Polarity effects of ionization chambers used in TBI dosimetry due to cable irradiation

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• 作者：Fiorino ; C. ; Mangili ; P. ; Cattaneo ; G.M. ; Calandrino ; R.
• 关键词：Polarity effects ; Cable effects ; Total-body irradiation ; Dosimetry
• 刊名：Medical Dosimetry
• 出版年：2000
• 出版时间：Autumn, 2000
• 年：2000
• 卷：25
• 期：3
• 页码：121-126
• 全文大小：100 K

文摘

This paper presents the results of an investigation on polarity effects in total-body irradiation (TBI) dosimetry. Thimble (NE2571, 0.6 cc) and plane-parallel (Markus NE2534 0.055 cc) chambers were investigated in a 30 × 30 × 30-cm³ acrylic phantom in TBI conditions (6-MV x-rays). The thimble chamber was positioned at the midline and at the entrance and exit D_max (1.5 cm from the phantom surface) levels. The Markus chamber, which is generally used for skin dose estimations, was positioned at various depths from the entrance surface of the phantom (from 0- to 2-cm depth). The polarity factor (P_pol) was defined as (Q⁺ + Q⁻)/2Q⁻, where Q⁺ and Q⁻ were the collected charges at positive and negative bias voltage, respectively. The variations of P_pol with many parameters (absorbed dose, dose rate, the presence or absence of a 1-cm acrylic spoiler, irradiated cable length) were investigated. Results show that P_pol is quite small (within 1.002 for on-axis measurements and 1.005 for off-axis measurements) for the NE2571 chamber when the beam spoiler is placed. P_pol was significantly higher without the beam spoiler (within 1.008 for on-axis measurements, up to 1.02 for off-axis measurements). Concerning the Markus chamber, for on-axis skin dose measurements, P_pol was found to be less than unity (around 0.988) or more than unity (around 1.0035), respectively, with and without the beam spoiler. Possible “directional effects” of the currents generated in the cable were investigated for both chambers and found to be insignificant. This shows that the application of P_pol correction has to be considered a reliable procedure in minimizing these effects. When the beam spoiler is placed, the cable has to be drawn to minimize the portion of cable just outside the beam; if this is not the case, P_pol may significantly vary (for the NE2571 chamber values up to 1.0035 were found for on-axis measurements).