The Effect of Aquaplast on Surface Dose of Photon Beam

Oh, Do Hoon; Bae, Hoon Sik

Abstract

PURPOSE: To evaluate the effect on surface dose due to Aquaplast used for immobilizing the patients with head and neck cancers in photon beam radiotherapy.
MATERIALS AND METHODS
To assess surface and buildup region dose for 6MV X-ray from linear accelerator(Siemens Mevatron 6740), we measured percent ionization value with the Markus chamber model 30-329 manufactured by PTW Frieburg and Capintec electrometer, model WK92. For measurement of surface ionization value, the chamber was embedded in 25 X 25 X 3 cm3 acrylic phantom and set on 25 X 25 X 5 cm3, polystyrene phantom to allow adequate scattering. The measurements of percent depth ionization were made by placing the polystyrene layers of appropriate thickness over the chamber. The measurements were taken at 10 cm SSD for 5 X 5 cm2 , 10 X 10 cm2 , and 15 X 15 cm2 field sizes, respectively. Placing the layer of Aquaplast over the chamber, the same procedures were repeated. We evaluated two types o Aquaplast: 1.6mm layer of original Aquaplast(manufactured by WFR Aquaplast Corp.) and transformed Aquaplast similar to moulded one for immobilizing the patients practically. We also measured surface ionization values with blocking tray in presence or absence of transformed Aquaplast. In calculating percent depth dose, we used the formula suggested by Gerbi and khan to correct overresponse of the Markus chamber.
RESULTS
he surface doses for open fields of 5 X 5 cm2 , 10 X 10 cm2 , 15 X 15 cm2 were 7.9%, 13.6%, and 18.7% respectively. He original Aquaplast increased the surface doses upto 38.4%, 43.6% and 47.4% respectively. There were little differences in percent depth dose values beyond the depth of Dmax. Increasing field size, the blocking tray caused increase of the surface dose by 0.2%, 1.7%, 3.0% without Aquaplast, 0.2%, 1.9%, 3.7% with transformed Aquaplast, respectively.
CONCLUSION
The original and transformed Aquaplast increased the surface dose moderately. The percent depth doses beyond Dmax, however, were not affected by Aquaplast. In conclusion, although the use of Aquaplast in practice may cause some increase of skin an dbuildup region dose, reduction of skin-sparing effect will not be so significant clinically.