fr
1SSI :1977-025
COMPARISON OF PHOTON BEAM QUALITIES FORTREATMENT OF DEEP SEATED TUMOURS
JAMES SSENGABI
NATIONAL INSTITUTE OP RADIATION PROTECTION
Fack, S-1O4 01 STOCKHOLM SWEDEN
INSTITUTE OP RADIATIONPHYSICS
Pack, S-104 01 STOCKHOLMSWEDEN
June 1977
rT API i: OF CONTENTS
Item
INTRODUCTION
1 COMPARISON OF BASIC MACHINE PARAMETERS
1.1 Collimator system
1.2 Radiation sources
1.3 Photon spectra
2 COMPARISON OF IRRADIATION BEAM PARAMETERS
2.1 Isodose curves
2.1.1. The central axis depth dose data
2.2 Beam flatness
2.3 Penumbra and dose gradient
3 COMPARISON OF "BEAM-PATIENT" INTERACTION PARAMETERS
,01
02
02
02
07
08
08
09
10
iY>
if,
3.1 "Patient-tumour"model
3-2 Dose planning
3.3 The integral dose
3.3.1 "Area" integral dose
3.3.2 Volume integral dose
k BEAM GEOMETRIES
^.1 Simulator technique
16
20
35
36
38
40
5 DISCUSSION
6 CONCLUSION
7 SUMMARY
8 ACKNOWLEDGEMENT
9 LIST OF REFERENCES
rLT^T i F IAM.F.S
i : i h i C N l i n i l » » ! '
I CPMPAF?!.-.. \ OF "ASIC MACHINE PARAMETERS
:i CFNTR.-U AXIS DEPTH DOSE DATA
•j 1 ' A l l F M - "TAI'GET AREA" DATA (TARGET VOLUME)
-'» CCMPAiM.SON ME MACHINE IRRADIATION PARAMETERS
3 COMPARISON r,F INTEGRAL DOSES (TWO BEAMS)
h COMf-'AfCMA' OF TNTFGRA1 DOSES (FOUR BEAMS)
03
1"-
18
I.I SI OF FTGt/KES
nu.nn >r
I Th° c o l 15 mat trig i-ysi'otn of t h v r a p y m a c h i n e s l a )
'1'h c c o l l i m a ' in^r s y s ! rni " lf " (b'l
<? [.sodor»o c u r v e s f u r 10 y 10 em > "am s i z r ( a )
" " " " " ( b )
'I The c e n t r a l a> i ^ d e p t h d o s e d a t a
h Thr bf.'an. f l a t n e s s a t 10 cm d e p t h ( 1 0 x 1 0 cm)
5 P a t i « n t and t a r g e t volurans ( d i f f o r e n t m o d e l s )
(• T »v p a r a l l e l o p p o s e d beam Lechr i ique Co:
•; " " " " " 6 MV 1 i n . ar.-o.
8 " " " " " 8 MV Lin . a c e .
9 *' " " " " 10 M\' M i r r o t r o n
JO " " " " '42 MV b e t a 11 :,r.
IL A & j mpl L t; i <>d ^l-br-am t c c h n i q u t 1 . ' >',<> f'lamma* ron ' j
12 " " •' " t, MV l i t i . a r c .
I'J " " " " " 8 MV I i n . zee .
O ~}
O r .
1 i
Ik
r ~i;.I.-T •'!•" i : . ! • : !> ( o •-»•it imiod)
': i ;ni t f 'i;::nt • • r :
1 * A .-• • mp i i i i cd 4-beam ti chiiicju»^. 10 >f\" M i c i o i r o n
1", ' f " "' ;4 2 MV b e t a t r o n
l'< '\5iiij^ri son of normalized a r e a s , "a rna ' 1
i tit.i>/^ra 1 rlo?es of Tivo mach ines
IT ( OTi-pari ~o'i o f hoam ^eonw-t .r ios f o r t tie f.l v*»
f i i c i a p \ ma nh i n o s
30
37
rr: )MPAr<T • >\ Oh PHOTON" I ' T A M
•<\ DVY.P - : - . A ! H " ' Ti.">::i'.*T-'S
- FOR TRFATMFN!
\XKS ssrv.-U-T
T VT !<'.>[>' _rjON
I t iv commoT! p r a c t i c e i n r a i n . o t h p r a p y t o c o m p a r e beam
i | u a l i M « - 5 f rnm hi^'li i-ncrfry m a c h i n e s by u s i n g t l i e c f i i t r a l
ax i i- d e p t h Jo<< r i a f a .
I h i s c o m p a r i s o n £-ive« l i m i * f d i n f o r m a t i o n r p R a r r i i n p
t h e M i i r a J . i l i t y oi ' a m-'.cliirio t o t r o a t f o r e x a m p l e ,
dt'ip jwat^cj t u i i i t u ! ' ? . "!"h*" njftno.i f r i l l s t o p o i n t o u t thf>
ci'icct of oi'iiT important dosimotric aspects of photon
beaaif such a? gftosii* t T ica 1 and physicaL pMiumttra
bf»am uniformity and "Lu! ia-'iowi>" i-onr? it i or.s at ihe
• xi t .
Th«* purpose of til l? paper i s to present the r e s u l t s
of n rr,orr> elaborated comparative study of photon bc
from n Coba 1. }-'•("» u::ir , 6 MV anJ 8 MY l inear acce l e r a to r s ,
a 10 MV* micfofrcn and a ^2 MV beta t ron used in rad io-
therapy. Manual done planning has be*;ti employed in these
inves t iga t ions a-irl romnuter ca l cu la t ions -J re unrlerwav
to study more coniplicated beam configurat ions in
r*diotherapy p r a c t i c e .
James -vsenpabi. H.^c . , M.Sc.
