Fundamentals Quiz 4

e- therapy

Most clinical useful energy range for electron therapy

6 - 20 MeV

electron therapy treats…

superficial tumors

• less than 5 cm deep

main uses of e- therapy

treatment of skin and lip cancers

chest wall irradiation for breast cancer

administering boost dose to nodes

treatment of head and neck cancers

2 inelastic collisions w/ atomic e-

ionization & excitation

inelastic collisions

KE can be lost producing:

• ionization or excitation

• converted to brems photons

ionization/excitation is dominant in

lower atomic number

brems is dominant in

high atomic number

high E

elastic collisions w/ atomic e-

electron-electron scattering

elastic collisons

KE is not lost but redistributed among particles after collision

collisional vs radiative processes

collisional: e- interacting w/ e- surrounding atoms

radiative: brems (e- passing by nucleus)

sum of collisonal and radiative losses

total mass stopping power

Gradient

rate of change of a value (dose) w/ a change position

(basically change in dose over change in distance)

beyond what range is the dose due to x-ray contamination of the beam

Practical range

E/2

(indicated by the tail on the depth dose curve)

most useful treatment depth of electrons is at what % of the depth dose

90%

estimate beam E based on depth dose curves

dmax for co-60

0.5 cm

dmax for 4 MV

1

dmax for 6 MV

1.5 cm

dmax for 10 MV

2.5 cm

dmax for 15 MV

3 cm

dmax for 18 MV

3.5 cm

skin sparing vs e-

e- do not provide skin sparing

(provide depth sparing)

increasing E will …… to percent surface dose for e-

increase

how is field size varied for e-

variable jaws are fixed

applicators (cones) change the size of the field

for smaller fields what happens to the dmax

dmax shifts toward the surface

as field size increase the output factor…

increases

due to extra scatter from sides adding to direct beam

virtual source

intersection point where the beam appears to diverge from a point source

• right before scattering foil

e- vs inverse square law

they don’t follow it

• because of heavy scattering and no true point source

scattering foil

widens the e- beam

(second may be used to improve flatness of beam)

most e- beam treatments are planned for

single field techniques

beam E is set to cover target volume at

90 to 95% isodose curve

E is decided on what 3 factors

• depth of target

• minimal target dose required

• acceptable dose to critical structures

block margins for photons vs e-

photon = 0.5 - 1 cm

e- = 1.5 - 2 cm

larger skin obliquity = ….. dose

larger dose

higher E = … photon tail

increased photon tail

sharp edges can be smoothed with a

shaped bolus

CET

coefficient of equivalent thickness

what is CET used for

approximates dose when there is tissue inhomogeneities by comparing it to a uniform slab (equivalent thickness) of water

bolus is used in e- beams to

• flatten out irregular surface

• reduce penetration of e- to parts in field

• increase surface dose

bolus is measured againist what

water

risks of overlapping/abutted fields vs seperated fields

overlapping - risks overdosing

seperated - risks underdosing

when e- and photon fields are abutted which side will have the hotspot vs coldspot

e- will have coldspot

photon field will have hotpsot because of how e- scatter laterally

what MeV range is used for TSI

2 - 9 Mev

2 types of TSI treatment categories

translational technique

large field technique

translational technique

pt lies on a motor driven couch and moved across the beam

(or pt stationary and beam is moved across pt)

• uses strontium-90

Large field technique

scattering e- through wide angles and large treatment distances

• pt treated standing w/ 4 to 6 fields equally spaced

• stanford technique

limiting factor for TSI

x-ray contamination

stanford technique

uses 6 fields (AP, PA, 4 obliques) (all 60 degrees apart)

3 dual fields per day

• AP, 2 posterior obliques

• PA, 2 anterior obliques

90% isodose at

E/4

80% isodose at

E/3

50% isodose at

E/2.5

Practical range (Rp) at

E/2

higher E = .. contamination in e- beam

more contamination

penumbra is more noticeable on what field size

smaller field sizes

rate of E loss of e- >= 1 MeV is

2 MeV/cm