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

C. Fiorino, P. Mangili, G. M. Cattaneo, R. Calandrino

Research output: Contribution to journalArticle

Abstract

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 x 30 x 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). (C) 2000 American Association of Medical Dosimetrists.

Original languageEnglish
Pages (from-to)121-126
Number of pages6
JournalMedical Dosimetry
Volume25
Issue number3
DOIs
Publication statusPublished - 2000

Fingerprint

Whole-Body Irradiation
spoilers
ionization chambers
cables
dosimeters
polarity
chambers
Skin
irradiation
X-Rays
dosage
entrances
unity
electric potential

Keywords

  • Cable effects
  • Dosimetry
  • Polarity effects
  • Total-body irradiation

ASJC Scopus subject areas

  • Oncology
  • Radiology Nuclear Medicine and imaging
  • Radiation
  • Health Professions(all)

Cite this

Polarity effects of ionization chambers used in TBI dosimetry due to cable irradiation. / Fiorino, C.; Mangili, P.; Cattaneo, G. M.; Calandrino, R.

In: Medical Dosimetry, Vol. 25, No. 3, 2000, p. 121-126.

Research output: Contribution to journalArticle

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abstract = "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 x 30 x 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). (C) 2000 American Association of Medical Dosimetrists.",
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PY - 2000

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N2 - 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 x 30 x 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). (C) 2000 American Association of Medical Dosimetrists.

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