Current A
Approach
in
n
Clinical Electron Beam Dosimetry
Clinical
Dimitris Mih
hailidis, Ph.D.
Ph.D.
Charleston Radiation
n Therapy
py Consultants
THANK
K YOU
PROGRAM DIRECTORS
D
David W.O. Rogers,
R
Ph.D.
Joanna Cyg
yg
gler,, Ph.D.
g
AAPM & SS
S SubCom
Local Arrangeme
ents Committee
Ms. Betsy Phelps
Phelpsh l -Medi
dicall Physics
h
Publishing
bl h
Participants
OUTL
LINE
Objectives
 Current calibration protocols
 E-beam quality specification
n
 Measurement of CA %DDs in water:
water:

– ion
i chambers,
h b
di
diodes,
d
fil
film
– practical rules
Output
p factors
 Non
Non--water phantoms: Relattive measurements

– ion chambers, %DDs, film
Smallll and
S
d iirregular
l fifield
ld d
dossimetry
i t
 Extended treatment distanc
ces
 Electron algorithms
 Some data and examples

Objec
ctives
 Address
the issues that currently influence the
dosimetry of clinical elec
ctron beams due to
changes recently introdu
uced by the new
dosimetry calibration pro
otocol (TG
(TG--51).
 Suggest how to approprriately modify and update
the widely accepted TGTG-25 electron dosimetry
protocol.
 Describe a detail proced
dure of converting
measured depthdepth-ionizatiion curves with ion
chambers into depth
depth--dosse curves making use of
recently published stopp
ping--power ratios and
ping
other conversion factors
s.
 Present the important po
oints of the upcoming
AAPM TGTG-70. (Gerbi et al. Med.
M
Phys. July 2009, issue!)
Objec
ctives
 Describe
the use of wate
er equivalent phantoms
to perform relative electrron dosimetry based on
recently published conve
ersions factors.
 Discuss small and irregu
ularly shaped electron
field dosimetry using the
e concept of lateral
buildup ratio (LBR) as an
n avenue to evaluate
electronic equilibrium an
nd compute dose per MU
for those fields
fields.
 Give some common clin
nical examples where
electron beam dosimetry
y is applied.
Important rep
ports to have
 TG
TG--25 (Clinical electron beeam dosimetry).
 TG
TG--51 (protocol for clinical dosimetry for high
high-energy photon and
d electron beams).
 TG
TG--69 (Radiographic Filmss for MV Beam
Dosimetry).
 TG
TG--39 (The calibration andd use of planeplane-parallel
ionization
ioni
ation chambe
chamberrs
s fo
for dosimetry
dosimet of
electron beams) .
 TG
TG--53 (Quality assurance for clinical radiotherapy
treatment planning
g).
 TG
TG--106 (Accelerator beam
m data commissioning and

procedures.
TG--70 (just published)
TG
Current calibra
ation protocols
ation
Current calibra
ation protocols
 TG
TG--51
of AAPM (19
999).
999)
 TRS
TRS--398 of IAEA (2003).
“Absorbed
“Ab
b d Dose
D
determin
d
ination
i
iin E
Externall B
Beam
Radiotherapy: An Interna
ational Code of Practice
based on Standards of Ab
bsorbed Dose to Water”
 Code
of practice off IPEM (2006).
“Code of practice for elec
ctron dosimetry of
radiotherapy beams with
h initial energy from 2 to 50
MeV based on air
air--kerma calibration
calibration”
”
Main objectivves of TGTG-51
 Defines
Dose at one poiint, dref.
 Proposes to do two thin
ngs:
1. Incorporate the new abs
sorbed dose standard
– Absorbed dose is more robust than the AirAir-Kerma Std.
Q
N D ,w

60 Co
k q N D ,w
– Dose to water is closer to the dose to tissue
2. Simplify the calibration formalism
f
(as much as
possible)

Makes use of realistic waterw
water
-to
to--air restricted
w
SPRs
SPRs.
 L 
   air
Because of TGTG-51
1 (and other recent
dosimetry data), do
o we need to modify
TG--25 ?
TG
YES! TGTG-70 was charged!
E-Beam Qualitty
ty Specification
Electron Beam Qu
uality Specification
New beam quality specifie
er*:: R50 (depth in water
er*
at which dose falls to 50% of maximum for
large field size (>10x10 cm2) (TG
(TG--51).