I'ad i of ysiska "Ins f i tut i onen , S.f".
r1 COMPAHISON OF BA?IC MACHINE PARAMETERS
Table 1 is a summary of the basic physical parameters
of the radiotherapy machines used in this study.
1 .1 Col i ;Lms.+ irift systems
Fig. 1 illustrates the col limat ing systems Tor the units
under study. With the exception of the microtron, all
machines are used at the Radiumhemmet. Stockholm. A
source-to-axis, usually equal to source-to-skin (SSD)
distance of 1O00 mm is standard for the accelerators and
it is 750 mm at t tie Cobalt-unit . The h2 MV botatron normally
operates at 1200 mm SSF) in order to obtain lar^pr beam sizes
and to reduce contamination of the photon beam with
electrons from thn block diaphragm.
1 , 2 Rad i at i on source.1
Data concerning phot.on sources a;id radiation output
are summarised in Table 1. Radiation output, for the
Cobalt-^>0 unit is quoted for a source with the activity
t U2'2'fO Fq (about 6000 Ci). Gold is predominantly used
as target material for bremsstrahlun^ production while
platinum is used by Siemens in their older (^<W>tt) betatron.
The effect h e source size (^^mm) cited in Table 1 for
the Siemens betatron has not, been measured but only
estimated. Data on effective source sizes for other
machines have been obtained by direct measurements at
Radiumhemmet or supplied by the machine manufactures,
Diameters for beam f Latieninf? filters refer to base
of filter.
r
TABLE 1 COMPARISON OF BASIC MACHINE PARAMETERS
Machine type
6 0 .Co
Gamma tron 3(Siemens)
6 MV Lin .ace.(Varian)
8 MV Lin.ace.(M.E.L/SL75-10)
10 MV Mi c rot ron(ScanditronixMM 10)
42 MV Betatron(Siemens)
Target or source dota
Mate-rial
60-Co-
pelletsinstainl.steel
Goldf=19.32
Gold
GoldandTungst.
Platin»andTitan.
Thick-nessmm
~10
2.93
3
1.2
3
0.02
Dia-metermm
20
^ 3
6
10
7
Effect,sourcesize mm
20
4.0
2
Beam flatten,filterMate-rial
-
Stainl.steel€=7.75
Tungst,.alloye=i7.iStainl.steel£=7.75
Leade=n.35
Thickn.at centremm
-
14.2
10.6
30
27
Beamflat,filter% maxmm
-
40
43.7
45
97
SSDmmnormal
750
1000
1000
1000
1000
1200
GeometricalpenumbrammAtsurf.
x)
26.4
2.2
4.1
2.2
2.6
4.3
At 10cm dpt
x)
34.4
2.7
5.2
2.6
3.4
5.1
Physicalpenumb.mm8O%-20%level at10 cm dpt
xx|
15.5
5
9.5
4
11
11
xxx)Dose ratemax(per minute)Gray
1.75
4.50
10
4
1
0.7
^Without penumbra trimmers.xx)
Small due to penumbra trimmers.xxx) At a depth of moximum dose.
CORRECTED VERSION, CORRECTING COLUMNS 6 AND 8O
r
A comparisop. of the " oo '. 1. i rn*i t. i U R system" lor the
Oobalt-^O Gamma f ron ?• ( ?ionn.-np ) , f' M:> Hii^ar
accpltrator (Variati), .Q. MA" linear accele ru t: or
(>!.E.L./.^L75-1O) , 70 MY Miorotron I :-ca:id i t i-onix-MM10)
and h2 MV Betatron (::;ipi!inns~modf 1 IQf'.ft). All
dimensions are in mi 1 lim«;1 >n••.
4.--
60Co Gammatron 3
C Siemens >
440
6 MV Lin. ace
(Variant
42MV Betatron
J
10 MV Microtron(Scanditronix MM 10)
FIG.l(b)
8MV Lin. ace.(M.E.L./SL 75-10)
ON
/,• /7. s s :r/
r1 . 3 Photon spectra
Experimental data pertaining to bremsstrahlung spectra
on medical electron accelerators above 3 MeV are
limited to a few reports (Ward and Dolphin 1958,
Bentley et al. 196? ,Jessen 1973, Levy 197k).
Theoretical bremsstrahlung spectra can be calculated
using formulas developed by Kramers (1923) Schiff (I951)
and Koch and Motz (1959). However the studies of
Levy (197^) have shown that measumed bremsstrahlung
spectra of medical electron accelerators differ
markedly from calculated spectra especially for
photon energies above 5 MeV. This finding confirmed
previous results reported by .lessen (1973) on a
6 MV Varian linear accelerator of the same make as
ours, .lessen noted that for photon energies below about
2 MeV the measured spectrum was influenced by thick-
target effects. For photon energies above 2 MoV, his
measured spectrum agreed better with the calculated
thin-target spectrum.
Knowledge of the photon spectra is necessary in the
interpretation of various dosimetric measurements.
Levy (1974), Levy et al ( 19? '-* , 1975) reported that
calculations of the weighted C. values (the absorbed
dose conversion factor lor photons) from measured
spectral distributions of energy fluenco of five
different machines, were slightly but consistently
higher than the recommended values. Tn studies on doso
r 8
build-up in phantoms Manson et al (*975J noted
differences between a 35 MY clinac operati-ig at
25 MV and betatrons operating at the same nominal
energy. These findings were explained by a statement
made earlier by Rawlinson and John» ( 1973) that »he
output bremsstrahlung spectrum of the linear
acceleravor has a lower average energy than that
of the betatron.
Photon spectra of Cobalt-60 machines have been
reported in the literature (lCRU 19?0, Löfroth et a I
1973). In the studies by Löfroth et ai ( 1973) a
Cobalt-60 Gammatron 3 (Siemens) similar to
ours, was used. Exposure calculations of the photon
spectra revealed that, exposure from scattered
photons with energies below 1.0 MeV amounted to
between 10 to 17 per cent of the total exposure.
