Skin damage probabilities using fixation materials in high-energy photon beams

Introduction: Patient fixation, such as thermoplastic masks, carbon-fibre support plates and polystyrene bead vacuum cradles, is used to reproduce patient positioning in radiotherapy. Consequently low-density materials may be introduced in high-energy photon beams. The aim of the this study was to measure the increase in skin dose when low-density materials are present and calculate the radiobiological consequences in terms of probabilities of early and late skin damage. Method: An experimental thin-windowed plane-parallel ion chamber was used. Skin doses were measured using various overlaying low-density fixation materials. A fixed geometry of a 10 x 10 cm field, a SSD = 100 cm and photon energies of 4, 6 and 10 MV on Varian Clinac 2100C accelerators were used for all measurements. Radiobiological consequences of introducing these materials into the high-energy photon beams were evaluated in terms of early and late damage of the skin based on the measured surface doses and the LQ-model. Results. The experimental ion chamber gave results consistent with other studies. A relationship between skin dose and material thickness in mg/cm(2) was established and used to calculate skin doses in scenarios assuming radiotherapy treatment with opposed fields. Conclusion: Conventional radiotherapy may apply mid-point doses up to 60-66 Gy in daily 2-Gy fractions opposed fields. Using thermoplastic fixation and high-energy photons as low as 4 MV do increase the dose to the skin considerably. However, using thermoplastic materials with thickness less than 100 mg/cm2 skin doses are comparable with those produced by Variation in source to skin distance, field size or blocking trays within clinical treatment set-ups. The use of polystyrene cradles and carbon-fibre materials with thickness less than 100 mg/cm2 should be avoided at 4 MV at doses above 54-60 Gy. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.