Background and purpose: The underdose near air cavities in the head and neck region at photon energies of 4 MV and 6 MV was studied in search for clinical advantages of the 4 MV over 6 MV treatments.
Materials and methods: The on-axis and off-axis dose distributions were measured with a parallel-plate ionization chamber and films in polystyrene phantoms containing an air cavity of appropriate size based on the results of computed tomography scans.
Results: Although most results are similar for both energies, the 4 MV photon beams give a somewhat smaller underdose effect and a faster re-build up than the 6 MV. For both energies a significant underdose effect was observed at the edge of the field in the larynx phantom. This proved to be true for small and large fields, for smaller and larger cavities, for one-beam as well as parallel-opposed beams.
Conclusion: For most clinically relevant situations there is no remarkable benefit in the use of either of the two energies.