Radiation biology

Radiation
- Absorbed radiation is measured by Gray (Gy)
  - It does not take into account the biological effect of radiation
- Health effects of radiation is measured by Sievert (Sv)
  - It does take into account the biological effect of radiation
- Types of radiation used for therapy
  - Gamma rays (?)
    - Used in stereotactic radiosurgery with gamma knife
  - Alpha radiation
    - Radium-223
    - For example for bone metastases or prostate cancer
  - Beta radiation
    - Iodine-131
    - For example for thyroid cancer
- Types of radiation used in diagnostics
  - Positron
    - PET scan
  - Gamma
    - X-ray
    - CT
    - Many nuclear imaging studies
- X-ray interactions with matter
  - Compton scattering
    - Makes x-rays scatter off the patient -> the patient becomes the source of scattered radiation
      - This scattered radiation can hit personell or equipment
      - Personell and equipment should be protected
    - Reduces image contrast
  - Photoelectric effect
    - It’s what makes contrast agents work
  - Coherent/Rayleigh scattering
  - Pair production does NOT occur
    - Only at energy levels much higher than medical x-ray
- Biological effects of radiation
  - Deterministic effects (nonrandom)
    - Examples
      - Skin erythema
      - Hair loss (3 Gy)
      - Sterility
      - Death (3 – 5 Gy)
    - Occur when the radiation-induced cell damage exceeds the cell’s ability to repair the damage
  - Stochastic effects (random)
    - May occur at any level of exposure
    - Probability for occuring increases with increasing dose
    - Severity is independent of the dose
    - Due to DNA and free radical damage
    - May occur years after exposure
    - Examples
      - Cancer
        10 mSv increases risk for cancer (0,04 mSv per x-ray)
  - Most radiosensitive organs
    - Organs with rapidly dividing cells
    - Bone marrow
    - Colon
    - Lung
    - Breast
    - Stomach
- Protection from radiation
  - Radiation protection involves 3 parts
    - ALARA
    - Justifiable exposure
    - Dose limits
  - ALARA principle – as low as reasonably achievable
    - High quality images should be obtained by using the lowest possible dose
    - Factors contributing to reducing radiation
      - Beam collimators
        Decrease scattering
      - Lead apron
      - Careful indications
        Asking yourself whether the benefits outweigh the risk, and whether a non-radiating modality could be used instead
      - Standing far away from the patient as possible
      - Accurately setting the field of examination
  - Three major safety practices
    - Time – limiting exposure duration
    - Distance
      - According to the inverse square law one can reduce their exposure to 25% by standing twice as far away from the source
    - Shielding
      - Using lead to limit the amount of radiation exposure
  - Yearly occupational limit – < 20 mSv
  - Personal monitoring = dosimetry
    - Every person can carry a dosimeter which measures the radiation dose received‎