Absorbed Dose: Rad and Gray

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Questions and Answers

What is the relationship between gray (Gy) and rad?

  • 1 Gy = 100 rads (correct)
  • 1 Gy = 10,000 rads
  • 1 Gy = 10 rad
  • 1 Gy = 1000 rad

How is absorbed dose calculated?

  • Absorbed dose = kerma - bremsstrahlung (correct)
  • Absorbed dose = kerma
  • Absorbed dose = kerma + bremsstrahlung
  • Absorbed dose = kerma x bremsstrahlung

What is the SI unit of absorbed dose?

  • Gray (correct)
  • Rad
  • Kerma
  • Roentgen

What does 1 milligray (mGy) equal in gray (Gy)?

<p>1/1000 Gy (B)</p> Signup and view all the answers

Which of the following describes kerma?

<p>The energy transferred to the medium from radiation (C)</p> Signup and view all the answers

What is the energy associated with 1 roentgen in air?

<p>8.5 x 10^-3 J/kg (A)</p> Signup and view all the answers

What factor is used to convert roentgen to rad?

<p>Roentgen-to-rad conversion factor (D)</p> Signup and view all the answers

How many microgray (µGy) are in one gray (Gy)?

<p>10,000 µGy (D)</p> Signup and view all the answers

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Study Notes

Absorbed Dose: Rad and Gray

  • Introduced by ICRU in 1962, the rad (r) is a unit of absorbed dose.
  • 1 rad equals 100 ergs per gram, independent of radiation type and medium.
  • Radiation exposure leads to energy transfer defined as kerma, but some energy may not be absorbed due to bremsstrahlung and delta rays.
  • Absorbed dose calculation: Absorbed dose = Kerma - bremsstrahlung X-rays.
  • The SI unit for absorbed dose is gray (Gy), defined as 1 Gy = J/kg = 10^7 erg.
  • Conversion: 1 Gy = 100 rads; common submultiples include:
    • 1 milligray (mGy) = 10^-3 Gy
    • 1 microgray (µGy) = 10^-6 Gy.
  • Relationships: 1 mGy = 100 mrad; 1 µGy = 100 urad.
  • Rad or gray units describe energy delivered to the medium and are suitable for quantifying biological effects in patients.

Roentgen-to-Rad Conversion

  • The absorbed dose (D) relates to roentgen through a conversion factor (f) specific to materials.
  • F-factor values vary with photon energy (in keV) and material type (specifically for fat, air, and muscle):
    • 30 keV: f = 0.53
    • 50 keV: f = 0.64
    • 100 keV: f = 0.58
    • 150 keV: f = 0.53

Conversion of Exposure to Absorbed Dose

  • Dose in air (Dair) relates to the charge of 1 roentgen, which is 2.58 × 10^-4 Coulombs/kg.
  • Average energy for a photon causing this charge is approximately 33 J.
  • Total energy absorbed per kilogram of air equates to 8.5 × 10^-3 J/kg, numerically equal to 1 roentgen.

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