EL ADIOTHERAPY & ONCOLOGY ELSEVIER Radiotherapy andOncology 41 (1996)277-280 Beam flatness perturbation effects of a perforated thermoplastic irnmobilization device on 6 and 12 .MeV electron beams’ Ricky L. Van WayaYb, Revlon 0. Williams”‘*, Peter AS. Johnstonea, Patrick J. Papinb “Radiation Oncology Division, Naval Medical Center, San Diego, CA 92134-5000, USA “Department of Physics, San Diego State University, San Diego, CA 92182.1233, USA Received 15 January 1996; revised10 September 1996; accepted 20 September 1996 Abstract 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 effect of perforated thermoplastic on 6 MeV and 12 MeV electron beams was describedin terms of optical density differences in a comparative film study. Results: Variations of beam flatness were documentedof 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 consideredinsignificant (meanoptical density differences <2%) is approximately 10 mm for both beam energies. Conclusions: For clinical situations where the target volume is superficial, someconsideration should be given to beam inhomogeneity causedby the mask. Keywords: Beam modification; Electron beam flatness; Dose distribution; Electrons; Immobilization devices; Perforated thermoplastic; Radiation therapy 1. Introduction Radiotherapy of head and neck malignancies requires accurate and reproducible setup techniques. To this end, several brands of perforated thermoplastic immobilization masks have become available; these facilitate setup repro- ducibility by providing rigid landmarks and minimizing potential patient movement during treatment. In providing these necessary functions, the question arises as to the magnitude of beam perturbation or attenuation contributed by the mask. Some treatment centers cut beam portals, or windows, in the mask to prevent this potential problem. However, cutting the mask is not optimal as its rigidity decreases with increasing size and number of such cutouts. Conversely, not cutting portals preserves mask rigidity but All tests were conducted in the Radiation Oncology Clinic at the Naval Medical Center, San Diego, CA. The brand of thermoplastic used at this facility is Aquaplast (WFR Aquaplast Corp., Wychoff, NJ); the 3.5mm thick- ness was used throughout this experiment. Film was chosen as the measurement modality primarily because of its ability to record minute changes in electron flux in a small spatial arrangement. The film used was Kodak X- Omat V Therapy Verification film in the Ready Pat format (emulsion number CAT- 1655927, 24X30 cm size.) Expo- sures were made on aVarian model 2100 C medical linear *Corresponding author. ‘The opinions or assertions contained hereinare those of the authors and are not to be construed as official or representing the views of the United States Navy or Department of Defense. 0167-8140/96/$15.00 0 1996 Elsevier Science IrelandLtd. All rights reserved PII SO167-8140(96)01848-8 adversely reduces the skin-sparing effect for megavoltage photon fields [2]. This paper examines the extent to which perforated thermoplastic disturbs the beam flatness of 6 and 12 MeV electron beams at various depths in tissue since these beam are used to treat more superficial tumors. 2. Materials and methods