IB Objectives - Radiation in Medicine

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.

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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

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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

=

=

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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

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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 =

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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

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Quality Factor

Also called Relative Biological Effectivenessrelative to 250 keV X-ray

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Activity to Equivalent Dose

Exposure (C/kg) Dose (Gy)

Equivalent Dose (Sv)

Activity(Bq - Disintigrations/s)

Geometric factorsTime factors

Qualityfactor

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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

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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

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Precautions and Risks

Patient Practitioner

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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)

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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

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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

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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)

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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)

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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?

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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

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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

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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)

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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

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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

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Key Ideas for Radiological Medicine