Dosimetry

• What is the unit of absorbed dose?

  Gray (Gy)
• What does the absorbed dose measure?

  Energy deposited by ionising radiation per mass
• How is 1 Gray defined?

  1 joule of energy per kilogram
• What is the relationship between Gy and J/kg?

  1 Gy = 1 J/kg
• What does dose rate refer to?

  Rate of radiation dose delivery
• How is dose rate expressed?

  In Gray per minute (Gy/min)
• What happens if the dose rate falls below tolerance?

  The machine will switch off
• What is the total absorbed dose at a point in a patient composed of?

  Primary dose and scatter contributions

  
• What does PDD stand for?

  Percent Depth Dose
• How is PDD expressed mathematically?

  PDD = $\frac{D_d}{D_0} \times 100\%$
• What does the prescribed dose signify?

  100% of the intended radiation dose
• If a patient receives 50Gy in 25 fractions, what is the daily dose?

  2 Gy/fraction
• What is the purpose of normalising the absorbed dose?

  To provide a standardised measure of dose distribution
• What are common reference depths in radiotherapy?

  Dmax, isocentre, specific depth
• What does the ICRU reference point specify?

  Point for reporting and specifying dose
• At what depth does the reference point occur for MV energies?

  At Dmax (d0 = dm)
• How does beam energy affect PDD distribution?

  PDD increases with beam energy to Dmax
• What is KERMA?

  Energy transferred from photons to electrons
• Why does skin-sparing occur at MV energies?

  Due to dose reaching a maximum at depth

  
• What is the Dmax for an electron beam?

  Does not follow a linear relationship with energy
• What is the therapeutic range for MV electron modality?

  Broad region covering Dmax
• How is the Dmax for a 6 MeV electron beam calculated?

  Dmax = $0.46 \times (6 \text{ MeV})^{0.67}$
• What is the average energy of an electron beam?

  Typically estimated as 2-5%
• How is the average energy of X-ray photons in a beam calculated?

  Approximately a third of the energy
• What defines the field size in radiotherapy?

  Lateral distance between points on the 50% iso line
• What does the 50% isodose line represent?

  Region where dose is half of the max dose
• How do ES fields help in dose calculations?

  They simplify complexities of irregular fields
• How does beam energy influence isodose curve shape?

  Depth of isodose curve increases with energy

  
• What is the penumbra in radiotherapy?

  Measured between 80% and 20% dose
• How does scatter differ between low and high energy beams?

  Low energy beams have greater lateral scatter
• What happens to isodose curves outside the primary beam for low energy beams?

  They bulge out due to increased scatter

  
• How does MV energy affect scatter outside the treatment field?

  Minimised due to forward scattering
• What is the dose gradient in the penumbral region?

  Changes rapidly with distance
• What are the factors that affect PDD distribution?

  • Beam energy
  • Depth
  • Field size and shape
  • SSD
• How does absorbed dose behave outside the primary beam for low energy beams?

  Greater than for higher energy MV beams
• What are the characteristics of isodose curves?

  • Penumbra depends on beam quality
  • Dose gradient changes rapidly in penumbral region
  • Isodose curves outside primary beam are distended for low energy beams
  • MV beams minimize scatter outside the field

  
• How does the depth of a given isodose curve change with beam quality?

  It increases with beam quality

  
• How does beam energy influence isodose curve shape?

  It affects the shape near field borders
• What effect does very low energy beams have on isodose curves outside the field?

  They cause the curves to bulge out

  
• How does the absorbed dose in the medium outside the primary beam compare between low and high energy beams?

  It is greater for low energy beams