electron beam therapy - jpneylon.github.io
Post on 21-May-2022
10 Views
Preview:
TRANSCRIPT
Electron Beam Therapy
Dimitre Hristov
2
Electron Beam Therapy 2009
ASTRO’s view on physics teaching
3
Electron Beam Therapy 2009
Select the correct answer:
The percent depth dose at 5 cm for a 6 MeV beam is approximately: 100%, 90%, 80%, 50%, <5%?
To treat a lesion extending to a depth of 5 cm you will use: 6 MeV, 10 MeV, 15 MeV, 20 MeV?
What is the approximate range of a 6 MeV beam in lung overlaying 1 cm of tissue: 1-3 cm, 7-9 cm, > 12 cm?
A 4x8 cut-out is placed in a 10x10 beam cone for 20 MeV treatment. What changes is: 90% depth, cGy/MU, d_max, skin dose, all of the preceding?
4
Electron Beam Therapy 2009
Outline
• Production of electron beams
• Interactions of electron beams with matter
• Electron beam characteristics
• Treatment planning
5
Electron Beam Therapy 2009
Electron Beam Production
6
Electron Beam Therapy 2009
Field Flatness and Uniformity
Primary scattering foil
Secondary scattering foil
Primary collimator
Secondary collimator
Dual foil system for producing a uniform electron beam
Monoenergetic e– beam
Energy spectrum broadened
Energy spectrum broadened
7
Electron Beam Therapy 2009
Primary Scattering Foil
Secondary Scattering Foil
Ionization Chamber
Photon Jaws
Electron Cone
Electron Beam Production
8
Electron Beam Therapy 2009
Charged particle interactions
Incoming radiation
Bremsstrahlung
Main e- track
9
Electron Beam Therapy 2009
Electron-Matter Interactions
Collisional losses Inelastic (excitation, ionization)
Elastic
col
S
ρ
10
Electron Beam Therapy 2009
Electron-Matter Interactions
Radiative losses :Bremsstrahlung
Total mass stopping power radcoltot
SSS
+
=
ρρρ
rad
S
ρ
11
Electron Beam Therapy 2009
Electron Scattering
Pencil electron beam tracks in a bubble chamber
12
Electron Beam Therapy 2009
Relation of Absorbed Dose to Fluence
med
Φ is the differential fluence in energy of identical
charged particles L/ρ is the restricted mass collision stopping power with a
cutoff energy
( ) dEELDcol
E
E∆∆
Φ= ∫
,
max
ρ
13
Electron Beam Therapy 2009
Electron Dose Measurement
colkcolkk dx
dTSD,,
1
=
Φ=
ρρ colmedcolmedmed dx
dTSD,,
1
=
Φ=
ρρ
colkcolmedk
med SSD
D
,,
=
ρρ
med med
Bragg-Gray Cavity Theory
colkcolkk dx
dTSD,,
1
=
Φ=
ρρ colmedcolmedmed dx
dT1SD,,
=
Φ=
ρρ
14
Electron Beam Therapy 2009
The Bragg-Gray Cavity Theory
The ionization produced in a gas-filled cavity placed in a medium is related to the energy absorbed in the surrounding medium.
When the cavity is sufficiently small, electron fluence does not change.
colkcolmedk
med SSD
D
,,
=
ρρ
15
Electron Beam Therapy 2009
The Bragg-Gray Cavity: stopping power ratios
17
Electron Beam Therapy 2009
Energy specification and measurement
18
Electron Beam Therapy 2009
Energy specification and measurement
Measure PDD
for a broad beam
=> 20 cm x 20 cm
19
Electron Beam Therapy 2009
Energy specification and measurement
( )2
21
21
2p3p210p
MeVcm00250CMeVcm981CMeV220C
RCRCCE−− ===
×+×+=
. ;. ;.
,
20
Electron Beam Therapy 2009
Energy specification and measurement
( ) 500 RcmMeV332E ×= /.
