3/19/2009ib physics hl 21 ib objectives - radiation in medicine i.3.1state the meanings of the terms...
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3/19/2009 IB Physics HL 21
IB Objectives - Radiation in Medicine
I.3.1 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 22
IB Objectives - Radiation in Medicine
I.3.5 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 23
Radiological Medicine
Terminology Activity
Source-related Exposure
Amount of ionization Dose
Amount of energy absorped Equivalent dose
Biological effects
Image from: reactor.reed.edu/pictures.html
3/19/2009 IB Physics HL 24
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)
N dt
dN λ−=
t-0
1/2
eN N
2ln t
λ
λ
=
=
3/19/2009 IB Physics HL 25
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
(kg) m
(coulombs) Q X Exposure =
Image from:www.rogerwendell.com/nukes.html
3/19/2009 IB Physics HL 26
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 10-19 J x 6.25 x 1018 electrons = 34 J/kg= 34 Gy
Other materials would differ (energy to create pair of ions)
(kg) m
(J) E (Gray) D =
3/19/2009 IB Physics HL 27
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 FactorD 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 28
Quality Factor
Also called Relative Biological Effectivenessrelative to 250 keV X-ray
3/19/2009 IB Physics HL 29
Activity to Equivalent Dose
Exposure (C/kg) Dose (Gy)
Equivalent Dose (Sv)
Activity(Bq - Disintigrations/s)
Geometric factorsTime factors
Qualityfactor
3/19/2009 IB Physics HL 210
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 211
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 212
Precautions and Risks
Patient Practitioner
3/19/2009 IB Physics HL 213
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 214
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 215
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 216
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 217
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 orλE = λR + λB (decay constants)
3/19/2009 IB Physics HL 218
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 219
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 220
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 radiotherapyRadioactivity from source outside body (e.g., accelerator or radioactive source)
BrachytherapyWhere radioactive source implanted in body near locale to receive radiation
3/19/2009 IB Physics HL 221
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 222
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 223
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 224
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 225
Key Ideas for Radiological Medicine
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