First find I50 (50% of ionization maximum).
R50  1.029I 50  0.06
cm
m for
2  I 50  10
cm
or
R50  1.059I 50  0.37
c for
cm
I 50  10
cm
*(TG--25, be
*(TG
eam quality specifier: (Ep)0)
Energy at
a depth
Harder’s Eq. still valid!
v
(Harder 1968)

Mean energy at depth: E d



d
 ( MeV )
 E 0 1 


R
p


M
Mean
energy att surface
f
off phantom
h t
: (IPEM 2003)
2
E0  0.656  2.059 R50  0.022 R50
( MeV )
or
2
E 0  0 .818  1 .9355 I 50  0 .040 I 50
( MeV )

Practical range: (Rogers 1996)
R p  1.271R50  0.23(cm)
Energy at
a depth
Harder’s Eq.
q still valid!
v
((Harder 1968))
 Mean
Ed
?
energ
gy at depth:


d
 ( MeV )
 E 0 1 


R
p 

?
Energy at surface and
a practical range

Mean energy at surface
e of phantom : (IPEM 2003)
2
E0  0.656  2.059 R50  0.022 R50
( MeV )
or
2
E 0  0 .818  1 .935 I 50  0 .040 I 50
( MeV )

Practical range: (Ro
ogers 1996)
R p  1.271R50  0.23(cm)
Comm
ment
Harder’s Eq.. still valid!

Mean energy at depth: E d

U off ((as off TGUse
TG-25):
25)


d
 ( MeV )
 E 0 1 


R
p


E0  2.33 R50 ( MeV )
or
E0  2.40 R50 ( MeV )
Produce values within
n 0.4 MeV of previous
relationships
Measurements
Measureme
nts of %DDs
Phantoms an
nd detectors
 WATER
for relative measurements
m
(%DDs, OFs, etc) such as large data
collection..
collection
 Plastic
ast c (
(non
(nono -Water
ater)
) for
o limited
ted relative
e at e
measurements of clin
nical setups (%DDs,
OF,, etc).
O
)
 Detectors (cylindrical, pp-p, diodes, film)
Cylindrical cha
ambers
ambers in water
% de
epth ioniz
zation orr dose
Step 1:
1: Shift chamber deeper by 00.5r
0.5
5rcav
5r
Step 2:
2: Correct reeading for Piion, Ppoll
M ( d )  M raw( d )  Pion ( d )  Ppol ( d )
.
• Apply TGTG-51 requirem
ments for Pion and Ppol
(For %DD: measure att I50 and Rp depths)
M (d )
% di ( d )  1000
M ( I max )
Step
p 3:
3: Use realisttic SPRs water/air
 From Burns 1996
(in water)
w
a  bln
n R50   cln R50   d  z R50 
L
  R50 , z  
2
3

