Clinical Beams II

Question 1

How is the beam width and penumbra defined on an isodose plot? (2 marks)

Answer 1

Width = FWHM (50% of max)
Penumbra = defines useful extent of beam (usually defined from 80-20%

Question 2

How does the beam profile on an isodose plot vary with depth? (3 marks)

Answer 2

• The presence of the flattening filter should cause the profile to be flattest at 10 cm depth.
• At shallower depths the profile appears more horny due to the softer beam at the edges not being sufficiently attenuated
• At greater depths the beam appears more rounded due to the harder central portion of the beam being more penetrative than the softer outer edges, arising from the shape of the flattening filter

Question 3

What are the three sources of penumbra? (6 marks)

Answer 3

• Geomtric penumbra, varies with finite source size and distance
• Transmission penumbra from beam penetration through the edges of the collimators, varies with energy
• Scatter penumbra from lateral scatter in patient, varies with beam energy (more at lower energies)

Question 4

How does increasing the field size affect the head and phantom scatter factors? (3 marks)

Answer 4

• Increase in FS results in an increase in both the head and phantom scatter factors, hence more dose.
• The increase tails off with increasing FS due to the finite range of the electrons
• At low doses the head scatter dominates but phantom scatter is higher at larger field sizes

Question 5

On a plot of FS and S_p (phantom scatter) how does beam energy affect the plot? (2 marks)

Answer 5

• Both energies increase gradually with increasing FS, but in a exp(-x) manner.
• Higher energies have more S_p initially but then the lower energy dominates at higher FS values, due to increased volume to generate lateral scatter.

Question 6

What is meant by a monitor unit? (3 marks)

Answer 6

• unit of measurement used to link the exposure (charge collected) by the ionisation chamber to the amount
• usually defined as 100 MU = 1 Gy at reference conditions (100 cm SDD, 10 cm deep and 10 x 10 cm FS)
• number of MUs linked to beam control such that the beam terminates based on the number of MUs recorded