Human Biology Human Response To Ionizing Radiation



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STUDY GUIDE – RT 244 (Notes from Bushong’s text) pg pg #

Radiation Protection & Radiobiology Notes

Human Biology

  • Human Response To Ionizing Radiation

  • Early Effects of Radiation: When response occurs within minutes or days after exposure.

  • Late Effects of Radiation: When response is not observed for many months or years after exposure.

  • Effects of fetal irradiation: Death, malformation, malignancy, diminished growth.

  • Composition of Human Body

  • Cell Theory: 85% of the body is hydrogen and oxygen.

  • Hydrogen: 60%

  • Oxygen: 25.7%

  • Carbon: 10.7%

  • Nitrogen: 2.4%

  • Molecular Composition

  • Water: 80% - A simple molecule.

  • Protein: 15% - A macromolecule.

  • Fat: 2% - A macromolecule.

  • Other: 3%

  • Tissues and Organs: (The most radiosensitive cells in the human body are Lymphocytes & Spermatogonia.


Fundamental Principles of Radiobiology

  • Law of Bergonie and Tribondeau

  • 1) Older cells are more radioresistant.

  • 2) Younger cells are more radiosensitive.

  • How these principles apply to radiation safety: The fetus is more radiosensitive to radiation than a child or adult.

  • Biologic Factors Affecting Radiosensitivity

  • Age: Humans are most sensitive before birth.

  • Gender: Females are less radiosensitive than males.


Molecular and Cellular Radiobiology

  • Irradiation of Macromolecules: (p.450) Follows a Linear, Non-threshold dose/response relationship.

  • Radiation Effects on DNA: DNA is the most radiosensitive macromolecule.

  • 1) Cell death

  • 2) Malignant disease

  • 3) Genetic damage

  • Radiolysis of Water: Indirect Effect. Most prevalent in humans.

  • We are mostly water: 80% Water

  • The breakdown of water by radiation is harmful.

  • Breakdown products, called free radicals, cause DNA damage.

  • DNA damage can cause somatic and genetic outcomes.

Direct and Indirect Effect

  • Direct Effect: Radiation hits DNA causing cell damage.

  • Indirect Effect: 2 Step process The principle action of radiation on humans is the Indirect Effect.

  • 1) Radiation breaks down water molecules.

  • 2) Break down products (free radicals) damage DNA.

Single-Target, Single-Hit Model

  • Poisson Distribution: A statistical law. If 100 rain drops fell on 100 squares, 63% will be wet, 37% will be dry.

Early Effects of Radiation

  • Acute Radiation Lethality: X-ray beams are not large or intense enough to cause radiation lethality.

  • Early Effects: See a radiation response within a few days.

  • Local Tissue Damage:Requires a higher dose to produce a response. Results in tissue atrophy.

  • Skin: Erythema- a sunburn-like redness. SED50 (Skin Erythema Dose to affect 50% of pop.) at 600 Rad.

  • Gonads: Ovaries & Testes are very sensitive to radiation. Permanent sterility at 500 Rad.

Late Effects of Radiation

  • Local Tissue Effects

  • Skin: High doses of radiation causes discolored and weathered appearance of skin.

  • Chromosomes: Damage to lymphocytes can produce both an early and a late response.

  • Cataracts: The eye's sensitivity is age dependent. Increase in age, increases effects. Cataracts at 1,000 Rad

Life Span Shortening

  • 10 days per rad: You can loose 10 days of life for every rad received.

  • Relative safety of occupation: Radiologic Technology is considered a safe occupation.

Risk Estimates

  • Relative Risk: Observed cases ÷ Expected cases. Estimate used when you don't know the original dose.

  • Excess Risk: Observed cases - Expected cases. Estimate using the excess cases induced.

  • Absolute Risk: Estimate using 2 known dose levels.

Radiation Induced Malignancy

  • Leukemia: Linear, Non-threshold. 4-7 year latent period. 20 year "at risk" period.

  • Thyroid Cancer: Linear, Non-threshold dose/response relationship.

  • Bone Cancer: Radium watch-dial painters developed bone cancer.

  • Skin Cancer: Follows a threshold dose/response relationship.

  • Breast Cancer: Atomic blast survivors who received 10 Rads plus, exhibited a risk factor of 4:1.

  • Lung Cancer: Uranium miners show a risk factor of 8:1.

  • Liver Cancer: "Thorotrast" contrast agent, caused liver cancer after 15-20 year latent period.

  • Total Risk of Malignancy

  • Overall Absolute Risk of Malignancy: (p.487) Approximately 10 cases/106 persons/rad/year, 25 years after exposure.

  • Radiation and Pregnancy

  • Irradiation in Utero: High dose may interrupt fertility.

  • First two weeks: The first two weeks of pregnancy may be the safest. A high dose may produce a spontaneous abortion. This is an all or none dose/response relationship.

  • Major organ formation: From the 2nd - 10th week of gestation. High rad dose will produce skeletal, organ and CNS abnormalities.

  • Summary of effects

  • Health Physics

  • Cardinal Principles of Radiation Protection: Time, Distance and Shielding.

  • Minimize Time: Keep exposure time as short as possible.

  • Exposure = exposure rate x time

  • Maximize Distance: Use the longest SID possible from the source.

  • Doubling the distance drops the exposure by 4 times.

  • Technologist should be as far from the source as practical.

  • Maximize Shielding: Insert shielding between the source and person.

  • Half Value Layer of Lead (at 80kVp): .19mm Lead.

  • Tenth Value Layer of Lead (at 80 kVp): .64mm Lead.

  • Dose Limit (DL)

  • Definition: Ensures that radiation workers have the same risk as those in safe industries. There is no patient DL.

