Background: This study aimed to compare high-grade gliomas radiation therapy methods (high energy three-dimensional conformal radiation therapy [3D-CRT] vs. low energy intensity-modulated radiation therapy [IMRT]). Materials and Methods: Out-of-field photons resulting from 15- and 18-MV 3D-CRT and 6-MV IMRT techniques were measured with thermo-luminescent dosimeters in a head and neck homogeneous phantom. Moreover, the dose to the left and right eye lenses, parotid glands, the thyroid gland, and the tongue was determined for each of the treatment techniques. Additionally, the risk of secondary thyroid cancer was estimated according to the biological effects of ionizing radiation BEIR VII model. Results: Errors in the treatment planning system (TPS) increased with increasing distance from the field edge and varied in different treatment techniques. The 6-MV IMRT technique increased the photon dose to all of the organs. Further, the excess relative risk for thyroid cancer was obtained higher in the 6-MV IMRT technique than in the 15- and 18-MV 3D-CRT techniques, as the photon dose to the thyroid gland was higher. Conclusion: Although 6-MV IMRT better improves local control and dose distribution than 3D-CRT, it increases the total dose equivalent in out-of-field organs independently of beam energy and exacerbates the risk of secondary thyroid cancer more significantly.