Mor<9 and better spectral calculations and measure-
ments for deep therapy beam qualities are still
reeded.
2
2.1
COMPARISON OF 1KHADTATTON UK AM 1'AfMMKTKRci
1sodose curves
Fig 2 is a comparison of isodose curves for thr five
machines studied for a nominal 10x10 cm beam size
which is defined to be t.hc area at the surface of a
plain phantom between thr; $()'% i sodose curves
extrapolated to the surface. The shape of the isodose
curves (isodose surfaces) between the 2O'> and zero
r for che betat r-.>-.\ i* soon from Viff. 2, to bo influenced
by contamination of the photon beam by electrons from
the air and the block diaphragm. The effect is still
noticeable even at 12O0 mm SST).
2.1.1 The central axis depth dose data
Fig. 3 compares the central axis depth dose data for
the five machines studied and for a beam size, 20 x 20 cm.
The choice of 2O x 20 cm beam size has been made to
illustrate better, the conditions of dose decrease at
the exit of the beam, for a water or water-equivalent
mix-D phantom, 20 cm thick. For a standard beam size
10 x 10 cm at the surface, the central axis depth doses
are compared in Table- 2 column 3, at a depth of 1O cm.
They are obtained from the isndos<- curves of Fig. 2.
In Table 2 the term "build-up" ratio refers to dose at depth
of maximum dose divided by the dose at the surface. The
inverse of this ratio (flWf or %BUR~1, ir.pp .-I al. 1<>"H)
characterizes the fractional decrease in surface absorbed
dose from its value if electronic equilibrium existed.
For a correspond.! ri£ situation at the beam exit , the terms
"build-down" (]3DH~ or %hDR~ Koskineii and Spring 1073)
and "build-down" ratios have boon introduced to characterize
the fractional decrease in (exit) surface dose from its
value if full backseat tor conditions existed. "P.ui Id-down"
appears to be well understood in the dosimetric context, but
it remains questionable as to its dramatical correct ne.ss.
Details of dose "build-up" and "build-down" studies in
phantoms will be further discussed in a paper to bo
published (Ssengabi 1977). In the same paper we will
examine also discrepancies in measured relative surface
doses which have boen reported in the 1 i * e ra l.u r" by
vorkers us i up ionizfil ion <.-lint-iber s ! 'nckson 1<r/1. Cray 1973,
r 10 ~iVelkley et al 1075. Månson et al 1975, Gannon et al 1975)
on one hand and by those using thermolurainescencn
dosemeters (Jackson 1971, Rao et al 1974) on the other.
From Fig. 3 and from Table 2 (column 3) it is seen, as
expected, that the k2 MV betatron shows the largest
value of central axis depth dose while the cobalt-fiO
unit shows the smallest value at equal depths beyond k cm.
2.2 Beam flatness
Fig. h is a comparison of beam flatness in a plane
perpendicular to the bean axis at a depth of 10 cm for
a 10 x 10 cm beam size at the surface of the phantom.
Normalized data are obtained from the isodoso curves,
Fig. 2. The lateral distance from 'ho central axis that
corresponds to the 90efc level of the relative dose
(normalized at a depth of 10 cm) is denoted by Xq and
is shown in Table 2, column 6. This distance is seen to
be lcrgest for the miciotron, indicating the best beam
flatness of all the machines at a depth of 10 cm, followed
by the (> MV linear accelerator (Varian) the 8 MV linear
accelerator (M. K.L./SL7t>-10) the »̂2 MV betat.roti (Siemens)
and the Cobalt-60 Gammat^-on 3 (Siemens). However,
beam flattening filters are designed to give the best
beam flatness at a predetermined depth and for a particular
beam size. This differs from machine to machine.
H r a h m e a n d S v e n s s o n ( I f>~'>) h n v e dn.scfilicrl a m e i d o d o f
d e s i g n i n g i r i h o m o g P i i e o u M b e a m f I ;\ I ' >TL j n g f i l t e r ; t,o cler-rrnsp
the; m e a n o n o r g y o f t h e p h o t o n s in t h e I'orvHnl d i r e c t i o n find
i n c r e a s e it in M I R p e r i p h e r y t h e r e b y p r m i u c i n g <lnsr ri j .; |. r - i -
b u t i O I L S o i' ,";M(KI u n i f o r m i t y al a l l i it • |. t h.» in t b e plwi n : <>in ,
60U Ä
Co Gammatron 3CSiemens )SSD:75 cm
42 MV BetatronCSiemens^jSSDtOOcm
6 MV Lin.ace.CVarian)SSD:100cm
40
30
20
FTt;.i?(a) !"'onipiri son of isodosecurves for 10 x 10 cm beam-i/p at the surface. Note wellth" '.'O- to 10 isodosc curves fortii»' '•" "!\ !>etatroii botweeu rhu'•inTari1 and 10 cm depth.
5cm
J
r 8MV Lin. ace. 10 MV Micrctron
(M.E.L./SL75-10) (Scanditronix MM 10)SSD:100cm
12 ~1
30
• 5 cm
•30
FIG.2 (b)
Central Axis
42 MV Betatron (Siemens)
10 MV Microtron (Scanditronix MM 10)
8MV Lin. acc. (M.E.L./SL 75-10)
80
60
40
20
Depth Doses 6 MV Lin. acc. (Varian)
. Co Gammat ron 3 (Siemens)
Beam size: 20x20 cm \
20 cm"
6 8 10 12 14 16
cm Depth5 4 3 2 1mm Depth
l i t . , ) C o m p a r i s o n o f t h e c o n t r a t a x i s d e p t h d o s e d a t a , i n c l u d i n g f h r " b u i l d - u p " a n d " b u i 1 d - d o w i i ' 1
I ' R g i i m s , f o r a 2 0 x 2 0 c m b e a m s i z e a t t h r - s u r f a c e 111 i n T i l ) a n d d i o d e d o s n r n r t I T S u s e d i nmoasuremont of the "bui'd-up" and ''build-down" regions.
