1. IB Objectives - Radiation in Medicine
State the meanings of the terms exposure, absorbed dose, quality factor (relative biological effectiveness) and dose equivalent as used in radiation dosimetry
I.3.2
Discuss the precautions taken in situations involving different types of radiation.
I.3.3
Discuss the concept of balanced risk. (Students should appreciate that codes of practice have been developed for conduct involving the use of radiations.)
I.3.4
Distinguish between physical half-life, biological half-life, and effective half-life.
3/19/2009
IB Physics HL 2

2. IB Objectives - Radiation in Medicine
Solve problems involving radiation dosimetry.
I.3.6
Outline the basis of radiation therapy for cancer.
I.3.7
Solve problems involving the choice of radio-isotope suitable for a particular diagnostic or therapeutic application.
I.3.8
Solve problems involving particular diagnostic applications.
3/19/2009
IB Physics HL 2

3. Radiological Medicine
Terminology
Activity
Source-related
Exposure
Amount of ionization
Dose
Amount of energy absorped
Equivalent dose
Biological effects
3/19/2009
IB Physics HL 2
Image from: reactor.reed.edu/pictures.html

4. Activity Number of decays per unit time
Depends on atomic type and amount
Other equations:
What is the activity of 5 g of 131I (t1/2 of 8 days)
3/19/2009
IB Physics HL 2

5. Exposure
Measures amount of ionizing radiation something is “exposed” to
Ionization -> charge separation
Taken relative to charge separation in air
~ Charge separation in tissue
For X-rays and -rays
3/19/2009
IB Physics HL 2
Image from:

6. Absorbed Dose
Measures how much radioactive energy actually deposited in an object (relative to mass)
In air, 34 eV needed to create one ion pair, so
1 C/kg in air is 34 eV x 1.6 x J x 6.25 x 1018 electrons = 34 J/kg = 34 Gy
Other materials would differ (energy to create pair of ions)
3/19/2009
IB Physics HL 2

7. Equivalent Dose
Absorbed dose does not relate the biological effect of specific radioactive decay particle.
Equivalent dose does take biological “effectiveness” into account
H (equivalent dose) (seivert) = Q D(dose) N where Q is Quality Factor D is absorbed dose (Gy) N is other factors (set to 1)
Q = 1 for X-rays, -rays (at 250 keV), protons = 5 for thermal (slow) neutrons = 10 for fast neutrons,  particles = 20 for recoiling nuclei
3/19/2009
IB Physics HL 2

8. Quality Factor
Also called Relative Biological Effectiveness relative to 250 keV X-ray
3/19/2009
IB Physics HL 2

9. Activity to Equivalent Dose
Activity (Bq - Disintigrations/s)
Geometric factors Time factors 
Quality factor
Exposure (C/kg)
Dose (Gy) 
Equivalent Dose (Sv)
3/19/2009
IB Physics HL 2

10. Radiation Effects on Humans
X-ray, -ray less harmful than electrons,  particles
Differing effects on different cells
Reproductive cells very radiation sensitive
Nerve cells not so sensitive
With increasing exposure:
Topical burns
Nausea, diarrhea, fever (radiation sickness)
Loss of hair
Cancer, leukemia
Death
3/19/2009
IB Physics HL 2

11. Radiation Effects on Humans
Exposure Level (mSv)
Effect
0.3
Background radiation (per year)
0.03
Chest X-ray
0.14
Dental X-ray
7.2
Abdominal CT
8,000
Thyroid therapy 50
U.S. Maximum annual dosage
3,000 – 4,000
50% Mortality after 30 days
3/19/2009
IB Physics HL 2

12. Precautions and Risks
Patient
Practitioner
3/19/2009
IB Physics HL 2

13. Precautions and Risks Patient Monitor exposure time carefully
Use only procedures that convey net benefit
Keep exposures as low as reasonably achievable
Do not exceed recommended limits for dose
Lead aprons (reduce stray radiation)
3/19/2009
IB Physics HL 2

14. Precautions and Risks Practitioner
Procedures to limit or minimize risk of contamination or exposure
Monitor radiation exposure (film badge)
, -ray, X-ray, and neutron monitoring
3/19/2009
IB Physics HL 2

15. Precautions and Risks Practitioner procedures to minimize risk:
Use lab coat in locations where radioactive material used, handled, or stored
Use disposable gloves
Monitor hands before and after leaving work area
No eating, drinking, or smoking in work area
Clearly label radioactive material
3/19/2009
IB Physics HL 2

16. Precautions and Risks Shielding (patient and practitioner)
Distance (1/R2 fall-off)
Lead, concrete, water: X-rays and -rays
Neutrons: mass (lead, steel)
Lead aprons (patient)
3/19/2009
IB Physics HL 2

17. Half Lives Radiological half-life (physical half-life): TR
Time for half of radioactive isotope to decay
Biological half-life: TB
Time for the body to get rid of half of the radioactive isotope
Effective half-life: TE
Effective half-life of isotope, including both radiological and biological effects
1/TE = 1/TR + 1/TB or E = R + B (decay constants)
3/19/2009
IB Physics HL 2

18. Half Life Example
Thallium-200 has a radiological half-life of 26 hours, and a biological half-life of 42 hours.
What is its effective half-life?
How long will it take for its activity to fall to 1/10th its initial value?
3/19/2009
IB Physics HL 2

19. Radiation Treatment of Cancer
Laws of Bergonie and Tribondeau regarding sensitivity of cells to ionizing radiation
More sensitive when cells are:
Young
Simple
High metabolism
Dividing rapidly
These characteristics make cancer cells more susceptible to radiation damage than normal cells
3/19/2009
IB Physics HL 2

20. Radiation Treatments of Cancer
Three main classifications
Internal radiotherapy Radioisotope is in body, and becomes localized in affected organ (e.g., I-131 and thyroid cancer)
External radiotherapy Radioactivity from source outside body (e.g., accelerator or radioactive source)
Brachytherapy Where radioactive source implanted in body near locale to receive radiation
3/19/2009
IB Physics HL 2

21. Internal Radiotherapy
Use isotopes which emit -rays, -particles (electrons)
Deposit energy close to where radioisotopes are
Examples
I-131 and thyroid treatment
Yt-90 - liver cancer
P-32 - bone marrow
Sm-153 and breast and prostrate cancers
3/19/2009
IB Physics HL 2

22. External Radiotherapy (Teletherapy)
Collimate and shape beams to illuminate target tumor
Reduce illumination of healthy tissue
Beam from accelerator or radioactive source (e.g., Co-60)
3/19/2009
IB Physics HL 2

23. Brachytherapy Use , -emitters to localize energy deposition
Radioactive source is local
Reduce illumination of healthy tissue
Place catheters for placement of wire
Example: Ir-192 and breast cancer, mouth cancer
3/19/2009
IB Physics HL 2

24. Radioactive Tracers (Diagnostics)
Use to determine functioning of physiological processes, location
Usually use -emitter
Emerge from body easily
Can be detected by scintillation camera
Want short radiological half-life (<~ day)
Examples:
Cr-51 and bleeding
Sr-90 and bones
Rb-86 and muscles
Tc-99 and multiple organ imaging
3/19/2009
IB Physics HL 2

25. Key Ideas for Radiological Medicine
3/19/2009
IB Physics HL 2