RT4 - Measuring PDDs & Beam Profiles

Question 1

Answer 1

Thimble ion chambers:

PDD1: scan direction down

PDD2: scan direction up

Always scan upwards.

If you go down, the surface tension in the water pulls the water down with it, leading to shallower than expected water, hence overmeasure of dose.

Question 2

Answer 2

Thimble chambers:

1. 6MV
2. 10MV

• Deeper dmax ⇒ Beam is more penetrating, i.e. it takes longer to reach electronic equilibrium for higher energies because secondar electrons produced have more energy and hence greater range
• Low energy beam drops off faster at depth ⇒ PE decreases with increasing photon energy & scatter is more forwardly directed
• Lower surface dose for 15MV beam - artefact of thimble chamber poking out the top of the water (increased dose). Better to use paralell plate in build-up region with better res

Question 3

Answer 3

Thimble chambers

• SSD = 90
• SSD = 110

Extended SSD curve falls off slower, dose at depth relative to Dmax is higher.

Due to attenuation & inverse square:

D1/D2 = ((SSD+d2)/(SSD+d1) )^2

Larger SSDs effect of inverse square is smaller. In absolute terms, greater SSD = less dose, but because you normalise to Dmax, it looks like you get more dose at depth.

Sneaky

Question 4

Answer 4

1. Thimble chamber (250V)
2. Photon diode (0V)

Thimble chamber measures lower dose at depth because:

• Diodes are not water equivelent (higher Z), increased PE, over-respond to low energy photons at depth due to scatter

Question 5

Answer 5

Varied depth

• Height of profile decreases as dose drops off with depth
• Shallow profile has peaks, deeper is more rounded and convex.
  
  • Conical flattening filter os designed to give flat beam at 10cm. f
  • flattening filter is high z, hence PE is important.
    
    • deeper beams, dose drops off faster off-axis because beam is softer
    • Peaks appear at shallow depths because it has been normalised to the 10cm value
• Penumbra widens
  
  • Divergence of beam & increased lateral scatter

Question 6

Answer 6

Beam energy (6 vs 15MV)

• 15MV penumbra is wider due to increased range of secondary electrons.
• NB// At low energies (10MV) increasing energy sharpens penumbra due to more forwardly directed scatter, but up to a point (15MV) the electron range becomes the dominant factor

Question 7

Answer 7

Diode vs Thimble Chamber

• Diode - smaller sensitive volume = better spat res and more suited to measurements of hifh dose gradients (e.g. penumbra)
• Ion chamber has larger sensitive volume = measuring average dose over larger area & penumbra appears wider than it really is (blurring out of edges with alrge pixel size)

Question 8

Name & describe 3 types of penumbra

Answer 8

1. Geometric - Finite source, inherent
2. Transmission - Beam making it through corners of collimator jaws
3. DOsimetric - Secondary electron range & scatter

Question 9

What happens to penumbra at high and low energies?

Answer 9

At low energies (4-6MV)

• Penumbra narrows as energy increases
  
  • Decreased probabilit of compton scattering
  • Tighter angle of compton scatter

High energies (15-25MV)

• Penumbra widens as energy increases
  
  • Range of secondary electrons is increased