r% Relative Dose
100
90
80
70
60
50
40
30
20
10
Beam Flatness
and Gradients
lOMVMicrotron(ScanditronixMMIO)
6MV Lin.acc.(Varian)
8MVLiaacc.(M.E.L./SL 75-10
Beam size
i42MV Betatron (Siemens) ii \ . \
60Co Gammatron 3 (Siemens) A \ \ \
ize: 10x10 cm V \ S>
1 6 7 8 cmLateral Distance
FIG.k Comparison of beam flatness in a plane perpendicular tothe central axis at a depth of 10 cm. Comparison ofphysical penumbra widths is made in the same figure betweenthe 80^ and 20^ levels. The beam size refers at surface ofthe phantom. Lateral distance is measured from central axis.
A slight adjustment of original data was done to make the
lines coincide at 50?' level. The actual differences do notexceed 2mm,
TABLE 2 CENTRAL AXIS DEPTH DOSE DATA
Machine type
Co Gamma*ron 3(Siemens)
6 MV Lin .ace ,(Varian)
8 MV Lin ,acc .(M.E.IVSL75-10)
10 MV microtron(Scanditronix-MM 10)
42 MV Betatron(Siemens)
% BUR"1
10x10cm
36
28
25
25
20
°'o Dosedepth10 cm
55
67
71
73
84
% BUR"1
20x20 cm
52
32
35
32
33
% BDR"1
20x20 cm
80
84
86
86
90
XOQ (cm)10 cmdepth
4.25
4.75
4.oO
5.0
4.45
BoMd-updepthmm
3 - 5
U - 15
18-20
20 - 22
40'-45
Build-downdepthmm
2 - 3
- 2
1 - 2
1 - 2
1 - 2
°o dose is t!ie percentage depth dose at 10 cm (in a 10x10 cm beam size «f the surface)
% BUR is the ratio of surface dose to dose at maximum depth
% 3DR is the ratio of measured exit dose fo dose at central axis with full backscatter
X90is the lateral distance (cm) from central axis (100 %) to 90 % level (10x10 cm beam size at the surface)
_l
r If'l
2. "j Penumbra and dose gradient
Comparison of geometrical penumbra widths at the surface
and at a depth of 10 em has been made in section 1. It is
seen from Table 1 (columns 10 and 11) that without penumbra
trimmers, the CobaLt-oO Gammatron 3 would have geometrical
penumbra widths of !»£.•'» mm and 3•'».'» mm at the surface and
at a depth of 10 cm respectively. With the first pair of
penumbra trimmers placed at K» cm while the other pair is
placed 13 cm from the collimator (Fig.l)fthe geometrical
penumbra is reduced appreciably. The t> MV linear accelerator
(Varian) and the 10 MV Microtron (scanditronix MM-LO) have
the lowest value of geometrical penumbra width at the
surface, 2.2 mm under conditions given in Fig. I. Fig. '»
compares the physical penumbra widths and the dose gradients
As there is so far no agreed definition of physical penumbra
width, we have chosen the distance (mm) between the HfV-i
and 20% levels to define this parameter. The results arc
shown in Tanl.e 1, column 12. It is seen from these results
that the physical penumbra is not always directly related
to the geometrical penumbra; althougn a rough correspondence
can be noticed as in the case of the 6 MV linear accelerator
(Varian) or the 1O MV Microtron (Scanditronix MM-lo).
Compare columns 11 and 1? of Table I.
3 COMPARISON OF "RF.AM-PATTKNT" INTERACTION' PARAMETERS
3•1 "Patient-target-volume" model
In order to compare high energy photon beams for their
suitability to treat deep seated tumours, a "pationt-
target-volume" model was sought.
r 17
From the registr;, of Radiumhemmet, data of patients
previously treated for the tumours of the urinary
bladder were uerd to set-up such a model.
The mean values of twenty patients and target-volumes
are shown in Table 3 under Radiumhemmet. Table 3 compares
also our "paticnt-target-volump" data with that of
Almond et al. (l97^)» for patients previously treated
for thp carcinoma of the cervix at. M.T). Anderson Hospital
Texas, U.S.A.
,5 shows cross-sections of "patient-target-volume"
models basec* on data in Table 3- Our choice of an
oval or '*quasi-ellipse" figure is in agreement with
the conclusions reached by other institutions that
the average axial dimensions of a patient are
37 x 22 cm, 32 x 20 cm, 19 x 15 cm and 12 cm diameter
for the four representative parts of the body,
namely, the abdomen, thorax, head and neck respec-
tively (Cohen and Martin 19»>6). The compromise axial
dimensions adopted by Cohen and Martin (19'>M in their
atlas, for small abdomen and thorax (30 x 20 cm) arc
in fairly f;or>d agreement with ours (33*2 x 21).7 cm).
rTABLE 3 PATIENT - "TARGET-AREA" DATA
x)
18
Pa tie nt- "Targe b-Areo "data in cm or cm
1 PA
TIEN
T1
TARG
ET A
REA
DATA
Dx-Sm
Ventral-Dorsal
Area (cm )
Dx-Sin
Ventral-Dorsal
Area (cm )
Ven tro I-Tumour
Ventra l-T-centre
Dorsal-T-centre
Dorsal-Tumour
Area (cm )
U-blodder tumourRadiumhemmetStockholm
35.2
20.7
572
11.6
10.1
92
3.8
8.85
11.85
6.8
92
Cervix tumourM . D . Anderson HospitalTexas
33.1
21.9
633.1
13.8
8.8
121.4
6.75
10.95
10.95
6.35
121.4
*)„Torget-Area" refers to the oreo of the cross-section in the transverse piane, as the
some plane intersects the "Target-Volume". The word "tumour" in the table implies
the entire "Target-Volume".
rA.
