is due to the relatively high linear energy transfer (LET)of these electrons and primary electron beam leaking from the linac head, leading to surface doseenhancement. Thus, when assessing the dose delivered to a patient\\\'s skin, it is necessary to consider theseelectrons from a radiological protection perspective.

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