  • Summary of DL:

  • Summary of NCRP Dose Limit Recommendations. (See the last page of this guide.)

  • Annual DL is still 5 rem/yr (50 mSv/yr)

  • Cumulative DL is 1 Rem x age (10 mSv x age)

  • Educational Considerations

  • ALARA principle: Maintain radiation exposures As Low As Reasonably Achievable.

X-Rays and Pregnancy

  • Radiobiology of Pregnancy: Time dependence (first 2 weeks are safest) & dose dependence (200 Rad dose will certainly cause radiation effects).

  • The Pregnant Radiographer: DL = 5 mSv/9 months. Should wear 2 film badges, one at collar, one at waist.

  • Pregnancy in Diagnostic Radiology

  • Fetal Dose by Exam: An AP L-Spine exam of the mother gives the highest fetal dose.

Designing for Radiation Protection

  • Design of X-ray Apparatus

  • Requirements of the Control Panel: Must show two things.

  • 1) Display conditions of exposure.

  • 2) Positively indicate when x-rays are being produced.

  • Radiation Protection Designs For X-ray Equipment

  • Source-to-Image Distance Indicator: A tape measure. Must be accurate to within 2% of SID.

  • Collimation: Light localized, variable aperture. Must be within 2% of SID.

  • 1) Light Localizer: Light bulb.

  • 2) Variable Aperture Collimation: Can be adjusted by the Technologist.

  • 3) X-ray field/ light field agreement. Must be within 2% of the SID.

  • Positive Beam Limitation (PBL): Automatic Collimators. Must be accurate to within 2% of the SID.

  • Beam Alignment Device (cross-hair target): Line up the cross-hair target for proper tube/film alignment.

  • Filtration: 2.5mm Aluminum at 70 kVp.

  • Design of Protective Barriers

  • Types of Radiation:

  • Primary Radiation: The useful beam.

  • 1) Most of the radiation comes from primary beam.

  • 2) Most dangerous.

  • 3) Hardest to shield against.

  • Primary Barriers: Any wall to which the useful beam can be directed.

  • Secondary Radiation: 2 types

  • 1) Scatter Radiation: Comes from the patient.

  • 2) Leakage Radiation Comes from the tube.

  • Secondary Barriers: The control booth barrier is a secondary barrier. Do not have to contain lead.

  • Radiation Detection and Measurement

  • Types of Dosimeters: 1) Film Badges: Used most often. Have a limited range. Not reusable.

  • 2) Thermoluminescent Dosimeter (TLD): Have a wide range. Are reusable.

Radiation Protection Procedures

  • Estimation of Patient Dose

  • Skin Dose: Easiest to measure using TLD's.

  • Gonadal Dose: Important because of suspected genetic response. GSD- Genetically Significant Dose.

  • Bone Marrow Dose: Important because bone marrow is the target organ believed responsible for leukemia.

  • Reduction of Occupational Exposure: Do not aim the tube toward the control booth barrier.

  • Personnel Monitoring: Required when you will receive more than 1/4th the DL.

  • Film Badges: Cheap, easy to use, easy to process, accurate and have been used for decades.

  • TLD: Newer, can be worn for up to 3 months.

  • Pocket Ionization Chamber: Must be charged daily before use.

  • Patient Holding: Radiology personnel should never be used to hold patients. Use family or friends of patient.

  • Reduction of Unnecessary Patient Dose

  • Unnecessary Examinations: The technologist has no control over this.

  • Repeat Examinations: The # 1 reason for high patient dose. Due to patient positioning over the photo-cells.

  • Radiographic Technique: Use high kVp technique to reduce patient dose.

  • The Image Receptor: Use the fastest film/screen combination to reduce patient dose.

  • Patient Positioning: Be sure the useful beam does not intercept the gonads if patient is seated for extremity films.

  • Specific Area Shielding: The lens of the eye, breasts and gonads are frequently shielded from the primary beam.


From NCRP Report No. 116 Limitation of Exposure to Ionizing Radiation Issued:

Summary of Recommendations

A. Occupational exposures



  • 1. Effective dose limits

  • a) Annual 50 mSv/yr (5 Rem/yr)

  • b) Cumulative 10 mSv x Age (1 Rem x Age)

  • 2. Equivalent dose annual limits for tissues and organs

  • a) Lens of eye 150 mSv (15 Rem)

  • b) Skin, hands and feet 500 mSv (50 Rem)

  • B. Guidance for emergency occupational exposure (not listed)

  • C. Public exposures (annual) are 1/10th the occupational exposures.

  • 1. Effective dose limit, continuous or frequent exposure 1 mSv (0.1 Rem)

  • 2. Effective dose limit, infrequent exposure 5 mSv (.5 Rem)

  • 3. Equivalent dose limits for tissues and organs

  • a) Lens of eye 15 mSv (1.5 Rem)

  • b) Skin, hands and feet 50 mSv (5 Rem)

  • 4. Remedial action for natural sources:

  • a) Effective dose (excluding radon) >5 mSv

  • b) Exposure to radon decay products >7 x 10-3 Jh m-3

  • D. Education and training exposures (annual)

  • 1. Effective dose limit 1mSv (0.1 Rem)

  • 2. Equivalent dose limit for tissues and organs

  • a) Lens of eye 15 mSv (1.5 Rem)

  • b) Skin, hands and feet 50 mSv (5 Rem)

  • E. Embryo-fetus exposures (monthly)

  • 1. Equivalent dose limit 0.5 mSv (.05 Rem)

  • F. Negligible individual dose (annual) 0.01 mSv (.001 Rem)





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