VENT
B.
DXi:VENT
SIN
DOR
FIG. 5 Two different ways of presenting the patient and
target volume data. The "quasi-ellipse" or oval shape
is recommended by many institutions (Cohen and
Martin 1966), and has been used in this study.
Fig.5 (b) is by Almond et al. (197&).
The cross-sections are in the transverse plane
as shown in Table 3.
r 20 " I3.2 Dose planning
In order to compare the photon beams under identical
conditions of patient treatment, manual dose planning
was done using the following criteria:
1. A "plateau" of high and uniform dose in the "target
volume" was aimed at and its average value designated
the "target dost" and normalized to 100?é.
2. The maximum occurring dose in the "target volume"
was not to exceed the- "target dose" by more than
5% i.e. 1O59fc max dose.
3» The isodose surface forming the boundary of the
"target volumo" was not to be less than 95?é of
the "target dose".
The results obtained for a two-dimensional planning in
a si tion through the centre of the target volume are
shown in Figs 6-10 for two parallel-opposed beams and
in Figs H-15 for a simplified four be»m ("box") technique,
The required beam sizes thus obtained together with
the reference dose per beam are indicated in the
figures. Table U (column 3 and 6) shows results of the
parallel-opposed beams while columns 8 and 11 shows
the results of a simplified four-beam technique.
r Beam size: 13.6 x 13.6 cm2 1
Co Gammatron 3
~i
FIG.6 Two parallel-opposed Anterior-Posterior (A-P) beams,
Bladder carcinoma treatment plan. Target volume is
shown shaded.
r ANT.
Beam size: 12 x 12 cm
~i
• MV Lin. ace(Varian)
SSD: 100 cm
Ref. dose: 78
Beam size: 12x12
POST.
ITf r .7 Two p a r a l L«.-1 - op
t. r r-s tnrrr i p "ir
(A-P) beams. Rtadd*?r carcinoma
r ANT.Beam size: 12.3x12.3 cm
1
8MV Li a ace.
(M.E.L./SL 75-10)
SSD: 100 cm
Ref.dose: 71%
t Beam size: 12.3 x 12.3 cm
POST.FIG.8 Two parallel-opposed beams (A-P). Bladder carcinoma treatment
plan.
r ANT.
1 Beam size: 11.8 xii.8 cm
10MV Microtron
CScanditronixMM 10)
SSD: 100 cm
Ref dose: 71
I Beam size: 11.8x11.8 cm
POST.
FIG.9 Two parallel-opposed beams,
r ANT.Beam size: 12.6 x 12.6 cm ;
~1
42 MV BetatronI Siemens)SSD:120 cm
Ref. dose: 60 %
FIG.10 Tvo parallel-opposed (A-P) beams from a 42 MV betatron(Siemens), for SSD 120 cm. Bladder carcinoma treatmentplan.
L. J
r ANT.
Beam size:114 x 12.6 cm
~l
Co Gammatron 3(Siemens )SSD 75cm
Ref. dose: 59 %
i \ Beam size: 11.4 x12.6 cm
POST.FIG. 11 A simplo type of '•-beam, "box" technique for the
carcinoma of the bladder. Target volume shown shaded.
ANT.Beam size: 11.5 x 11.9 cm
6 MV L i n . a c c / ^ N ^( Var ian ) '
AWWWW
S S D : 100cm
Ref. dose: 45%
| Beam size: 11.5 x11.9 cmPOST.
FIG. 12. Simple type of ^-beam technique. Bladder carcinoma treatment plan.
L .
r ANT.Beam size: 11.6 x 12.1 cm
8 MV Lin. ace.( M.E.L./SL75-10)
SSD: 100 cmRef. dose: 42 %
Beam size: 11.6 x 12.1 cm
POST.FIG. 13 Simple type of '»-beam technique. Bladder carcinoma
treatment plan.
r ANT.
Beam size: 11.3 x 11.7 cm
10MV Microtron
(Scanditronix
1 Beam size: 11.3 x 11.7 cm
POST.FIG. 1*» Simple type of '•-beam technique. Bladder carcinoma
treatment plan.
r ANT.
Beam size 11.7x12.2cm
30
42 MV Betatron(Siemens)SSD 120 cm
Ref. dose: 34 %
Beam size.-11.7* 12.2cmPOST.
FIG. 15 Simple type of '•-beam technique» Bladder carcinoma
treatment plan.
TABLE 4 COMPARISON OF MACHINE IRRADIATION PARAMETERS
Machine type
60Co Gamma tron 3
(Siemens)
6 MV Lin .ace.(Varlan)
8 MV Lin .ace.(M.E.L/SL75-10)
10 MV Microtron(Scanditromx-MM 10)
42 MV Betatron(Siemens)
42 MV Betatron(Siemens)
SSDcm
75
100
100
100
100
120
Beam sizecmxcm
•13,6 *X
13.6
12.0X
12.0
12.3X
12.3
11.8X
11.8
12.0X
12.0
12.6X
12.6
VolumeIntegra 1dose ^% of WCo
100
75.0
74.0
71.0
87.0
89.0
"Area"integraldose Q
% of °UCo
100
95.0
94.0
94.0
89.0
89.0
Ref. doseper beam
%
91
78
71
71
63
60
XX
Beamgeom.
