Beam flatness perturbation effects of a perforated thermoplastic immobilization device on 6 and 12 MeV electron beams.

Purpose: Perforated thermoplastic masks are widely used in radiotherapy of head and neck malignancies. They provide for patient immobilization and increase setup reproducibility. Some oncology treatment centers cut mask portals (windows) for the beam to pass through; for those centers that do not, the mask affects beam fluence. The extent to which beam flatness is altered by such a mask is investigated.

Materials And Methods: The effects of perforated thermoplastic on 6 MeV and 12 MeV electron beams was described in terms of optical density differences in a comparative film study.

Results: Variations of beam flatness were documented of up to 11.8% at 5 mm depth for 6 MeV, and 8.1% for 12 MeV electrons. The depth at which this effect may be considered insignificant (mean optical density differences < 2%) is approximately 10 mm for both beam energies.

Conclusions: For clinical situations where the target volume is superficial, some consideration should be given to beam inhomogeneity caused by the mask.