comparison of photon beam qualities for treatment …

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)


% 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)



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)



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)



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)


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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15-, 2^-MVp photons. Med. Phys. J, pp. 113-116, 1976.

comparison of photon beam qualities for treatment …

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