°?A5.18
3.43
3.52
3.38
3.43
3.01
Ref. dose4-beamtechnique
59
45
42
42
36
34
Volumeintegraldose
100
87
89
82
106
105
BeamsizeA-Pcmxcm
11.4X
12.6
11.5X
11.9
11.6X
12.1
11.3X
11.7
11.5X
11.9
11.7X
12.2
BeamsizeLATcmxcm
8.7X
12.6
8.8X
11.9
9.0X
12.1
8.5X
11.7
8.8X
11.9
9.1X
12.2
PATIENT AND TARGET SIZES: ANTERIOR-POSTERIOR (A-P)
PATIENT (CM): ^
LATERAL (LAT)
20 .7^3 .8
' s *ne °Pen 'n9
35.2 t 1.93
11.6-1.10
for a given beam size, thus defining the beam geometry
ANTERIOR-CENTRE
8.9±2.65
TARGET VOLUME (CM): 10.1-0.9 11.6-1.10
TABLE 5 COMPARISON OF MACHINE IRRADIATION PARAMETERS
Machine type
Co Gamma tron 3(Siemens)
6 MV Lin,ace *(Varian)
8 MV L in .ace.<M.E.L,/SL75-IO)
10 MV Microtron(Scanditronix - MM 10)
42 MV Betatron(Siemens)
42 MV Betatron(Siemens)
INTEGRAL DOSES BEFORE AND AFTER NORMALIZATION
Beam sizecm x cm
13,6x13,6
12.0x12.0
12.3x12,3
11.8x11 .8
12.0x12.0
12,6x12.6
SSDcm
75
100
100
100
100
120
Un-normalized totalvolume integral doses(2-beam technique)
5,3 Joules
4.6 Joules
5.0 Jouler
4,8 Joules
6,6 Joules
7,1 Joules
Normalizingtumour dose
%
no
128
140
140
158
165
Integral dosepei 1 Gytumour dose
4.8 Joules
3.6 Joules
3.5 Joules
3.4 Joules
4.2 Joules
4.3 Joules
J
TABLE 6 COMPARISON OF MACHINE IRRADIATION PARAMETERS.INTEGRAL DOSES BEFOREAND AFTER NORMALIZATION
Machine type
Co Gammatron 3(Siemens)
6 MV Lin.ace.(Varian)
8 MV Lin .ace.(M.E.L./S175-10)
10 MV Microtron(Scanditronix-MM 10)
42 MV Betatron(Siemens)
42 MV Betatron(Siemens)
SSDem
75
100
100
100
100
120
* )
Beam sizecm x cm
11.4x12.6
8.7x12.6
11.5x11.9
8.8x11.9
11.6x12.1
9.0x12.1
11.3x11.7
8.5x11.7
11.5x11.9
8.8x11.9
11.7x12.2
9.1x12.2
Un-normalizedtotal volume »integral doses(4-beam technique)Joules
4.3
3.4
4.4
4.5
4.7
5.1
4.4
4.6
6.2
7.5
6.4
7.8
Un-normalizedtotal volumeintegral doseJoules
7.7
8.9
9.8
9.0
13.7
14.2
Normalizingtarget dose
%
169
220
240
240
278
294
Total volumeintegral doseper 1 Gytarget dose J
4.6
4.0
4.1
3.8
4.9
4.8
Upper values for the same machine are for anterior-posterior direction. Lower values are for the lateral direction,
u>
rTable 5 and Table 6 show the normalizing target doses
as occur in the "plateaus" of the target regions for
each machine for the parallel-opposed beam technique and
for the "box" technique respectively. The beam sizes are
of course related to the physical penumbra sizes for the
two parallel-opposed beam technique, but not so much in the
case of the simplified four beam technique. In the latter
case, the cobalt-60 unit shows smaller beam sizes compared
to the case of two opposed beams. This results is due to
the roundness and divergence of the isodose curves
which seem to fit better the round shape of the target
region. If a "quasi-rectangular" target volume model
rather than an oval-shaped one had been used, the beam
size, in particular for the Cobalt-6o unit, would have
been different.
It is essential always to visualize treated volumes
in three dimensions. This is important because
intersections of different planes with the same
treated volume result in cross sections that
could easily be misleading in the choioe of adequate
beam sizes to cover the treated volume. As an example
we note that the intersection of the bladder-
"target-volume" with the Transverse plane leads to
a "quasi-ellipse" shape having a smaller diameter
of 10.1 cm and a larger diameter of 11.6 cm. On the
other hand, the intersection of the Frontal plane with
the sane bladder rai*rht well result in a circular cross
section with a diameter of 11.6 cm. For this reason
adequate beam sizes in the Anterior-Posterior
direction, in the two-parallel-opposed beam technique
are found to be squares, for all machines. But in
the case of a simplified, four-beam "box" technique
both the Anterior-Posterior and the lateral beams
were found to be rectangular, for all machines. Table U
columns 10 and 11. Ignoring the circular cross-section
of the bladder-target volume in the Frontal plane could
have led to beam sizes that are smaller and square in
cross-section, and the criteria stated above would not
have been fulfilled at all boundaries of the target
volume.
3 • 3 The integral dose
Topics concerning the integral dose and methods to
calculate and estimate it in an irradiated medium,
have appeared in literature over the years, (Maynoord
Johns et al 19^9, Loeffler 195^, Sharpa I960, Dutreix
and Tubiana 19^1, Johns 19^>9). We have attempted to
compare the integral doses for each machine? subject
to the same treatment criteria and to relate the integral
dose obtained for each machine to that obtained from
using cobalt-60 gamma rays.
r3.3.I "Area" integral dose
A very simple and approximate method of estimating
and comparing integral doses for different beam
qualities is to evaluate the areas under the central
axis depth dose curve for the thickness of the
irradiated medium (Scarpa 19<io). To illustrate the
incorrectness of this method, areas under the central
axis of two-parallel opppsed 10x10 cm beam sizes have
been evaluated. Fig. 16, for a"patient" 20 cm thick.
The "target area" is assumed to occupy a region at the
centre of the "patient". For this reason all areas under
the curves are normalized at a depth of 10 cm. By
definition (Mayneord 19-M). the "area" integral dose
for each machine is a product of the ar«>a under the
curve and the "centre of gravity" oi the dosp which
in this case is 100^ target dose for all machines.