1  eln R50   f ln R50   g ln R50   h z R50 
  air
w
2
Where:
a = 1.0752
e = - 0.42806
b = - 0.50867
f = 0.064627
c = 0.088670
g = 0.003085
d = - 0.08402
h = - 0.12460
These coefficients give an rms
s deviation of 0.4% and a max.
de iation of 1
deviation
1.0%
0% for z/R
/R50 betw
between
een 0
0.02
02 and 1
1.1.
1 The max.
ma de
deviation
iation
increases to 1.7% if z/R50 value
es up to 1.2 are considered.
.
Step 4:
4: a) Com
mputation of %DD
b) Corre
ect for Pfl and Pwall
w
%ddw (d )  %idw (d ) 
L
  ( R50 , d )  Pfl ( Ed )  Pwall (d )
  air
a
w
L
  ( R50 , dmax )  Pfl ( Edmax )  Pwall (dmax)
  air
Errors associa
ated with SPRs
Fitted SPRs vs.
vs individually calculated
Error:
% of Max dose=
%error
%
off th
the fitted
fitt d
SPRs vs. individually
calculated values.
R
Rogers,
2004
Rogers
g
has shown that
the %DD can be
determined to within
1% of the dose at dmax.
(Rogers 2004)
Reminder:: For
Reminder
F electrons
Prepl  Pgr  Pfl
Pgr: addressed with chamber shift
f
Pfl: Use Table V in TG
T -25
TG-
Pfl from
m TGTG-25
Inner diameter (mm)
Ed
.
((MeV))
3
5
6
7
2
0.977
0.962
0.956
0.949
3
0.978
0.966
0.959
0.952
5
0 982
0.982
09 1
0.971
0 96
0.965
0 960
0.960
7
0.986
0.977
0.972
0.967
10
0.990
0.985
0.981
0.978
15
0.995
0.992
0.991
0.990
20
0.997
0.996
0.995
0.995
*where

Ed  E0 1
1 d / Rp

Pwall
w ll
Pwall=1:
1 for
f electron
l t
beams
b
and
d llowlow-Z thinthin
thi -walled
ll d
chambers (Johansson 1978)
Pwall: Recent data (Buckley&Roggers 2006) show change up
to 2
2.5%
5% at 0
0.5
5 cm
cm--R50 for 6 Me
eV.
eV
Recommendation: make
Recommendation:
e sure your chamber
introduces less than 2%
% error at depths by
comparing its response to published data.
p-p chamb
chambers
ers in water
Use of pp-p cham
mbers for %DD
• In general, front of window is point of measurement
Front window thickness ((1
(1--2mm)) should be taken into
account.
Pggr=0 for all p
p--p chamberrs.
Pfl=1 for most chambers except Markus and Capintec
PS--033 (use TG
PS
TG--39
9 for correction factors).
Pwall corrections are conssidered negligible in most
cases.
Pwall:
Recent data (Buckley&Rogers 2006)
2
have shown strong
depth dependence for NACP-02, Roos, Markus and
Capintec PSPS-033.
Buckley&Rogers
s: ps:
p-p chambers
Recommendation:
Recommendation:
C
Compare
your chamber’s
h b ’
response with data from
lit t
literature
tto be
b <2%
2%
Know your chamber!
diodes in water
Use of diod
des for %DD
Diodes specifically
p
y desig
g
gned for electron beams.
The effective p
point of me
easurement is the dye
y
(get specs from manufacturer).
Validate %DDs measure
ed with diodes against
curves measured with
w chambers (TG
(TG--25).
Have a QC program for diodes before taking large
amount of data.
Use of diod
des for %DD
1) Look at new TGTG-106 (Daas 2008)
2)) Look at this Summer Sc
chool’s chapter
p
QUES
STION
STION
1.Ridiculous
1
Ridiculous
2.Frightening
3.Horrific
4.Abominable
5.Hungry
ABOMINABLE (A
A-Bomb-In-A-Bull)
A-Bomb-In-A-Bull)
Courtesy of J. Gibbons, PhD
General Characte
eristics of Eeristics
E-%DDs
definitio
ons….
Practical Range
Therapeutic Range
ICRU 35 (1984)
As Energy increases
dose (at 0.5
5mm) ↑.
 dmax ↑ for lower Es and
a stops at
moderate/high Es.
 Beam penetration ((d90, d50 and Rp) ↑.
 Distance between d90-d10 ↑.
 x-ray contamination
n ↑.
 Surface
As beam energ
gy increases….
gy
Hogstrom (2003)
As field size
e decreases
dose (at 0.5m
mm) ↑.
 dmax and d90 ↓ (shift tto surface)
surface).
 Distance between d90
0-d10 ↑.
 Rp unchanged.
h
d
 Surface
As field size decreases….
d
9 MeV
Hogstrom (2003)
20 MeV
Outputt factors
Defin
nition
D d max ra , ra , SSD treat 
S e d max ra , ra , SSD  
D d max r0 , r0 , SSD nom 
ra: treatment field
f
att SSD
SS treat
r0: reference / cal fie
eld at SSDnom
Non--water phantoms
Non
Relative measurements
Use of non
non--wa
ater phantoms

