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 φ_pl^w, and chamber ionization ratio k_pl, of water to SW457 and water to SW557 for 4-15MV photons were calculated as a function of depth using Monte Carlo simulations, and compared with measured values. Values of φ_pl^w for SW457 were in the range of 1.004-1.014 for 4MV, and 1.014-1.018 for 15MV photons. The φ_pl^w for SW557 ranged from 1.005 to 1.008 for 4MV and from 1.010 to 1.015 for 15MV photons and the variation of φ_pl^w with depth for each beam energy was within ±0.5%. Values of k_pl were obtained with a PTW 30013 Farmer-type ionization chamber. The k_pl for SW457 ranged from 0.997 to 1.011 for 4-15MV photons. Values of k_pl for SW557 were almost unity for 4 and 6MV photons, while in the case of 10 and 15MV photons they were less than 1.006, excepting the build-up region. The measured and calculated k_pl values of water to SW557 were in the range of 0.997-1.002 and 1.000-1.006, respectively, for 4-15MV photons, at a depth of 10cm with a source-to-axis distance of 100cm. The measured and calculated k_pl 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 4-15MV photons, and is more water-equivalent than SW457 in megavoltage photon beams.