Dosimetric properties of a Solid Water High Equivalency (SW557) phantom for megavoltage photon beams

The dosimetric properties of the recently developed SW557 phantom have been investigated by comparison with those of the existing SW457 phantom in megavoltage photon beams. The electron fluence ratio φplw, and chamber ionization ratio kpl, of water to SW457 and water to SW557 for 4-15 MV photons were calculated as a function of depth using Monte Carlo simulations, and compared with measured values. Values of φplw for SW457 were in the range of 1.004-1.014 for 4 MV, and 1.014-1.018 for 15 MV photons. The φplw for SW557 ranged from 1.005 to 1.008 for 4 MV and from 1.010 to 1.015 for 15 MV photons and the variation of φplw with depth for each beam energy was within ±0.5%. Values of kpl were obtained with a PTW 30013 Farmer-type ionization chamber. The kpl for SW457 ranged from 0.997 to 1.011 for 4-15 MV photons. Values of kpl for SW557 were almost unity for 4 and 6 MV photons, while in the case of 10 and 15 MV photons they were less than 1.006, excepting the build-up region. The measured and calculated kpl values of water to SW557 were in the range of 0.997-1.002 and 1.000-1.006, respectively, for 4-15 MV photons, at a depth of 10 cm with a source-to-axis distance of 100 cm. The measured and calculated kpl values were in agreement within their uncertainty ranges. As a water-equivalent phantom, SW557 can be used with a dosimetric difference within ± 0.6%, for 4-15 MV photons, and is more water-equivalent than SW457 in megavoltage photon beams.