Water substitutes shou
uld mimic water across
th whole
the
h l electron
l t
enerrgy range
– mainly in stopping an
nd scattering powers
thus,
h
both
b h the
h electro
l
on density
d
i and
d the
h effective
ff
i
atomic number should be matched to water
 in practice for some phantoms,
phantoms
p
this is difficult to
achieve (due to the ca
arbon in plastics)

– offoff-thethe-shelf material can have large
g
variations in density and
a
scattering power

Must be careful in using these materials
SO:
USE WATER
R WHENEVER
POSS
SIBLE!
Use of nonnon-wa
ater phantoms
di
discussion
i of corrections
i
 Depths
need to be sc
caled.
 Chamber
Ch
b readings
di
ne
eed
d to
t be
b multiplied
lti li d b
by
an appropriate fluenc
ce--ratio correction.
ce
 Stopping
Stopping--power ratio
os should be taken at
the scaled depth.
 Charge storage effec
cts should be kept in
mind using
gp
polystyre
y y ene and PMMA.
Correction
ns needed
Depth correction:
d w  d med  eff
w
 R50
 d med  med
R
 50




Corrected dose (Ding
(D
1997):
D w ( d wmax
)
maax
D med ( d med
d
Electron fluence correction factor
)L /  

w
coll med
w
 med
Depth correc
ction factors
Material
Mass densiityy
(g cm-33)
Recommended effective
density, eff
Water*
1.00
1.00
Clear polystyrene*
1.045
0.975
High-impact polystyrene
((white)*
)
1.055
0.99
Electron solid water*
1.04
1.00
Polymethylmethacrylate
(
(PMMA)*
)*
1.18
1.115
Epoxy resin water
substitute, photon
formulation**
1.02
0.98
*from TG-25, Table VIII, p. 84.
**From IPEM 2003, p.
p 2945.
Output in non
non--water
w
phantom
Se,w (d
w
 med
max
w
,r)
r)  Se,medd (d
max
m
md
med
,r)
r) 
 (d
 (d
max
med
max
med
w
,r) med
w
,r0 ) med
: Use data from Ding
D g 1997.
1997.
NOTE: Since corrections appe
ear as RATIOS might be
neglegible. CHECK it!
Measurements PDDs
iin
n nonnon-water Phantoms
Ph
 The
SSD and field size are not to be scaled.
 Chamber
must be positio
oned with its effective
point of measurement att the equivalent
equivalent--scaled
depth in the non
non--water phantom.
p
 Correct
w
for electron fluen
nce med
PDDs in nonnon-water
w
phantoms
• Cylindrical chambers
% dd w ( d w )  % di med ( d med ) 
L


L


w

w
 ( R 50
, d w )  P fl ( E d med )
 airr
w

w
w
 ( R 50
, d max
)  P fl ( E d med )
max
 air
• p-p chambers:
h b
Pfl=1
1
• Film XV
XV--2 ((TG(TG-25))
Large energy dependence

w
 med
( d med )
w
med
 med
( d max
)
Dosimetry
y of small
an
nd
irregula
ar fields
Dose determination in
n small/irregular fields
 Inherent
problems in
n dosimetry of small
electron fields
– depth of dmax become
es shallower.
shallower
– the output factor may
y be significantly
different than the con
ne factor if the field size
is small enough for la
ateral scatter equilibrium
((LSE).
)
– isodose coverage is re
educed in all directions
as the field shrinks.
Square--Ro
Square
oot Method

