Attenuationcharacteristics
of a new compensatormaterial:Thermo-Shield
for high energyelectronand photonbeams
BhudattR. Paliwal,a)
StephenRommelfanger,
and RupakK. Das
University of Wi5vo,t.ti,,ConVrehutsive Ctucer Cilter. illudivttr, lfi.rrorrsirr -!-17(.tJ
(Received25 August 1997;acceptedfor publication14 January1998)
A new thermoplasticmaterialwith extremelydesirablephysicalandradiationshieldingproperties
The muerial softensbetween108'F and 132"F andcanthenbe easilymoldedto any
is presented.
desiredshape.As it coolsdown it hardensat about 102"F, retainingits moldedshape.lt is very
light (p:1.66glcc), comparedto most othercompensating
and shieldingmaterialsusedin the
clinic. Its photonand electronattenuationcharacteristics
have beenmeasuredand arecompared
with otherrnaterialsrelevantto radiotherapy.
Possibleapplicationsasa bolusmaterial.compensator
partial
. @ 1998Atnerican
and
or totalshieldingmaterialin clinical radiationtherapyarediscussed
Associationof Physicists
in Medicine.[500%-2405(98)00304-6]
Key words: Thermo-Shield,
compensator,
bolus
INTRODUCTION
In clinical situationsinvolvingextemalpatientcontourvariations and internalinhomogeneities,
routineuseof compensators to achievehomogeneous
dosedistributionsin the planning treatmentvolume (PTV) hasbecomepracticalbecause
of advances
in computertechnology.
Three-dimensional
anatomical data can be acquiredby CT, leadingto dosimetric
evaluationof tissuedensitiesin the treatmentplanningsystem (TPS) which can be programmedto design a
compensator.lAutomationin the fabricationof thesecompensatorshas been achievedby computerdriven milling
machines.2
The compensator
shapedesignedby the TPS can
milled
directly
be
in compensator
materialor a complimentary image of the compensatorshapecan be milled into a
mold, which can then be filled with compensator
material.
Compensators
havebeenfabricatedfrom aluminum"3
brass,o
wix,5 lead,GeLipowitz metal,l0'llgypsu,m
and metal,12
and
tin granulemixtures-l]The objectiveof this technic4lnoteis
to introducea new material,"Thermo-ShieldrM,"(Med-Tec,
Inc.), highly suitedfor theseapplications.
To use clinically,Thermo-Shield
is warmedin a water
bath at about 108-l32,oF and molded to the patient's
anatomythat is to be shieldedfrom electronor photonradiation therapy (seeFig. l). The shield hardensat approximately102oFandsinceit doesnotstickto thepatient'sskin,
it canbe easilyremoved.Theshieldbecomesrigid for accurate anatomicdetail and can be placedrepeatedlyfor multiple radiationtherapysessions.
It can also be handcarved,
shapedwith rotary instruments,
or cut with hot wire or Exacto hot scalpeltechniqueif desired.With a specificgravity
of only L66, the thermoplasticmatedalis very light com.
paredto its shieldingcounterpart
like lead and cerrobend.
Clinlcalsuitabllity
The bio-compatibleand shielding attributes of the
Thermo-Shieldallow its usa-se
for protectinghealthy,radiosensitivecells,tissues,hair,lips, eyes,and organellesfrom
radiationtherapy,while allorvingradio+eletherapy
to reach
the desiredtumorfield. lt is equallywell suitedfor electron,
orthovoltageandmegavoltage
externalbeamsas well as for
Physlcal properties
Thermo.Shieldis a highly attenuatingplastic that is
manually moldableand conformsto any shapeat 108'F132"F, It hardensas it cools, setting to a rigid form at
102"F. The constituentsusedin the productionof this thermoplasticradiationshieldare:(i) FederalDrug Administration approveddental hydrocarbonimpressioncompoundfor
intraoralusage,and (ii) elementalbismuth(100 mesh).Production of this thermoplasticis done by homogeneously
blendingthe finely powdereddentalcompoundwith the 100
meshbismuthin a volumeratio of appmximately2:1 The
mixture of this metal and thermoplastichydrocarbonis then
heatedin an oven to approximately2000oF for 30 min and
then cooled.The dentalcompoundbondsto the rhombohedral bismuthspheres,thus forming the densethermoplastic
radiationshield.
Med.Phys.25(4),April1998
Ftc. l. Clinical applicationof Thermo-Shield(photo counesy of MedTec, Inc.).
0094-2405/98/25(4)/484/4/$1
0.00
@1998Am. Assoc. Phys. Med.
