Comparison between TG-51 and TRS-398: electron contamination effect on photon beam-quality specification

Abstract

Two dosimetry protocols based on absorbed dose to water have recently been implemented: TG-51 and TRS-398. These protocols use different beam-quality indices: %dd(10)x and TPR20,10. The effect of electron contamination in measurements of %dd(10)x has been proposed as a disadvantage of the TG-51. For actual measurements of %dd(10)x in five clinical beams (Primus 6-18 MV, SL-75/5 6 MV, SL-18 6-15 MV) a purging magnet was employed to remove the electron contamination. Also, %dd(10)x was measured in the different ways described in TG-51 for high-energy beams: with a lead foil at 50 cm from the phantom surface, at 30 cm, and for open beam. Moreover, TPR20,10 was determined. Also, periodic quality-control measurements were used for comparing both quality indices and variation over time, but D20,10 was used instead of TPR20,10 and measurements in open beam for the %dd(10)x determination. Considering both protocols, S(w,air) and kQ were calculated in order to compare the results with the experimental data. Significant differences (0.3% for kQ) were only found for the two high-energy beams, but when the electron contamination is underestimated by TG-51, the difference in kQ is lower. Differences in the other cases and variations over time were less than 0.1%.

From the limits of the classical model of sensitometric curves to a realistic model based on the percolation theory for GafChromic™ EBT films

“…This depth guarantees 39 that electron contamination of the photon beam can be neglected. The quality specifier for the 6 MV photon beam 40 is TPR 20,10 = 0.6738Ϯ 0.0013.…”

Characterization of electron contamination in megavoltage photon beams

“…For the actual measurements of electron contamination in a clinical beam a purging magnet was performed, 8 based on two permanent magnets of an alloy of Nd-Fe-B, of remanent field B r = 1.23 T, and coercitive field of H c = 860 kA/ m ͑10.8 kOe͒. Each magnet measures 2 ϫ 10ϫ 10 cm 3 .…”

“…In the center of the magnet, the magnetic field was similar to previous measurements with permanent magnets-0.12 T ͑20 MV͒ 11 and 0.14 T ͑4-25 MV͒ 34 -or with electromagnet, which ranges 24-26 from 0.08 to 0.39 T. More details concerning this magnet can be found in previous publications. 4,8…”

“…Separation of the contaminant electron dose at shallow depth is more and more important due to the increasing use of convolution-based three-dimensional models in TPS [1][2][3][4][5] and it plays an important role in beam-quality specification as defined in TG-51. [6][7][8] The influence of electron contamination on absolute calibration has been studied elsewhere 8 and is usually overridden in TPS as the beam calibration is made at points deeper than the maximum range of contaminant electrons.…”

“…In the literature, three methods have been described for the determination of electron contamination: ͑a͒ direct measurements with a magnet, 8,11,[24][25][26] and/or helium bag, 11 ͑b͒ variations of PDD with the field size based on analytical methods, [27][28][29][30] or ͑c͒ Monte Carlo simulations. [12][13][14] In the present paper, we compare the first two methods: experimental and analytical ones.…”