For rectangular fields off X & Y dimensions:
%dd (d , rX ,Y )  [%dd (d , rX , X )  %dd (d , rY ,Y )]1/ 2
S e (d m , rX ,Y )  [ S e (d m , rX , X )  S e (d m , rY ,Y )]1 / 2
(Hogstrom (1981) & Mills (1982))
Approximate irregular shapes
shapes with rectangles
Hogstrom (2000)
Comparison of the percent differrence between measured and
calculated output factors for com
mmon methods of calculation
in the literature.
literature The measured and
a
calculated output factors
were for cutout shields that were
e shaped as squares,
rectangles, circles, ellipses, and arbitrary shapes used in the
clinic
Calculation method
Equivalent square
Square root
One-dimensional
Pencil beam
Sector integration
(Jursinic 1997)
Percent difference
(%)
2.7*, 5.9*, 3.0*
3 0*, 2.3
3.0
2 3*, 4.6
4 6*, 3.0
3 0*
3.0*, 2.0*, 2.1*
2.7*,, 2.0*
3.0, 1.5, 1.0*
Lateral Buildup Ra
atios (LBRs) Model
(Khan ett al. 1999)
THE WRATH
H OF KHAN
When is special do
osimetry required?

When the minimum field dimension is less than
the
th minimum
i i
radius
di
off a circular
i
l fi
field
ld that
th t
produces lateral scatte
er equilibrium (LSE)
R eq  0 . 88 E p , 0
or
a  1 . 58 E p , 0
(Khan&Higgins 1999)
where
E p , 0  0 . 22  1 . 98
9 R p  0 . 0025
2
Rp
Do you remember what it has been?

From Lax&Brahme (198
80) we have been using
th criterion
the
it i
ffor LSE:
LSE
E0
R eq 
2 .5
wherre
E0  E0
or
E 0  E p ,0
(also in Khan’s book)
Pencil beam…
b
Broad field
field--same surface fluence as pen
ncil
beam
 r2


exp 
2
 r ( z) 

d p ( r , z )  D (0, z ) 
 r 2 ( z )
Due to multiple
p
Coulomb Scattering
Khan et al. (1998)
LBR de
efinition

LBRs are related to  r2 d  mean square radial
spread
d off pencil
il beam
b
(independent
(i
( d
d t off fi
field
ld
size).
DEFINITIION of LBR:
D(d , rx )  i (r , E )

LBR (d , r ) 
D(d , r )  i (r , E )
rx: s
small
a field
d (e.g.,
( g , 2 cm
c radius)
ad us)
r∞: broad field (e.g., 20
0x20 cm2)
(…): incident fluence (normalize at 0.5 mm
depth)
(Khan et al. 1998)
Measured PDDs norma
alized at 0.5 mm depth
(surfface)
Diode data
1:
2:
3:
4:
2 cm
2.8 cm
3.7
3 7 cm
5.8 cm
Khan et al. (1998)
Deterrmine 

2
r
d 
Take a small circular field rx=2 cm diam. to
measure PDDs and ratio tho
ose to PDDs from a
20x20 cm2 ,normalized both
h at 0.5 mm, to
determine LBRs.
LBRs
 r2 (d )

rx2


1
ln 

LBR
r
d
1

(
,
)
x


w
with
LBR<1
(Khan et al. 1998)
Sigma vs. field size
size and energy
LBRmin
Khan et al. (1998)
In which applicator to use the rx field?
6 MeV
12 MeV
It does NOT
matter!
20 MeV
2 cm diam. insert in
10x6, 10x10 and
20x20 cm2
applicators
For irregularly shaped
s
fields…

Sector integration summing
g the pencil beam
contributions.
LBR eff
 ri 2
 n
(d , r )  1  (
)  exp[ 2
]
2 i  1
 r (d )
ri
x
θ
O
y
(Khan&Higgins 1999
1999)
Can we predict dmax
m for small fields?