485
485
Pallwal,Rommelfanger,and Das: Gharacterlsticgof a new comPensatormatsrlal
colrimators -'
=
->
Th€rmo.shlold
Dotoc'loral dsplh
ol modmum dose
(
)etector d€pth: 5 cn
-------->
SolUvYarer ->
ffi
I l-------->5crnx5
cm
o
.t the lsocenlel
|
|
tl
l-l
Frc. 3. Setupusedfor photon anenuationmercur€ments.Detector placedat
isacenterat depthof 5 cm in solid water,Thermo'Shieldslabswerc stacked
on a standardblock uay mountedto dre linac head
Distanceis lff)
FIG. 2. Setup used for electron attcnuationmeasurements.
cm from source to top of solid water. For each electronenergy,detector
placed at depth of maxirnumdose.
MATERIALS AND METHODS
brachytherapy.It is a good substitutefor lead strip types of
shieldsand unlike highZ materialslike leadandcenobendit
photonsin electrontreatments.
produceslessbremsstrahlung
It is reusableby reshapingas well asby changingthe thickness,and it may be disinfectedwith glutaraldehydesprayfor
asepsis.
Potentlal appllcatlon sltes
(l) Majority of treatmentswith externalelectronandphoton
beam irradiation in the intraoral head and neck sites.
Example: electron shieldingof 3 cm or less,as in shaping the electronfield by blocking at the end of the treatment cone.Field contouringin suchcasesusingThermoShieldis extremelysimple,convenientandeffective.
(2) Intraoral canceniof the tongue,floor of mouth, palate,
tonsillar arch, intraoral buccalmucosa,mandiblemaxilla
and parotid. Intraoral shielding prctects ompharyngeal
tissueswhich are otherwiseunshieldeddue to their position either within the primary beamor adjacentto the
brachytherapysite.
(3) Customizedimpressiontrays loadedwith shielding materials usedfor tongueimmobilizationin brachytherapy,
elecEonand orthovoltages.
(a) Shielding of tooth extractionsites or arcaswhere periodontal surgery has been performed. Since these
"wound-healingcells't with theirgreatermitotic activity
are particularly radiosensitive,shieldingcould lessenthe
probability of osteoradionecrosis,
allowing for a more
normal healing processand hencelessenthe untimely
delaysin radiation therapy.
The vendorcurrently marketsthe Thermo-Shieldmaterial
in slabsof l2x 12cr* andapproximaie
thicknessof lJ cm.
For our experimentaldeterminationof the attenuationcoefficients of the material. variableslab thicknessof less than
1.7cm wasachievedby warmingthe Therrro-Shield slabsin
a hot water bath and then pressingthem in the desiredthicknessusing a vise.
Electronattenuationpropertieswere measuredutilizing a
l0 cmX 10cm electronconeat 100cm SSDsetup,relativeto
the top of a stackof Solid Waterru (RadiationMeasurements
Inc., Middleton, WI) (Fig. 2). A Farmer-type ionization
chamberwas placedin the solid water at the depth of maximum dose,with l0 cm of solid water below for adequate
backscatter.The amount of chargecollected by the ionization chamberwas measul€dwith a Keithley 35040electrometer. For each electron energy, measurementswere made
with 0, 0.5, 0.7, 1.0, 2.0, 2.5, and 3.0 cm of Thermo-Shield
stackedabovethe solid water.
Photon attenuation properties were measured in a
5 cmX5 cm field anda 100cm SSDsetup(Fig. 3). The same
Farmer+ypeionization chamberwas placedin the solid water at 5 cm depth, with l0 cm of solid water below for adequatescatter.For eachphotoninergy, measurementswere
takenwith 0, 1.71,3.33,4.94,and6.63cm of Thermo-Shield
stackedon the block holder tray.
Relative transmissionvalueswere determinedby normalizing the readingsof eachenergyto their 0 cm reading. Uncertainty of the readings(67% confidencelevel) was determined from the sandard deviation of multiple measurements.The transmissionvalues for each photon and
electron energy were plotted againstThenno-Shield thick-
TABtt I. MeasurcmenBof olectrqr transmission(percentage)at depth of maximum dosedrough incrrasing
thickncssesof ltcrmo.Shield. For each beam cnergy, the datt are normalized to its conespondingvalue
measurcdfor 0 cm thicknass.
