Characteristics of Clinical Beams LT1

Question 1

Where are isodoses normalised to?

Answer 1

10cm deep in patient

Question 2

What’s the build-up effect?

What’s it caused by?

Answer 2

Region of increasing dose between depth = dsurface(z=0) and dmax

• Photons interact at different depths in tussue, generates secondary electrons
• Electrons travel most forwards to deposit dose
• As more tracks overlap, dose is built up until charged particle eqm is reached
• Steady state reached because of attenuation & scattering
• Dose >0 at surface due to backscattered electrons from patient and LINAC scatter

Question 3

PDD vs Beam energy

Answer 3

As beam energy increases:

• Surface dose decreases
• Deeper dmax
• Dose at depth increases

Question 4

PDD vs Beam Size

Answer 4

As beam gets bigger

• dose at depth increases due to:
  
  • more photons reaching patient from extended source
  • more scattered electrons

Question 5

PDD-vs SSD

Answer 5

• Actual dose decreases with distance from source
• PDD (normalised) increases with distance from source as 1/r^2 has less of an effect

Question 6

What is TPR?

TMR?

Answer 6

• TPR(z,c) = D(z,c)/D(zref,c)
• Fixed SSD, add more water on top to increase z
• TMR is special case of TPR when zref = zmax

Question 7

What is RDD? Can you compare to TPR?

Answer 7

• RDD = relative depth dose, normalised to 1 instead of 100%
• Different shape, not comparable
• Inverse square law effect
• different scatter conditions

Question 8

Key features of Electron PDD

Answer 8

• high surface dose - electrons deposit energy immediately
• build-up region - electron path becoming more oblique due to scattering up until dmax
• After dmax, steep dose fall off as electrons are not as penetrating
• Brehmstrahlung tail

Question 9

Electron PDD vs energy

Answer 9

With increased energy

• Increased surface dose
• Depth of dmax increases
• Gradient of fall off decreases
• Brehmsrahlung level increases