Relative values of the "area" integral doses are
obtained by normalization to that of the Cobalt-60
Gamma t ron 3 (Siemens) unit, and are shown in Table >L*
(column 5). It is seen from these results that this
simple method could lead to the conclusion that the
k2 MV betatron radiation has a smaller integral dose
than the cobalt-60 gamma radiation and qualities
between 5 MV and 15 MV. Such a conclusion is in direct
contradiction to findings based on other methods of
integral dose estimation as shown in section 3.3.2.
r
100
50
Co Gammatron-3 (SIEMENS)
6 MV Lin. ace. (VARIAN)
8MVLin.acc. (M . E . L / S L75-10)
—.-.-10MV Microtron (Scanditronix MM 10)
•42MV Betatron (SIEMENS)
10 15 cm
FIG. 16 Comparison of normalized areas under the central axis for parallel-opposed beams,( 10 x 10 cm boara sizes at the surface of the phantom). Thickness of phantom (patient)is 20 cm. This is an incorrect method when used to compare "area" integral doses.Each area is normalized to that for the Cobalt-^O Gammatron 3. (Table k, column 5).
r ~1
3.3.2 Volume integral dose
Evaluation of the volume integral dose by direct
mathematical formulas has been attempted and
reported in the literature (Mayneord I0-**2*» Scarpa 19^0,
Johns and Cunningham I969) even for radiations
having a build-up region. However, all these
methods tend to neglect the effect of penumbra on
the beam size required to treat a given tumour
size as seen from the results of this investigation. See
Table k (columns '3,10 and ll). For this reason we found it
bettor to evaluate the volumes enclosed between each pair
of adjacent isodose surfaces for the single beam sizes
obtained from manual dose planning, and to multiply each
resulting volume by a mean absorbed dose, in it.The density
is taken as unity. Mayneord (19M) has recommended the usi;
of the "centre of gravity" of the dose distribution in the
volume (or area) as a multiplying factor. For the photon
energies under this study, it is seen from Fig.'i that the
central axis depth doses are represented by straight or almost
straight lines between two isodose levels with a difference
of 10 units. The mean dose between two such levels approximates
very closely the "centre of gravity" of the dose distri-
bution function. Positions for the 20̂ > to IJjd isodose surfaces
in planes perpendicular to the central axis at different
depths were obtained by extrapolation method, Fig.k, for
example. Divergence of the isodose surfaces was taken into
r ~lconsideration in evaluating volumes. A mean value of the
volumes obtained bv adding the maximum and minimum calcu-
lated volumes between the same isodose levels, was used.
This method introduces an uncertainty in the integral doses
estimated to be +296. The error estimation is based on
considerations that the irradiated volumes are bounded by
curved isodose surfaces, but in the evaluation of volumes,
the curved portions of the isodose surfaces are taken to
be straight lines (Mayneord 19^4). Tables 5 and 6 show
results of un-normalized and normalized integral doses
together with the normalizing dose per machine and beam
technique used.
The integral dose of the Cobalt-60 Gammatron 3 (Siemens)
has been used in all beam techniques to compare integral
doses of the machines being studied. Relative integral
doses so obtained are summarized in Table k (columns k and 9).
Tn Table *» (columns h and 5), comparison is made
between the "volume" and the "area" integral dose for the
two parallel-opposed beam technique. It is seen as
pointed out above, that the "aren "integral dose concept,
incorrectly compares beam qualities. The magnitude of the
"area" integral dose is seen to be too large for all
machines and differences between beam qualities tend
to disappear.
r ~lThe k2 MV Betatron (Siemens) shows a slightly higher
"volume" integral dose than that of the Cobalt-60
Qammatron 3 (Siemens) unit. It is seen from Fig. 2
that the betatron hat- badly shaped 20"̂ to lr;i isodose
surfaces, between the surface and a depth of about
10 cm. The volume irradiated by these "oadly shaped isodose
surfaces for th*» betatron is much larger than the volume
irradiated by corresponding isodose surfaces of the
cobalt-60 unit. It is unlikely therefore that the use
of a purely mathematical expression and the concept
of beam size, to evaluate volume integral doses could
detect this integral dose difference between these two
machines. The volume integral dose of the betatron
is larger also because thr photon b<?am in the lateral
direction traverses a thickness of 35»2 cm which is
raised to high dose by "large-valued" isodose surfaces.
In spite of this, the four beam "box" technique is
frequently and favourably used for more irregular target
volumes than assumed in this study.
b BEAM GEOMETRIES
Fig. 17 shows the beam geometries of the five machines
relative to a common central axis and subject to
treating a deep seated tumour. The target volume ia shown
shaded. The angles formed by the central axis of each
rmachine and a ray from the photon source through the
50% isodose curve as extrapolated to the surface are
shown in Table b, column 7. These angles are based
on the results of the beam sizes obtained from manual
dose planning as shown in Table k, column 3, for the
two, parallel-opposed beam technique. Different therapy
machines thus require different beam sizes to treat
the same target volume. This difference in beam sizes
is explained by differences in beam geometries of
the therapy machines.
k.1 Simulator technique
Because of the differences in beam geometries and beam
sizes required by different therapy machines, to treat
even the .same target volume (Table k, columns 3,7110
and 11, also Fig. 17) a treatment set-up made using
a simulator for a particular therapy machine will not
automatically be considered valid for use by another
therapy machine with different physical parameters
(Table 1, sections 1 and 2). It is seen from this
study, that if for example a treatment set-up is made
first for a Cbbalt-60 Gftmmatron 3 (Siemens) unit for
a two parallel-opposed beam technique, a beam siza of
13.<> x 13«6 cm should be set up. If a 6 MV linear
accelerator (Varian) is used, the new beam size 12 x 12 cm
must be applied. ( A reduction of 25$ in volume integral
dose would also be achieved this way, Table h, column k).
