Characteristics of Clinical Beams: Photons

Question 1

Define the Tissue-Phantom Ratio.

Answer 1

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

Question 2

How does the beam profile change with increasing depth?

Answer 2

• 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 3

How does a geometric penumbra arise?

Answer 3

• Photon source is not a point source

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

Question 4

What does the width of the geometric penumbra depend on?

Answer 4

• width of extended source

- position of collimators

Question 5

What does the width of the transmission penumbra depend on?

Answer 5

energy of the beam

Question 6

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

Answer 6

• at larger field sizes the path through the collimators is shorter
• penumbrae becomes larger

Question 7

How do dosimetric penumbra change with increasing energy?

Answer 7

• lower energies have more lateral scatter

- dosimetric penumbrae reduce at higher energies as ther is more forward scatter

Question 8

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

Answer 8

• incident beam (more photons incident to patient)

- scatter within patient

Question 9

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

Answer 9

3-4%

Question 10

Where does head scatter come from?

Answer 10

mainly the flattening filter, but some contribution from collimators

Question 11

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

Answer 11

Sp = St/Sc

Question 12

What is a monitor unit?

Answer 12

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

Question 13

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

Answer 13

Greater penetration

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

Question 14

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

Answer 14

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

Question 15

How does an increase in energy affect the PDD?

Answer 15

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

Question 16

What are the major characteristics of a clinical photon beam?

Answer 16

• 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 17

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

Answer 17

• 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 18

How dose increasing the beam size affect the PDD?

Answer 18

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 19

How does increasing the SSD affect the PDD?

Answer 19

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 20

How does the TPR vary with increasing field size?

Answer 20

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

Question 21

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

Answer 21

• TPR does not have an Inverse Square Law effect

- Scatter conditions of the TPR and PDD are different.

Question 22

How is the beam size defined on a beam profile?

Answer 22

Defined at the Full-Width Half-Maximum

Question 23

How are the penumbra defined on a beam profile?

Answer 23

Difference between the 80% and 20% dose points.

Question 24

How is the wedge angle defined?

Answer 24

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

Question 25

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

Answer 25

• Geometric
• Transmission trough collimators
• Scatter in Patient.

Question 26

Concerning MV photon percentage depth doses, Which one of these statements is true?

a) The depth of maximum dose is a function of beam energy and field size
b) The increase in PDD with beam energy is due to increased lateral scattering of the incident photons
c) A 30cm increase in SSD has a negligible effect on the PDD
d) Surface dose decreases with increasing field size
e) Changes in PDD with field size are mainly due to differences in collimator/head scatter

Answer 26

a

Question 27

Concerning radiation beam geometry, which of these is true?

a) The dose at any depth is greatest on the central axis
b) At 100cm SSD, field size is defined by the width of the 95% isodose line at the patient surface
c) Only scattered photons can reach outside the geometric field edge
d) An increase in source to secondary collimator distance increases the geometric penumbra
e) Penumbra widths are expected to decrease as the beam energy increases from 4MV to 8MV

Answer 27

e