Circular fields as example, within
w
±0.5% of
maximum dose.
dose
rx
d max

 0.174rx
0.67
E 1p.,67
0

2 0.33
 0.06225E p ,0
rx ≤ Req
For broad field
Req
d max

0.67
0.46 E p ,0
rx ≥ Req
e
Compute output factor:
Relative applicator
Output Factor
S e ( d , r j )  S e  LBR eff ( d , r j )  % dd  ( d , r )
(Khan&Higgins 1999
1999)
Data vs. predictions for
f small circular fields
dmax
(Khan&Higgins 1999)
Data vs. predictions for
for small circular fields
Dose
MU
(Khan et al. 1998)
Treatm
ments
a
at
extended distances
Electron beam featurres at extended SSDs


Differences in PDDs resulting from InvSq. effect are small
because electrons do not pene
etrate that deep and because
the significant increase of pen
numbra width with SSD restricts
the SSD to 115 cm or less in clinical
c
practice (Hogstrom 2003).
The depth of dmax at extended
d SSD is a complex function of
field size and beam energy.
energy For
F small field sizes
sizes, the dmax
depth changes little. As the fiield size increases (>10x10
cm2), the dmax depth increases
s very slowly for electron
energies below 12 MeV.
MeV For higher
h
energies the dmax depth
energies,
increases with increasing SSD
D (Das et al. 1995, Cygler et al.
1997).

However, this effect is clinically insignificant since higher
electron energies have broad dmax ranges. In addition, beam
flatness decreases and penum
p
mbra width increases at
extended SSD, an effect more
e pronounced at smaller field
sizes and lower electron energ
gies.
(also in Khan’s book)
Treatment planning sysstems (without Monte
Carlo) differ in their ability to accurately depict
the effects of extended SSD and individual
institutions should invesstigate the limitations
of their planning system
ms before use on
patients.
Use TGTG-25 reccommendations.
Beam o
output
a
at
extended distances
The Effective
Effective--SS
SD (SSDeff) method
How to determine SSDeff

For every applicator and
d energy plot:
Gap:
g  SSDext  SSDnom
Io: Rdg
d at SSD
SS nom
I: Rdg at SSDextt
SSDeff
SSDext: extended SSD
((typically
yp
y <115 cm))
SSDnom: 100 cm
Typically dmax
(TG--25 & in Khan’s book)
(TG
Data from Sie
emens Primus
7 MeV
CONE
INSERT
dmax
SIZE
SIZE
[cm]
10x10
2x2
0.9
"
3x3
"
14 MeV
S
SSDeff
CONE
INSERT
dmax
S
SSDeff
[cm]
SIZE
SIZE
[cm]
0.791
33.1
10x10
2x2
1.2
0.893
58.7
1.2
0.870
46.6
"
3x3
1.6
0.918
66.4
4x4
1.4
0.957
55.4
"
4x4
2.0
0.944
62.5
"
6x6
1.4
1.001
61.5
"
6x6
2.6
0.987
71.5
"
8x8
1.6
0.998
73.6
"
8x8
2.9
0.995
79.1
5cm
5cm
1.5
0.817
57.3
5cm
5cm
2.7
0.922
82.0
10x10
10x10
16
1.6
1 000
1.000
83 9
83.9
10x10
10x10
29
2.9
1 000
1.000
94 2
94.2
15x15
15x15
1.6
0.998
99.4
15x15
15x15
2.9
0.984
103.5
20x20
20x20
1.6
1.001
100.4
20x20
20x20
2.9
0.957
104.6
25x25
25x25
1.6
0.992
106.0
25x25
25x25
2.9
0.957
106.9
[cm]
(Mihailidis, et al.)
Weak dependence of SSD
S eff on applicator size
(Varian 2100 Data)
Insert
size
i
(cm2)
Avera
age
Beam Energy
gy (MeV)
(
)
6
9
12
16
20
4
46.2
61.1
72.5
76.4
76.6
6
62.2
74.7
80.2
81.8
80.6
8
77.6
83.8
83.2
83.6
82.7
10
82.9
85.9
86.8
85.5
83.8
15
90.7
90.9
90.1
90.0
89.7
20
90.0
91.8
91.4
90.9
92.0
25
90.7
91.9
91.0
92.5
93.3
(Roback et al. 1995)
Output at exten
nded distances
S e ( d m , SSDext )  S e ( d m, SS
SD nom )  GF
with
 SSSDeff  d max 

