Electron beams

Question 1

Photons have a distinct point source, what about electrons?

Answer 1

With electrons beams we are unable to determine whey they originate. So we use the term ‘Virtual source position’ to indicate the virtual location of the treatment source.

Question 2

Why was the term ‘virtual source distance’ introduced?

Answer 2

To indicate the virtual location of the electron source

Question 3

What is the effective source to skin distance for electron beams? (SSdeff).

Answer 3

is defined as the distance from the virtual source position to the point of the nominal SSD (usually the isocentre of the linac)

Question 4

What happens to the dose beyond 90%?

Answer 4

he dose decreases abruptly beyond the 90% dose level, the treatment depth and the required electron energy must be chosen very carefully.

Question 5

What is the maximum depth electron beams can treat at effectively?

Answer 5

disease within 6cm on the surface, sparing deeper normal tissues.

Question 6

Why are custom made cut outs needed?

Answer 6

For irregular shaped tumours. To conform the shape of the radiation field, while sparing radiation to surrounding tissues.

Question 7

When are electron beam treatments performed at an extended SSD?

Answer 7

As the body anatomy may obstruct the positioning of the applicator.

SSD of 101-120 cm

Question 8

What is the ideal irradiation condition for the electron beam?

Answer 8

for the electron beam to be incident normal to a flat surface with underlying homogeneous soft tissues.

Question 9

Why is having the the correct angle of incidence important?

Answer 9

Electron beams are effected significantly by the presence of air bone.

Ideal for electrons to be incident on a relatively flat surface, and the underlying tissue most be homogenous.

Question 10

How does irregular surfaces and internal heterogeneities affect the dose distribution?

Answer 10

They create changes in side-scatter equilibrium , producing volumes of increased dose (hot spots) and decreased dose (cold spots).

Question 11

Why do we need to create a wax mould for the nose?

Answer 11

As the area being treated on the nose is not flat and contains inhomgenous tissue. We need to create a wax mould, to create a more even dose distribution.

Question 12

What is stand off?

Answer 12

The distance between the electron cone and the patient.

Question 13

How should electron energy be chosen?

Answer 13

The depth of the 90% isodose line needs to cover the deepest portion of the region to be treated.

In addition to an approximate 5mm additional depth beyond the treatment region.

Question 14

How should electron energy be chosen?

Answer 14

The depth of the 90% isodose line needs to cover the deepest portion of the region to be treated.

In addition to an approximate 5mm additional depth beyond the treatment region.

Question 15

When is the 80% idosdose level commonly used?

Answer 15

For chest wall treatments where the D80 would be placed at the lung-chest wall interface

Question 16

How is the depth of the isodose line related to beam energy?

Answer 16

The depth of the D80 is approximately a third of the electron beam energy. This means that if we have a 9MeV the D80=3cm

Question 17

What are common breast electron treatments?

Answer 17

Tumor Bed boost
Scar Boost in Chestwall treatments
IMC chain treatment

Question 18

What are common electron boost fractionations?

Answer 18

16Gy in 8 daily fractions
given in 1 1⁄2 weeks.

or

• 10Gy in 5 daily fractions given in 1 week.

Question 19

Why should the electron cone be greater than the PTV?

Answer 19

There needs to be a margin to account for penumbra

Question 20

What is an electron photon junction?

Answer 20

As electrons have lateral scatter and photons have forward scatter, there will be a region of overlap and subsequent high dose.

Question 21

What is MHD?

Answer 21

Maximum heart distance

Question 22

What are factors to consider when planning electrons?

Answer 22

Patient position
the location of the treatment
the depth of coverage required

Question 23

What are the different types of treatment techniques?

Answer 23

100cm SSD
extended SSD
best contact
electron art irradiation

Question 24

Why is an extended SSD used for electrons?

Answer 24

extended treatment distances from 100 to 130 cm to avoid collision between the patient and the applicator.

Question 25

What is best contact?

Answer 25

Bevel the angulation with the best contact to provide field coverage with the best contact between the end of the cone and the treated surface.

Question 26

what is Electron Arc irradiation?

Answer 26

The treatment of post mastectomy chestwall and other areas of the body such as the ribs and the limbs.

Question 27

What are electron cut outs, made out of?

Answer 27

Cerrobend (more common due to lead poisoning)

lead

Question 28

What are eye shields and what are they made out of?

Answer 28

They are made of tungsten and plastic and inserted inside the eyelids.
Tungsten plastic allows us to account for back scatter that can contaminate organs at risk

Question 29

How is the energy chosen?

Answer 29

The electron energy should be selected so that the max depth dose curve is located at the centre of the PTV (ICRU71)

Question 30

How is the field size chosen?

Answer 30

based on adequacy of isodose coverage of the PTV

Question 31

What is oblique incidence?

Answer 31

When the electron applicator is on an oblique angle to the patient instead of perpendicular.

This effects the isodose curves inside the patient.

Question 32

As the angle of incidence increases what happens?

Answer 32

1. Angle of incidence >30 Central axis PDD changes significantly
2. Depth of dose increases and maximum dose decreases
3. Angle of incidence >60 the DMAX increases dramatically

Question 33

What are 3 examples where sloped or curved surfaces are encountered?

Answer 33

Nose
Breast
Ear

Question 34

Why is bolus used?

Answer 34

1. to increase the dose on the skin surface.
2. to replace missing tissue due to surface
   irregularities
3. as compensating material to shape the coverage of the radiation to conform as closely as possible to the target volume while sparing normal tissue

Question 35

What are some examples of bolus materials?

Answer 35

Paraffin wax
􏰀 Polysterene
􏰀 Solid water
􏰀 Superstuff
􏰀 Superflab,
􏰀 Wet cotton
balls

Question 36

What happens to penumbra at extended SSD’s?

Answer 36

It broadens

Question 37

What is the effect of treating patients at a sub-optimal angle?

Answer 37

By changing the gantry angle you will change the penumbra.

Penumbra broadens as we increase the SSD.

Question 38

What is the dosimetry of small fields?

Answer 38

1. the depth of dmax moves toward the surface,
2. the central-axis percentage depth dose decreases
3. the output dose/monitor unit decreases from that of the unrestricted applicator.

Question 39

What are questions to ask a RO for an electron markup?

Answer 39

1. At what depth do you want the dose?
2. What shape do you want to get this treatment dose to?
3. What skin dose do you want?

Question 40

What happens if best contact is not set up?

Answer 40

May compromise coverage at a depth

Question 41

Field plans set up not for best contact are most likely done so to?

Answer 41

a. avoid a structure
b. ensure a junction match
c. create a dose distribution.

Question 42

What are most electron fields prescribed to?

Answer 42

100% depth dose (R100)

However R100 depth varies with field size and electro energy