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

Physics in medicine and biology (2004-02-20)
Antonio Lopez Medina, Antonio Teijeiro, Francisco Salvador, Daniela Medal, Julio Vazquez, Manuel Salgado, María C Carrion
RESUMEN

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%.

MATERIALES
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Lead, foil, 150x150mm, thickness 0.5mm, as rolled, 99.95%
Lead, foil, 50x50mm, thickness 0.25mm, as rolled, 99.95%
Lead, foil, 50x50mm, thickness 0.1mm, as rolled, 99.95%
Lead, foil, 100x100mm, thickness 3.0mm, as rolled, 99.95%
Lead, foil, 100x100mm, thickness 3.0mm, as rolled, 99.99+%
Lead, foil, 100x100mm, thickness 6.0mm, as rolled, 99.99+%
Lead, foil, 150x150mm, thickness 0.125mm, as rolled, 99.95%
Lead, foil, 150x150mm, thickness 0.15mm, as rolled, 99.95%
Lead, foil, 150x150mm, thickness 0.1mm, as rolled, 99.95%
Lead, foil, 150x150mm, thickness 0.25mm, as rolled, 99.95%
Lead, foil, 150x150mm, thickness 0.55mm, as rolled, 99.999%
Lead, foil, 150x150mm, thickness 1.0mm, as rolled, 99.5%
Lead, foil, 150x150mm, thickness 1.0mm, as rolled, 99.95%
Lead, foil, 250x250mm, thickness 3.0mm, as rolled, 99.95%
Lead, foil, 25x25mm, thickness 0.125mm, as rolled, 99.95%
Lead, foil, 25x25mm, thickness 0.125mm, as rolled, 99.99+%
Lead, foil, 25x25mm, thickness 0.15mm, as rolled, 99.95%
Lead, foil, 25x25mm, thickness 0.1mm, as rolled, 99.95%
Lead, foil, 25x25mm, thickness 0.25mm, as rolled, 99.95%
Lead, foil, 25x25mm, thickness 0.25mm, as rolled, 99.99+%
Lead, foil, 25x25mm, thickness 0.55mm, as rolled, 99.999%
Lead, foil, 25x25mm, thickness 0.5mm, as rolled, 99.95%
Lead, foil, 25x25mm, thickness 0.5mm, as rolled, 99.99+%
Lead, foil, 25x25mm, thickness 1.0mm, as rolled, 99.5%
Lead, foil, 25x25mm, thickness 1.0mm, as rolled, 99.95%
Lead, foil, 25x25mm, thickness 1.0mm, as rolled, 99.99+%
Lead, foil, 25x25mm, thickness 1.5mm, as rolled, 99.99+%
Lead, foil, 25x25mm, thickness 3.0mm, as rolled, 99.99+%
Lead, foil, 300x300mm, thickness 0.125mm, as rolled, 99.95%
Lead, foil, 300x300mm, thickness 0.15mm, as rolled, 99.95%

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