Clinical Beams

Question 1

What components are there to a PDD e.g. primary beam? (4 marks)

Answer 1

Primary beam, head and phantom scatter, electron contamination.

Question 2

How is a PDD measured? (3 marks)

Answer 2

• use a water tank and move a detector up through the water while taking measurements
• use a small volume ionisation chamber or diode for adequate resolution
• can use a parallel plate chamber in the build-up region

Question 3

What is the build-up effect? (5 marks)

Answer 3

• occurs due to the finite mean free path of secondary electrons which is the range at which they deposit their energy
• electrons are mostly scattered forward
• as the tracks overlap the amount of dose increases until the number of tracks overlapping stops increasing
• them the dose gradually tails off due to attenuation of the beam
• dose is non-zero at the surface due to backscatter and electron contamination

Question 4

How does the PDD vary with beam energy? (3 marks)

Answer 4

Increasing the beam energy:

• increases D_max
• more gradual tail-off
• less surface dose due to reduced backscatter

Question 5

How does the PDD vary with field size? (2 marks)

Answer 5

Increasing the FS:

• more gradual tail-off due to increase in phantom scatter
• increase in surface dose due to increase in head scatter (mainly from flattening filter but also collimators)

Question 6

How does altering the SSD affect the PDD? (2 marks)

Answer 6

Increasing the SSD:

• causes a more gradual tail-off due to the ISL effect causing less difference at greater distances
• the actual abosrbed dose decreases

Question 7

What is the TPR and how is this different to PDDs? (4 marks)

Answer 7

TPR = tissue phantom ratio, differs from PDD due to:

• no ISL effect as fixed distance from source to detector
• different scattering conditions
• used in isocentric treatments

Question 8

How do electrons interact with tissue? (4 marks)

Answer 8

Mechanisms include:

• ionisation (forming ions such as hydroxl)
• producing characteristic and bremstrahlung radiation
• therma excitations (heat)

Question 9

How does an electron energy distribution change shape as it travels from the waveguide, to the patients surface and at a greater depth within the patient? (5 marks)

Answer 9

Exiting the waveguide:
- initially a narrow peak around a particular energy
At the surface of the patient:
- the peak has broadened due to the interactions with the low Z scattering foil, ionisation chamber and collimators
At a depth in the patient:
- the mean energy has decreased and further broadening has occurred due to interactions within the patient

Question 10

How does a PDD appear for electrons? (4 marks)

Answer 10

• most of the energy is deposited at a shallow depth
• there is a steep fall off
• followed by a flatter low relative dose region due to bremstrahlung radiation
• increasing the energy of the electron beam causes a deeper d_max and a more gradual fall-off, similar to photons