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

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

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

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

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:www.rogerwendell.com/nukes.html

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) 3/19/2009 IB Physics HL 2

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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