It is therefore necessary to apply the proper beam sizes
for each therapy machine. The adequate beam size cannot
be judged only from the size of a "satisfactory simulator
beam",
r ANTERIOR
BEAM GEOMETRIES
Co Gammatron 3(Siemens)
42 MV Betatron(Siemens)
8 MV Lin. ace.(M.E.L./SL-75)6MV Lin. ace(Varian)
10 MV Microtron(ScanditronixMMKD
~i
POSTERIOR
FIG.17 Comparison of beam geometries. Different therapy machinesrequire different beam sizes to reat even the same targetvolume. Simulators in radiotherapy should be used withthis point in mind.
J
In short, the anatomical structures to be incLuded in the
simulator beam depond on which therapy unit is to be used.
ICRU, Report .?•'» (1976) points out the problems related to
simulators in radiotherapy practice and notes that
simulators are at times operated under conditions below
optimal for the therapy machines for which treatment
set-ups they are intended to simulate.
5 DISCUSSION
ICRU Report 2̂4 ( 197*1) has pointed out several physical
methods already tried by other workers in radiation
physics, in order to analyze multiple-beam dose distri-
butions for deep radiotherapy. The ICRU Report 2h points
out, however, that better methods for comparison of beam
qualities are still needed.
An attempt has been made in this study to evaluate the
significance of physical parameters with regards to
finding a balanced comparison of photon beams of different
qualities as used in the treatment of .deep seated tumours.
This study shows that finding a single overriding physical
parameter upon which to base this comparison is difficult
on account of such strong interdependence of the parameters
that influence the photon beam qualities. A choice of a
machinr for the treatment of deep seated tumours
when based on the high penetration power of the
photon beam into a patient, would tend to favour the
k2 MV Betatron (Siemens), Figs. 2 and 3, Table 2, column ').
rHowever, considerations such as low dose rates (Table i)
and the large integral dose (section 3, Table 4),
outweigh its superiority from the point of view of
having a larger depth dose. It is seen from the study
of the integral doses that the "area" integral doses
(Table U, column 5̂ indicate that the h MV linear
accelerator (Varian) , 8 MV linear accelerator (M.E.L./SL75-IO)
and 10 MV Microtron (Scanditronix MM 10) have "identical"
beam qualities; but this is shown not to be the case when
the volume integral doses are evaluated and compared.
For the two parallel-opposed and for the four beam-"box"
techniques, the microtron has the; lowest volume integral
dose relative to that of the cobalt-<>0 unit. The "area"
integral dose is seen therefore to be an inadequate
parameter for comparison of beam qualities.
Since the volume integral dose depends on the volumo
irradiated, hence on the beam size for a given beam
quality, it is important to ensure that the photon
beam and the light-localizer are properly aligned
The tolerance limit in beam sizes larger than or
equal to 15 x 15 cm on the surface of a plain phantom
is 1 mm for all therapy machines used at Radiumhemmet,
Stockholm. This tolerance limit does not include the
simulators which show differences between tho photon
beam and the light-localizer up to 5 mm. However,
greater than 5 mm differences should not be toLnrated in
thn simulator photon-light locallzer br-ams.
rIf a photon beam is made larger by 2 mm than the optimal
beam size required to treat a target volume, the integral
dose can be made to increase by as much as J%i and 3 mm
increase in the optimal beam size would increase the
integral dose by 8$. It is unnecessary to increase the
integral dose by employing unnecessarily large beam sizes
for as seen from Tables 5 and 6, the integral doses from
the therapy machines are not negligible even with small
beam sizes.
Manual dose-planning was made on the basis of
equal weighting of the beams. Although this dose-
planning might be regarded as not being "optimal",
Wollin et al. (l9?6) have noted that unequal weighting
of parallel-opposed beams in the treatment of, for
example oesophagial cancer, has dosimetric advantages
over equal weighting only when certain treatment and
beam energy conditions are met. They noted that no
clear cut advantage could be pointed out as regards
to weighting if the maximum photon energy exceeded
15 MeV at a beam separation of 12 cm or with any photon
energy at a beam separation of more than 16 cm.
Since the beam separation exceeds 16 cm in our case,
weighting of beams would produce a negligible effect
in the results obtained by not weighting the beams.
6 CONCLUSION
The results of the intercomparison of photon beam qualities
for the treatment of deep seated tumours depend on many
beam parameters and also on the size and shape of the
patient as well as the target volume and its location.
For this reason, the use of any ore single beam parameter,
such as the central axis depth dose data, to compare
photon beam qualities is inadequate and can be misleading.
7 SUMMARY
Physical parameters that influence the quality of photon
beams have been examined.The interaction of photon beams
of different qualities from cobalt-60 gamma rays to h2 MV
X-rays, with a "patient-target region" system has been
investigated with a view to compare the photon beam qualities
under specified irradiation conditions. The concept of
integral dose and its use in photon beam intercomparison
has been investigated. The results of the study have shown
the inadequacy of a single beam parameter, such as
the central axis depth dose data, in the intercomparison
of photon beam qualities for the treatment of deep seated
tumours.
8 ACKNOWLEDGEMENT
Gratitude is expressed by the author to the Director of the
Institute of Radiation Physics, Professor Rune Walstam and
to Dr. Anders Brahme for their valuable suggestions, discus-
sions and good criticism related to this work. To Ing.Ulf
Wester for the valuable contribution in drawing, aftpr the
author's original data, most of the figures that appear in
this report. To Physicists and Engineers on external beam therapy
for helpful discussions on accelerators'data at Radiumhemmet.
Also to the Cancer Society of Stockholm for financial support
to carry out part of this investigation.
~l
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