Characteristics of Clinical Beams: Photons

Question 1

What happens to the isodose plot in the patient as the energy of the beam increases?

Answer 1

Greater penetration

Less side scattered dose - loss of side lobes on the isodose plot.

Question 2

How is a percentage depth dose (PDD) curve obtained from and isodose plot?

Answer 2

Linear profile of the dose down the central axis of the plot.

Question 3

How does an increase in energy affect the PDD?

Answer 3

• Greater depth of d(max)
• Shallower drop off after d(max)
• Shallower build up region
• Lower entrance surface dose

Question 4

What are the major characteristics of a clinical photon beam?

Answer 4

• Build up region
• Point of max dose
• Normalisation point (5, 7 or10cm, depends on centre)
• Dose reduction with depth
• Penumbra
• Beam profiles at depth
• Dose from scatter (linac head and patient) and leakage

Question 5

What causes the build-up effect in a clinical beam?

Answer 5

• Photons interact at different depths in the tissue and generate secondary electrons.
• At each interaction, the recoils electrons travel (mostly) forward and deposit dose
• As more tracks overlap, the dose builds up until Charged Particle EquillibriumM (CPE) is reached.
• A steady state would be reached if there was not photon attenuation (absorption or scatter)
• Dose>0 at the surface due to back0-scattered electrons and electron contamination.

Question 6

How dose increasing the beam size affect the PDD?

Answer 6

Increases the dose at depth due to:

• More photons reaching the patient from the source
• More scattered electrons to the measurement point from the irradiated volume.

Question 7

How does increasing the SSD affect the PDD?

Answer 7

Increases dose at depth > d(max) (i.e. shallower drop-off)

Absolute dose would decrease with increase of SSD, but percentage dose relative to a fixed point increases

Question 8

Define the Tissue-Phantom Ratio.

Answer 8

Ratio od the dose obtained at a fixed field size and SSD, but with different thicknesses of “tissue” between the surface and the detector.

Question 9

How does the TPR vary with increasing field size?

Answer 9

• Steeper build-up region
• Lower depth of TPR(max)
• Shallower fall-off after TPR(max)

Question 10

Why is there a steeper fall-off from the TPR compared to a PDD?

Answer 10

• TPR does not have an Inverse Square Law effect

- Scatter conditions of the TPR and PDD are different.

Question 11

How is the beam size defined on a beam profile?

Answer 11

Defined at the Full-Width Half-Maximum

Question 12

How are the penumbra defined on a beam profile?

Answer 12

Difference between the 80% and 20% dose points.

Question 13

How does the beam profile change with increasing depth?

Answer 13

• Beam and penumbra widen
• Flattest beam at 10cm (with flattening filter)
• • Before 10cm beam is “horny”
• • After 10cm beam is peaked
• • Horniness and Peakedness increase with distance from 10cm depth.
• Dose is reduced as with PDD.

Question 14

How is the wedge angle defined?

Answer 14

Wedge angle is the angle between the isodose line and the normal to the central axis at 10cm deep.

Question 15

What are the three causes of penumbrae in a clinical beam?

Answer 15

• Geometric
• Transmission trough collimators
• Scatter in Patient.

Question 16

How does a geometric penumbra arise?

Answer 16

• Photon source is not a point source.

- Photons from opposite edges of source can pass through the collimators giving beams in overlapping locations

Question 17

What does the width of the geometric penumbra depend on?

Answer 17

• Width of extended source

- Position of collimators

Question 18

What does the width of the transmission penumbra depend on?

Answer 18

• Energy of the beam

Question 19

What happens if the jaw face is not parallel to the divergent beam?

Answer 19

• At larger field sizes the path through the collimators is shorter
• Penumbrae become larger.

Question 20

How do dosimetric penumbra change with increasing energy?

Answer 20

• Lower energies have more lateral scatter.

- Dosimetric penumbrae reduce at higher energies as there is more forward scatter.

Question 21

What two factors affect the dose to patient when beam size is increased?

Answer 21

• Incident beam (more photons incident to patient)

- Scatter within patient

Question 22

Whar percentage of the dose to the patient comes from head scatter?

Answer 22

3-4%

Question 23

Where does head scatter come from?

Answer 23

Mainly the flattening filter, but some contribution from collimators.

Question 24

How does the patient scatter factor (Sp) relate to the head scatter factor (Sc) and the field size factor (St)?

Answer 24

Sp = St/Sc

Question 25

What is a monitor unit?

Answer 25

Dose measured in the monitor chamber in the linac head that gives a fixed dose (1cGy) under the departmental reference conditions (fixed SSD, Field size and depth)