week 7: electron beam review

in the AAPM task Group 25 report, extended SSD for electron therapy is defined as treatments that are not more than ____ cm beyond the standard SSD

15cm

in low atomic number material, such as water and soft tissue, electrons lose energy predominantly through ionizing events with

atomic electrons

as the energy of an electron beam increases, the amount of bremsstrahlung contamination

increases

when two adjacent electron fields abut on the surface, the hot spot or area of increased dose will exist in the overlap region at depth (below the surface of the skin)

true

an electron beam is calculated to deliver 250 cGy using 265 MU. if the equipment terminates after delivering 200MU, what dose was actually delivered

189cGy (250/265 = x/200)

an electron beam with a custom insert has a measured output factor of .954cGy/MU at dmax. IF 200cGy is prescribed to the 90% isodose, the MU setting is

233MU (200/(.954 x .9))

what effect will an increase in the nominal energy of an electron beam have on the depth of the 80% isodose line

increase

when an electron field and photon field abut, the hot spit will be on the side of the

photon field

given a mean energy of an electron beam is 6MeV, what is the depth of the 50% isodose line

2.5cm (6MeV= depth x 2.4)

a patient has a tumor at a depth of 4cm. what energy of electron beam should be used so that the 80% isodose line encompasses the tumor

12 MeV (4cm = x/2.8)

the output for an electron beam depends on

cone size, field size, beam energy, SSD

QUESTION 12: In this image an electron depth dose curve, Portion D represents

photon contamination

as electron beam energy increases, Bremsstrahlung contamination

increases

as field size increases, electron beam output

increases

what linear accelerator components serves to initially scatter the pencil beam of electrons into a wider, more clinically useful electron beam using one or two thin layers of high atomic number material

scattering foil

intraoperative radiation therapy generally utilizes electron beam energies of 9-16MeV

true

what type of interactions are the predominate mechanism by which electron beams interact with low atomic number materials and lose energy in clinical radiation therapy

collisional

as electron beam energy increases, surface dose and percent depth dose

increases

as distance (SSD) increases, electron beam penumbra

increases

what treatment energy for electrons is most likely to use a scanning beam technique rather than a scattering foil

25 MeV

unlike photons, electrons have a mass and a charge

true

bolus is used to increase the surface dose of the electron beam

true

bolus is used for a tissue compensator for electron beams

true

bolus is used to shape isodose distributions for electron beams

true

bolus is used to increase the effective energy at depth for an electron beam

false

electrons are considered

directly ionizing radiation

what is the depth of the 90% isodose line of a 12 MeV electron beam

3.75cm (12/3.2)

as electron beam energy increases, the surface dose

increases

in regards to electron beam chest wall arc therapy, the dose is typically prescribed to the depth of the isodose line

80%

if greater than ____% of an electron treatment field is blocked, it is recommended that measurements be taken to determine the impact of the blocking on beam output

25

photon beams are more significantly impacted by tissue inhomogeneities as compared to electron beams

false

the phenomenon associated with electron beams involving the lateral constriction of the higher isodose values, such as at the 80% isodose line, occurs around

15 MeV

surface dose from an electron beam is about the same as that of a photon beam with the same energy

false

surface dose is lower for an electron beam generated with a scattering foil as compared to a scanning beam

false

electron beam surface dose is about the same as that of a superficial xray beam

false

electron beam surface dose increases with increased beam energy

true

a legal document that a patient uses to make known his or her final wishes regarding life prolonging medical treatment is a

living will

documentation present in every department that provides information on handling precautions, safe use and disposal/cleanup of chemicals is termed

safety data sheets

if measuring an adult patient's respiration, which of the following would be within normal limits

16 breaths per minute

when patients arrive to the radiation therapy deparment with an infusion pump, the bag containing the medicine should be placed

above the patient's IV site

the ability of an infectious agent to cause clinical disease is termed

pathogenicity

What effect will an increase in electron beam energy have on the lateral spread (or bulging) of low-level isodose line

decrease

In clinical practice, large bolus with edges that are perpendicular to the surface across a portion of an electron field may lead to an increase and/or decrease in dose by 20-30%. This is referred to as the _ effect

edge

The range at which an electron beam essentially come to rest is calculated by:

energy / 2

what is the practical range of a 16MeV electron beam

8cm (MeV/2)

As the source-to-collimator distance (SCD) increases, electron beam penumbra:

decreases

The predominant way in which an electron beam loses energy in tissue in radiation therapy energy ranges is through _ interactions

collision

In reference to electron beam therapy, high-level isodose lines (80% or greater) demonstrate lateral _ which increases with increasing beam energy and depth

constriction

What is the danger to tissue in front of an internal shield when used with electron beam therapy?

electron backscatter

The mass stopping power is defined as the rate of energy gain per unit length divided by the density of the medium

false (LOSS)

In water, an electron beam loses approximately MeV/cm

2

electrons are considered low LET

true

What is the thickness of lead in mm needed to shield a 16MeV electron beam, per Khan?

9mm (16/2 + 1)

In regards to electron beam chest wall arc therapy, the dose is typically prescribed to the depth of the isodose line

80%

As electron beam energy increases, surface dose and percent depth dose:

increases

what linear accelerator component serves to initially scatter the pencil beam of electrons into a wider, more clinically useful electron beam using one or two thin layers of high atomic number material.

scattering foil