Topic 5 - Dose Units

Internally Deposited Radioisotopes

• Absorbed dose from internal emitters
  • Follows directly from definition dose
  • For particles with short ranges
    • all energy absorbed in tissue :

SEE Concept

• Specific Effective Energy
  • infinitely large medium
  • uniformly distributed radionuclide
  •  absorbed energy = emitted energy
• “Infinitely large” means
  • dimensions exceed the range of radiation
  • considers mass of tissue, m:

Effective Half-life

• Used to describe residence time of a radionuclide in a tissue or an organism
• Considers
  • radiological (physical) decay (T_R)
  • biological elimination (T_B)
• If biological elimination follows first-order kinetics,
  • then can describe with loss rate constant, l_B 
  • related to half life by l_B = (ln 2/ T_B)

Effective Loss-Rate Constant

• Biological Loss rate constant, l_B 
• Sums with radiological loss-rate constant , l_R
• l_B + l_R = l_E (effective elimination constant)
• related to half life by l_E = (ln 2/ T_E)

Effective Half Life

• Biological (T_b) and Radiological (T_r) half lives together eliminate the radionuclide from the body faster than either one alone
• Combined, they are the Effective half life

Where to Find Effective Half Lives (T_e)

• Dependent on chemical form
• General guidelines in ICRP 2, ICRP 30

Dose Commitment

Gamma Emitters: Calculating dose at a point, P

Gamma Emitters for a sphere

• Integrating with respect to each of the variables, the dose rate at the center of the sphere is:

Geometry factors for center of a sphere

Geometry Factors

• The multiplier is called g, the geometry factor

Geometry Factors

• g is for point in a volume of tissue
• what about average dose rate, rather than single point?
  
• For a sphere
  

Geometry Factors

• what about at any point a distance d from the center of the sphere of radius R?

MIRD & Other Dosimetry Methods

• Wait until after biological effects discussion