Photon beam energy selection for scalp dose reduction in whole-brain radiotherapy: a benchmark phantom study

To evaluate the effects of photon beam energy on region-specific scalp dose and target coverage in whole-brain radiotherapy (WBRT), with additional assessment of field configuration. This study was conducted as a benchmark dosimetric investigation using an anthropomorphic head phantom. WBRT plans were generated with a field-in-field technique and lateral opposed fields. Photon beam energies of 4, 6, 8, 10, and 15 MV were assessed under two field configurations: an open-field plan and a close-field plan without air space. The scalp was segmented into four anatomical regions. Dosimetric evaluation included planning target volume coverage, homogeneity index, scalp D_2cc, and normal tissue complication probability (NTCP) for permanent alopecia based on generalized equivalent uniform dose. Lens and cochlear doses were also analyzed. PTV coverage and dose homogeneity were comparable across beam energies. In all scalp regions, dose decreased with increasing photon beam energy, with the strongest dependence in the top and back regions. The lowest scalp NTCP was observed at 15 MV, although 8-10 MV yielded comparable reductions. The close-field configuration consistently reduced scalp dose relative to the open-field approach, with a maximum NTCP reduction of approximately 17%. Lens dose was lower with the close-field approach, whereas cochlear dose decreased with decreasing photon beam energy. Photon beam energy selection in WBRT has a clear region-specific impact on scalp dose. Energies of 8-10 MV, particularly with a close-field approach, may provide a practical balance between scalp sparing, OAR protection, and clinical feasibility.