Medical Physics and Dosimetry - Research Assignment

Last updated Jul 29, 2023 Edit Source

# ← Physics Home

Research and make notes on the following concepts:

• The use of radioisotopes in diagnostic imaging including gamma cameras and PET scans
• The use of radioisotopes in therapeutic treatments
• The main ways in which radioisotopes are produced for use in medicine
• Proton therapy and Neutron therapy
• Radiation Dosimetry including Absorbed dose and Dose Equivalent.

• A gamma camera (or SPECT camera) is a camera that detects flashing light (scintillons) produced by gamma rays interacting with a sodium iodide ($NaI$) crystal at the front of the camera
• The gamma camera detects the energy from a radiotracer in your body and converts it into an image, it has radiation detectors called gamma camera heads
  • A radiotracer (radioactive tracer) is a chemical compound in which one or more atoms have been replaced by a radioisotope
• The gamma camera itself does not emit any radiation
• Structure of a gamma camera: GammaCamera

• Positron emission tomography (PET) scans are a medical imaging procedure that provides information about how an organ or system in the body is working
• Procedure:
  • A PET scan involves the injection of a small amount of a positron-emitting radiopharmaceutical
    • Radiopharmaceuticals are pharmaceutical drugs containing radioisotopes
    • The radiopharmaceutical undergoes $\beta^+$ decay
  • Images of the body are then taken using a PET scanner
    • The PET scanner surround the patient, similar to a CT scanner, and it shaped like a donut
      • PET scans are usually combined with CT scans for a better image
    • The camera detects emissions coming from the injected radiopharmaceutical and the computer attached to the camera creates 2D and 3D images of the area being examined
• Why are PET scans used?
  • PET scans are mainly used to assess cancers, neurological diseases and cardiovascular disease
  • PET scans are considered to be generally safe
    • Only exposes you to radiation equal to the amount you would receive over three years naturally

• Two types of radioisotope treatment:
  • External → Teletherapy or external irradiation
  • Internal → Brachytherapy or internal radionuclide therapy
• Radioisotopes can be used to kill cancer cells
  • As the radioisotope begins to decay, it affects the targeted tissue or tumour, because cancer cells absorb more of the radiation than non-cancer cells do (see below)
  • The higher dose of radiation eventually destroys the cancer cells
  • Radioisotopes are used due to the enhanced damaging effect on cancer cells which can sometimes allow a lower dose of radiation, lowering the damage to healthy tissue

Why do Cancer Cells Take More Damage from Radiation?

Cancer cells, by nature, have broken their tumour suppressing genes. This means they cannot fix their DNA. Radiation causes damage to cell’s DNA. Since cancer cells cannot fix their DNA, they are more likely to not be able to prevent or fix fatal damage caused by radiation, while healthy cells may be able to recover.

• Medical radioisotopes are made in three different ways:
  • Materials bombarded by neutrons in a nuclear reactor
  • Materials bombarded by protons in an accelerator called a cyclotron
  • Materials bombarded by photons in an electron accelerator
• Nuclear Reactor
  • Stable isotopes are bombarded by neutrons produced by nuclear reactor (i.e. fission of Uranium-235)
    • Produces neutron-rich isotopes which undergo $\beta^-$ decay
  • Produces large amounts of radioisotopes cheaply
• Cyclotrons
  • Protons are accelerated in a spiral pathway towards the targeted material
    • Produces proton-rich isotopes which undergo $\beta^+$ decay
  • Some hospitals have their own cyclotrons, which are generally used to make radiopharmaceuticals with very short half-lives (seconds or minutes)
    • Cyclotrons are less dangerous than nuclear reactors
• Electron Accelerators
  • Photons are made by directing electron beam (a stream of electrons) from a particle accelerator onto a heavy metal and then are used to bombard material to form a radioisotope

• Radiation therapy is a cancer treatment where high-energy particles or x-rays are used to destroy cancer cells
• Proton and neutron therapy are examples of radiation therapy

• Proton therapy specifically uses protons as the radiation that is delivered to tumours
• Less radiation occurs outside the tumour in proton therapy, thus healthy tissue are exposed to less radiation
  • There are less side-effects associated with proton therapy

• Neutron therapy is a specialized and very powerful form of external beam radiation therapy
  • Neutron therapy is also known as fast neutron therapy because it uses fast neutron radiation which has higher ionising ability
• It is used to treat tumours that are *radioresistant
  • Radioresistant means that the tumour is very difficult to kill using conventional x-ray radiation therapy; it is resistant to radiation
• Neutrons have a greater biologic impact on cells than other types of radiation
  • However, neutrons do not damage all cells to the same extent
    • In some cases, they are more damaging to cancerous cells than to healthy cells surrounding the cancer which is very useful
  • The dosage for neutron therapy is reduced due to its strength and efficiency

• Radiation dosimetry is the science of calculating and measuring radiation dose
  • There are many fields where humans may be exposed to radiation
  • Radiation can be damaging to the body and we need a way to measure the severity of radiation exposure
• Radiation dose can be measured in two main ways: absorbed dose and dose equivalent

• The severity of radiation exposure depends on the amount of radiation energy that has been absorbed ($E$) and the mass of tissue involved ($m$)
• The radiation energy absorbed per kg of tissue is called the absorbed dose (AD) $$ AD = \frac{E}{m}$$
  • Absorbed dose is measured in joules per kilogram ($J\\ kg^{-1}$) or grays ($Gy$)
    • $1\\ Gy = 1\\ J\\ kg^{-1}$

• Absorbed dose is not widely used since it does not take into account the type of radiation involved
  • Different types of radiation have different ionisation ability
    • Alpha radiation is 20 times more damaging than beta and gamma radiation at the same absorbed dose
• The weighting of the biological impact of radiation is called the quality factor, QF, or weighting factor

• Dose Equivalent = Absorbed Dose × Quality Factor
  • $DE = AD \times QF$
  • Dose equivalent is measured in Sieverts ($Sv$)

• https://ed.toomwn.xyz/Medical-Physics-and-Dosimetry/ (Thanks Ed)

• https://www.sciencedirect.com/topics/medicine-and-dentistry/gamma-camera
• https://www.betterhealth.vic.gov.au/health/conditionsandtreatments/pet-scan
• https://www.radiologyinfo.org/en/info/gennuclear#:~:text=The%20gamma%20camera%20detects%20the,round%2C%20donut-shaped%20gantry.

• https://www.exceldiagnostics.com/blog/what-is-radioisotope-therapy#:~:text=Radioisotope%20therapy%20uses%20radioisotopes%20to,%2C%20bone%20metastases%2C%20and%20neuroblastoma.

• https://www.ansto.gov.au/education/nuclear-facts/what-are-radioisotopes#:~:text=How%20are%20medical%20radioisotopes%20made,an%20essential%20part%20of%20radiopharmaceuticals.

• https://www.cancer.net/navigating-cancer-care/how-cancer-treated/radiation-therapy/proton-therapy
• https://radiationoncology.uw.edu/radiation-treatment/treatment-options/neutron-beam-therapy/

• Textbook: Pearson Chapter 3
• https://www.snmmi.org/AboutSNMMI/Content.aspx?ItemNumber=31086