21
Electron Beam Therapy 2009
Energy specification and measurement
( ) ( ) )/( p0pzp Rz1EE −=
22
Electron Beam Therapy 2009
Electron and Photon Beams
Photon beams indirectly ionizing exponential attenuation
Electron beams directly ionizing definite range
23
Electron Beam Therapy 2009
Electron Depth Dose: Rules of thumb
90%
(E/3.2) cm
80% (E/2.8) cm
50% (E/2.33) cm
R_p (E/1.98) cm
24
Electron Beam Therapy 2009
Electron and Photon Depth Dose
25
Electron Beam Therapy 2009
Depth doses for 20 MeV electrons compared to 15 MeV X-Rays
Electron and Photon Depth Dose
26
Electron Beam Therapy 2009
6 MeV electrons 6 MeV X-rays
Electron and Photon Depth Dose
27
Electron Beam Therapy 2009
Central Axis PDDs: field size dependence 6 MeV electrons
2x2
25x25 4x4
Little changes beyond field radius exceeding
0pp E880R ,.≅
28
Electron Beam Therapy 2009
Central Axis PDDs: field size dependence 16 MeV electrons
2 3
4
6 10-25 Little changes beyond field radius exceeding
0pp E880R ,.≅
29
Electron Beam Therapy 2009
Central Axis PDDs: energy dependence
Energy
(MeV) 4 6 9 12 16 20
Surface Dose
(% of maximum) 73.8 74.3 80.0 84.9 90.5 90.8
30
Electron Beam Therapy 2009
Electron Dose Distributions
4x4 field
2x2 field 4x2 field
6 MeV electrons
31
Electron Beam Therapy 2009
Electron Dose Distributions
16 MeV electrons
4x4 field
10x10 field
32
Electron Beam Therapy 2009
Central Axis PDD: SSD dependence
33
Electron Beam Therapy 2009
Dose profiles, at 15 mm depth at Ep,0 = 9 MeV
Profiles: SSD dependence
34
Electron Beam Therapy 2009
Dose Distributions: SSD dependence
Increased penumbra
35
Electron Beam Therapy 2009
Central axis depth dose curves plotted as a function of the angle of incidence of the beam
Angular dependence
36
Electron Beam Therapy 2009
Curved Surface Dose Distributions
Must correct predicted dose distributions for:
• Scatter of electron beam in tissue (angular dependence) • Air gap (inverse square falloff in exposure)
* SSD
Air gap
Water-equivalent materials can be used as “bolus” to smooth out uneven target surfaces
37
Electron Beam Therapy 2009
Tissue Inhomogeneity
38
Electron Beam Therapy 2009
Irregular Surface Dose Distributions
39
Electron Beam Therapy 2009
Tissue Inhomogeneity
)( CET1zdDeff −−=
CET: Coefficient of Equivalent Thickness
P
d z
Compact bone CET: 1.65
Lung CET: 0.2-0.25
40
Electron Beam Therapy 2009
Tissue inhomogeneity
uncorrected calculation corrected calculation
41
Electron Beam Therapy 2009
16 MeV electrons
Tissue Inhomogeneity
42
Electron Beam Therapy 2009
Tissue Inhomogeneity
43
Electron Beam Therapy 2009
Lead shielding for electrons
Thickness (mm) = E (MeV)/2 +1
45
Electron Beam Therapy 2009
Electron Backscatter
Solution: place a low atomic number absorber (bolus) on the surface of the shield to absorb the dose due to backscatter
Beam matching
47
Electron Beam Therapy 2009
Summary
Electron beam depth dose characteristics offer advantages for treating superficial lesions
Tissue inhomogeneities, and surface irregularities alter dose distributions significantly.
Modern treatment planning systems and dose-calculation algorithms open the possibility of wider use of electron beams as a treatment modality.
48
Electron Beam Therapy 2009
Select the correct answer:
The percent depth dose at 5 cm for a 6 MeV beam is approximately: 100%, 90%, 80%, 50%, <5%?
To treat a lesion extending to a depth of 5 cm you will use: 6 MeV, 10 MeV, 15 MeV, 20 MeV?
What is the approximate range of a 6 MeV beam in lung overlaying 1 cm of tissue: 1-3 cm, 7-9 cm, > 12 cm?
A 4x8 cut-out is placed in a 10x10 beam cone for 20 MeV treatment. What changes is: 90% depth, cGy/MU, d_max, skin dose, all of the preceding?
top related