Electron beam dosimetry

Question 1

Describe the changes that need to be made in the treatment head of a medical linear accelerator to convert from delivering X-rays to delivering an electron treatment.

Answer 1

Remove the target and flattening filter, add a large scattering foil ( electron scatters- beam is scatter ) and apply the electron applicator

Question 2

Discuss the electron interactions and subsequent processes for electron beams travelling through matter.

Answer 2

Electrons propagate through any medium, then interact with atoms of the absorbing medium by different elastic or inelastic coulomb force interaction.

Question 3

Outer shell electron elastic collision.

Answer 3

The struck electron is loosely bounded effectively free, after collision the struck electron is knocked out of the atom, both electrons go on to have further collisions.

The kinetic energy of both electrons after the collision adds up to that of initial electron kinetic energy.

Question 4

Inner shell electron inelastic collision.

Answer 4

The struck electron is bound in its orbit and the binding energy must be supplied before the electron can be knocked out of the atom.

initial electron kinetic energy equals the final kinetic energy of both electrons plus the binding energy.

Question 5

Collisions with Nuclei - elastic

Answer 5

electrons bounces of the nucleus with no change in energy.

This increase with atomic number and decreases with increasing energy.

Question 6

Collisions with Nuclei - inelastic

Answer 6

The scattering results in the emission of an x-ray (i.e. photon; bremsstrahlung)

increases rapidly with energy and atomic number

( above 9Mev this is the main form of energy loss for high Z targets)

Question 7

Explain how isodose curves differs in electron beam from the isodose curve of a megavoltage X-ray beam.

Answer 7

As an electron beam penetrates a medium, the beam expands rapidly below the surface, due to electron scattering on atoms of the medium, this leads to bulging out of low isodose curves. Depth is a lot shallower than MV.

Question 8

Rule of thumb for PDD

Answer 8

R90 - E/4
R80 - E/3
Rp (practical range)- E/2

Question 9

Skin sparing in electrons

Answer 9

% surface dose increase with increasing electron energy

Question 10

Describe how the central axis percentage depth dose curves of electron beams differ from those of megavoltage photon beam.

Answer 10

Surface dose for electron beams is in the higher %, so a lack of build up region, with a rapid dose-fall off. So limited/no skin sparing.

Question 11

What is the effect of field size on PDD

Answer 11

For smaller fields, when the side of the electron field is smaller than the practical range there is reduction in depth dose as electrons are scattered out of the beam.

The depth of maximum dose decreases and the surface dose increases. the decrease in dose with increasing depth is less steep.

Question 12

Discuss the problems that occur in terms of the dose distribution at and below the skin surface for
two adjacent electron fields (discuss both fields that are abutted and separated at the surface);

Answer 12

Adjacent electron fields create hotspots and cold spots within the plan. When they are abutting at the surface this creates hotspot at the junction and with adjacent beams separated at the surface this creates cold spots in the junction region.

Question 13

Discuss the problems that occur in terms of the dose distribution at and below the skin surface for an electron field abutted at the surface with a photon field.


Answer 13

When adjacent to a photon field due to out scattering of electrons from the electron field there’re hotspots on the photon side and cold spots on the electron side.

Question 14

9MeV PDD

Answer 14

SD= 80%

DMAX/ R100 = 17mm