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Electrometer offset current due to scattered radiation.

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Summary
This summary is machine-generated.

Internal electrometers used with small ionization chambers show polarity dependence due to scattered radiation. Shielding the electrometer significantly reduces this effect, improving relative dose measurements in radiation therapy.

Keywords:
electrometermicro-ionization chamberspolarityrelative dosimetryscatter radiation

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Area of Science:

  • Medical Physics
  • Radiation Oncology
  • Dosimetry

Background:

  • Relative dose measurements in radiation therapy using internal electrometers and small ionization chambers exhibit significant polarity dependence.
  • This polarity effect, particularly on percent depth dose (PDD) curves, has been observed but not fully understood or mitigated.

Purpose of the Study:

  • To investigate the cause and impact of polarity dependence in internal electrometers used with small ionization chambers.
  • To identify methods for reducing or eliminating the observed polarity effects on dose measurements.

Main Methods:

  • An internal electrometer was placed 1m from the isocenter and exposed to varying scattered radiation levels by adjusting field size.
  • Measurements were taken with nine ionization chambers of different volumes, analyzing signal ratios (M20/M10) at varying bias polarities.
  • The electrometer was shielded and repositioned to assess the impact on current and PDD measurements.

Main Results:

  • Electrometer irradiation generated a consistent negative current (-1 pA), distorting measurements, especially for low-sensitivity detectors.
  • Smaller ionization chambers showed greater polarity dependence; the iba CC003 (3 mm³) exhibited a ~7% difference in M20/M10 for a 10x10 cm² field.
  • Shielding the electrometer or increasing its distance from the source reduced the extraneous current and aligned PDDs for both polarities, even for the smallest chamber.

Conclusions:

  • Scattered radiation-induced current in internal electrometers is the primary cause of polarity dependence in relative dose measurements.
  • Shielding the electrometer is an effective method to mitigate polarity effects and improve the accuracy of PDD and beam profile measurements.
  • Recommendations include monitoring setups with internal electrometers by testing both polarities and shielding the electrometer if necessary.