Absorbed Dose: Rad and Gray

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.