GF  
 SSD

 Deff  d max  g 
Monitor Units: MU 
2
D prescription
Se (d m , SSDext )
(Khan et al. 1998)
QUES
STION
STION
1. Sad
2. Disgusting
3. Noble
4. Horrific
5. Silly
NOBLE (No-Bu
ull)
ull)
Courtesy of J. Gibbons, PhD
Electron Beam
m Algorithms
g
(Simple discussion)
General C
Comments

What should be done to
o commission these
algorithms (being consistent with TG53)
– know the pitfalls and limiitations of electron
algorithms
– careful with normalization of dose distributions for
electron
l t
algorithms
l
ith
Restricted field
 Extended treatment distance
 Plans involving inhomogeneities


Attention to how data should be entered into
the program.
program
Some clinicallyy relevant tests

Inhomogeneities
o oge e t es in elec
e ec
ctron
ct o treatments
t eat e ts
– The effects of inhomogen
neities on dose distributions
– Computer representation
n of the effects of dose
inhomogeneities
Use of bolus
 Field abutment

– ElectronElectron-electron, with sa
ame or different energies
– ElectronElectron-p
photon,, with sta
andard or extended distances
– Tertiary shielding for field
d abutment

Library of clinical treatm
ment examples
Acknowle
edgment





Members of TGTG-70 group.
g
Faiz Khan.
Dave Rogers.
Ken Hogstrom.
Hogstrom
Bruce Gerbi (Chair of
o TG
TG--70).
Not do
one yet
Stay where
e you are!
The Duke
Prob
blem

A small area will be
e treated with 16 MeV
electrons and a cus
stom circular field of
5 5 cm diameter (average),
5.5
verage) at 100 SSD
SSD.
What are the neces
ssary dosimetric
parameters to be measured
m
in order to
determine the output of such field?
2.25 cm radius
Solu
ution




Check
Ch k for
f LSE first.
fi t In
I pra
actice
ti
cm and
d Ep,0=16.2
16 2 MeV
M V
(from (12.19)). Then, Req=3.5 cm (eq. (12.18)), thus
no LSE since a diam=7 cm
m will be required.
Need
N d to
t find
fi d new dmax tha
th t is
i shifted
hift d towards
t
d the
th
surface. Use either film dosimetry
d
with film placed
parallel to electron beam in solid water or planeplaneparallel chamber in solid water
w
with thin sheets of 1
mm to measure the dmax. An estimate of expected
dmax for the 5.5 cm diam. field is: 2.8 cm (from
((12.24)).
))
Measure the output factorr of 5.5 cm field at 2.8 cm
(new dmax) relative to the reference 10x10 at 3.4 cm,
both at 100 SSD.
From film or planeplane-paralle
el chamber dosimetry find
dmax, d90, d80 and at those
e depths measure isodose
lines with film perpendicular to beam in solid water.
S
Scan
films
fil
in
ini -plane
l
and
d crross
ross--plane
l
to get the
h widths
id h
of 95%, 90%, 80% and 50% isodose lines at the
above depths.
THANK
K YOU!
Now, let’s
let s go forr a brew or two!