INTERNAL RADIATION DOSIMETRY

Transcription

1 INTERNAL RADIATION DOSIMETRY

2 Introduction to Internal Dosimetry

3 Importance of External and Internal Exposures by Radiation Type Charged particle radiation (α, p, β) Generally important only for internal exposure Generally, charged particles (α, p, β) are not important for external exposure because they can not penetrate skin and give dose to organs and tissues in human body But, when they are in human body, they are very much concern because all the energy from the particle will be absorbed by human tissue In addition, radiation weighting factor of α particle is 20. Photon and neutron radiation Generally important for both external and internal exposures For external exposure, the radiations can contribute radiation doses to organs and tissues in human body For internal exposure, the radiations also can contribute radiation doses to organs and tissues in human body.

4 General Definitions for Internal Dosimetry Internal Dose Assessment A process of measurement and calculation that results in an estimate of the dose to tissue in the body from an intake of radioactive material Measurement can take one of several forms: Factors influencing internal dose calculations:

5 Intake and uptake Intake: Amount of radioactive material that passes through the mouth or nose during inhalation or ingestion Uptake: Amount of radioactive material which reach the blood stream

6 Pathways of Intake (The internal exposure results from radioactive materials in human body) (The radioactive materials can enter via several pathways) Pathways of intake Inhalation: Ingestion: Absorption H-3 and I can be absorbed through undamaged skin

7 Elimination Process Pathways of excretion from body Exhalation Urine and feces Elimination process of radioactive material from an organ

8 Dosimetry Quantities for Internal Dosimetry Committed dose (Once a radionuclides has been deposited in the body, the exposed person is committed to the dose from the radionuclides so long as it is present in the body.) (The long-time stay of radionuclides thus long-time exposure should be considered for internal dosimetry.) External Exposure Internal Exposure Radiation Source Exposure Time

9 Committed equivalent dose: H T (τ) Equivalent dose averaged throughout tissue T over τ years after intake of radioactive materials. Internal dose integrated over 50 years (adult) or Committed Effective Dose: E(τ) Sum of the products of the committed organ or tissue equivalent doses and the appropriate tissue weighting factors (w T ), where τ is the integration time in years following the intake.

10 Internal Dose Calculation Method

11 Calculation Method of Committed Equivalent Dose CED can be calculated by integrating equivalent dose rate Absorbed dose rate is proportional to Decay rate of a radionuclide Energy absorbed due to the decay. Based on this idea, H(50) is directly proportional to: (1) Number of decays in source tissue S over 50 years (2) Energy deposited per gram in target tissue T per decay in source tissue S (3) Tissue weighting

12 CED for a single source tissue with a single radionuclide

13 SEE (Specific Effective Energy) SEE = Equivalent dose in T per nuclear decay in S (Sv/decay) Radiation weighting factor (w R ) is already considered to calculate equivalent dose 4 possible irradiation geometries to consider Main difficulty in determine the SEE is in estimating the absorption fraction AF (T S) R for a given radionuclide type

14 AF (Absorption Fraction) Absorption fraction in T per decay in S for radiation R AF varies depending on (1) radiation type and (2) radiation energy Radiation Alpha Beta Absorption Fraction (AF) Photon

15 AF data Specific AF of energy at various ages from internal photon sources (Adult Male)

16 CED for (1) multiple source organs (2) with multiple radionuclide burdens (After intake of radioactive material, the radioactive material is distributed to many organs and tissues [not to a single organ]. Therefore, when calculating radiation dose, energies absorbed to a target T from multiple sources S should be summed.) (If more than 2 radionuclides are taken, radiation dose from each radionuclide should be summed.)

17 Calculation of U s Activity of radionuclide at initial stage after intake. (After intake of radioactive material, the radioactive material is distributed to organs and tissues ) (Therefore, activity of radionuclide in source region S will increase initially) Activity of radionuclide with time Radioactive materials in the body (or in source region S) will decrease by: Both radioactive decay and biological elimination is expressed by Activity in organ S Time

18 Effective half life Radioactive materials in the body (or in source region S) decreases by by both (1) Radioactive Decay and (2) Biological elimination To consider both removal rates, effective half-life is used. where λ= decay constant T = half-life e = effective r = radioactive b = biological Activity in organ S decrease exponentially with effective decay constant

19 Calculation of U s U s = total number of decays in source region S over 50 years Therefore, U s = area under activity line to 50 years

20 Internal Dose Calculation using Dose Conversion Factors

21 Internal Dose Calculation Using Dose Database (We have studied how to calculate radiation dose due to intake of radioactive materials.) (Dosimetry equation looks simple. However, dose calculation is not as simple as you think because of following reasons:) Many sources organs (S) contribute radiation dose to target T Many target tissues in body A radionuclide may emit multiple radiations with different energy In addition, we must know how the radioactive material is distributed to each organ and how they are removed from the organs. Therefore, it will be convenient if pre-calculate database of internal dose per unit intake is available for: (1) different types of intake (2) different radionuclide. For the case, we can simply calculate radiation dose using only intake amount of radionuclide

22 Internal Dose Database Pre-tabulated dose database is available in literature Database sources The intake pathways include: (1) Inhalation (2) Ingestion Exposed persons include: Dosimetry quantities include: (1) Committed Equivalent dose [H T (50)] (2) Effective Dose [E(50)]

23 List of ICRP Publications for Internal Dose Database ICRP Publication 59: Age-Dependent Doses to Members of the Public from Intake of Radionuclides: Part 1. (1989) ICRP Publication 67: Age-Dependent Doses to Members of the Public from Intake of Radionuclides: Part 2 (1993) ICRP Publication 69: Age Dependent Doses to Members of the Public from Intake of Radionuclides: Part 3. Ingestion Dose Coefficients (1995) ICRP Publication 71: Age Dependent Doses to Members of the Public from Intake of Radionuclides, Part 4, Inhalation Dose Coefficients (1995) ICRP Publication 72: Age Dependent Doses to Members of the Public from Intake of Radionuclides, Part 5, Compilation of Dose Coefficients from Parts 1-4 (1996) ICRP Publication 68: Dose Coefficients for Intakes of Radionuclides by Workers (1994)

24 Types F, M, and S in Inhalation Dose Coeffieicnet (Inhalation dose depend on solubility of radioactive material in lung) (Fast dissolving material [like sugar] will be absorbed to blood quickly.) (Slow dissolving material [like soil] will not be absorbed to blood but stay in the lung for long time.) Types of F, M, and S Type F: Fast dissolution Type M: Moderate dissolution Type S: Slow dissolution Solubility of radionuclides Solubility does not depends on radionuclide but on chemical compound. If solubility is now known, ICRP recommends type M for most radionuclides and type F for Th. Radiation dose by solubility Type S higher lung dose (due to long stay in lung) However, type S dose not necessarily result in higher effective dose.

25 Particle Size AMAD (Activity Median Aerodynamic Diameter) (Particles in the air dose not have the same sizes, ranging from small one to the large ones AMAD: Aerodynamic diameter such that 50% of the airborne activity in a specified aerosol is associated with particles greater than the AMAD f(x) AMAD Particle size 5 µm AMAD vs. 1 µm AMAD For dosimetry purpose, generally 5 µm AMAD is applied for workers Generally, 1 µm AMAD is applied for the public

26 Dose Calculation You can simply calculate radiation dose using: where i = radionuclide Dose can be: Equivalent dose [H T (50)] Effective Dose [E(50)] Intake Inhalation: concentration, inhalation rate, inhalation time Ingestion: concentration, ingestion amount DCF (Dose Conversion Factor) Inhalation (AMAD, Solubility) Ingestion