Characteristics of Clinical Beams LT2

Question 1

Define Beam Size

Answer 1

FWHM @ 10cm deep

Question 2

Define penumbra

Answer 2

Distance between 80% and 20% dose levels @ 10cm deep

Question 3

What happens to beam profile with increasing depth?

Answer 3

• beam widens (divergence)
• penumbra widens (more scatter)
• Dose decreases
• Flattening filter makes it flattest at 10cm
  
  • Horny at lower depths
  • Rounded & concave at deeper parts

Question 4

How is wedge angle defined?

Answer 4

Angle between isodose line and normal to central axis at 10cm deep

Question 5

Three causes of penumbra

Answer 5

1. Geometric (extended source)
2. Transmission (through collimator)
3. Dosimetric (scatter in patient)

Question 6

What does geometric penumbra depend on?

Answer 6

• Width of extended source
• Collimator position
• SSD

Penumbra at depth d = (s(SSD + d) -SSD) / SSD

s=width of source

d=depth

Question 7

What affects transmission penumbra?

Answer 7

Energy of beam

Question 8

How is dosimetric penumbera affected by energy?

Answer 8

Lower energy = more lateral scatter hence wider penumbera

Question 9

Which types of penumbra dominate at high energies?

Answer 9

Transmission & Geometric

Question 10

How do you define a field size factor?

Why do you need them?

What two parts is it made up of?

Answer 10

Ratio of dose at 10cm depth under reference conditions to different collimator condition

Beam size increases, scatter increases, dose increases

FSF is made up of phantom scatter factor and head scatter factor

Question 11

Head Scatter Factor

What does it depend on?

How do you measure?

What component causes it?

Answer 11

• Difficult to quantify
• Depends on make of LINAC & is fn of field size
• Measure by keeping irradiated volume constant (removing phantom scatter)
• Mostly from flattening filter (3-4% dose to patient)
  *

Question 12

Phantom Scatter Factor (S_P)

What does it depend on?

How do we measure?

Answer 12

• Depends on beam quality
• Difficult to measure, infer from S_c (head scatter) and S_T (total scatter)
• Mostly from flattening filter

Question 13

Monitor Units:

• Definition
• Why do we have them?

Answer 13

• Amplifier adjusted until:
  
  • 1MU = 1Gy under calibration conditions
  • (e.g. 1MU = 1Gy at dmax for a 10x10 field)
• Have them to stop the LINAC when required number of MUs is given from TPS. LINAC calibration defines reference Dose/MU for dosimetry

Question 14

When calculating Dose/MU to a patient, what deviations from reference conditions might you need to think about? (x6)

Answer 14

• Depth
• Treatment distance
• Collimator setting
• Shape/size of area?
• Attenuators e.g. wedges
• patient heterogeneities

Question 15

What equation system should you use for:

Isocentrix

fixed SSD

Answer 15

• Isocentric - TPR (i.e. you are always prescribing dose to the same point, but you might adhd more or less water on top hence TPR)
• Fixed SSD - PDD (i.e. you are always prescribing dose to the surface so you only really care about how the dose drops off with depth

Question 16

Define Equivelent Square

EQN

Answer 16

• Square field which has same central axis depth dose characteristics as given non-standard field
• (i.e. what size square field would you need to measure the same dose at depth as an XxY field. Account for difference in scatter factors, deal with this first and make the rest of the calculation easier)
• S_c = sxy/(x+y)
• Only applies to photons