Elecuon boamenergy
Thickrrss (cm)
0.00
0.50
0.70
1.00
2.00
2.50
3.00
4 MeV
lm.0
10.0
0.4
0.4
0.3
0.2
0.2
MedlcalPhyslcs,Vol.25, No.4, Aprll 1998
6 MeV
100.0
t8.0
1.2
0.9
0.6
0.4
0.3
9 MeV
100.0
28.0
13.3
9.0
1.4
l.l
1.0
12 MeV
15 MeV
r00.0
68.0
45.0
29.0
2.8
2.5
2.0
100.0
85.0
74.0
56.0
4.9
4.0
3.2
lE MeV
100.0
92.0
83.9
68.0
7.6
6.1
5.2
486
Pallwal,Rommeltangsr,and Das:Charactsrietlcsof a new compensalormaterlal
TABI! lI. Measurimentsof photon transmission(percentage)at depth of
maximum dosc through increxing thicknessesof Thcrmo-Shietd.For each
beam energr, the data are normalizedto its correspondingvalue measured
for 0 cm thickness.
Thermo-Shield
Photonenergies(MV)
o.7
o
E
0.6
o
A
0.5
t:
o.4
t
()
Thickness(cm)
E
o
0
l.?l
3.33
4.94
6.63
66.4
46.3
69.1
50.r
33.0
233
36.7
26.7
72.3
53.7
N.4
29.8
72.3
53.6
39.8
29.0
E
d
0.3
o.2
0.1
0.0
1,0
RESULTS
Electrons
Resultsfor electrontransmissionmeasurements
are tabulated in Table I. As it has been discussedin the previous
s@tion, the uncertain$ 67Vo confidencelevel) associated
with the experimentaldatais 5% of the averagevalue of the
multiple measurrements.
It can be inferrcd from Table II that
the TVL for this materialat 4, 6, and9 MeV rangesfrom 0.5
to 1.0 cm whereasfor lead it rangesfrom 2 !o 4 mm. For
higher energies(20 MeV) TVL for Therme.Shieldis about 2
cm comparedto leadwhich is aboutI cm.la
Photons
Results for photon fransmissionmeasurementsare tabuIated in Table II. The uncertaintyassociatedwith the experimental data is l%. Exponentialfits to the transmissiondata
as describedabovegave linear attenuationcoefficient of the
Thermo-Shield at different energiesand are tabulated in
Table ltr. A comparisonof the linear attenuationcoefficients
for the Thermo-Shield and for other materials relevant to
radiotherapyare plotted in Fig. 4.15The linear attenuation
coefficient for the Thermo-Shieldmaterialfalls benreen the
commonly usedmaterial like lead andcenobendand tissueTrrle III. Linear attenuationcoefficientsfor Thermo-Sfticldfor different
enorry photon beams.
Thermo-Shield
Lincar Ann. Coef.
(per cm)
Photonenergies(MV)
0.205
0.tE5
0.187
-r--Lead
3.0
4.0
s.0
6.0
7.0
(9.63 g/cc) --*--Polystyrene. (1,06 g/cc)
(1f33 g/cc)
---x--Lucltc (1.f9 g/cc)
--.--Wrter
(0.99 g/cc)
----+-Therno"Shlcld (1.56 g/cc)
Flc, 4. Comparisonof linearattenuation
coefficientof Thermo-Shieldwith
(Attix, 1986).
othermaterialsrelevantto radiotherapy
like materials,suchas lucite andpolystyrene.With the density ofThermo-Shieldasprovidedby the vendorand verified
by our measurements
to be 1.66 glcc, the massattenuation
coefficientfor Thermo-Shieldin the Megavoltagerange varies from 0.137u*lg at 4 MV to 0.113 cmzlgat 20 MV. In
comparisonto this, the most commonlyusedmaterial in the
clinic today like lead or cerrobendhas massattenuationcoefficient in thg sameMegavoltagerange varying from 0.059
to 0.045cm?g and0.056to 0.M2cr*tg, rcspectively.
CONCLUSION
The new themroplasticmaterialcurretrtly being marketed
by Med-TecInc. asThermo-Shield
hassomeuniquephysical
and radiationshieldingproperties.Since the material softens
in a warm water bath it can convenientlymoldedto the patient's anatomyor to any desiredshape.'At102"F it hardens
retaining its shapeand the mold can then be usedfor multiple radiationtherapysessigns.It can also be shapedor cut
with rotary instnrmentsor hot wire cutter making it an extremely convenientmaterial to be used in routine clinical
use.With is high massattenuationcoefficient and low density it makesan extremelydesirablematerial for usein clinical radiation therapyas a compensatoror shielding material
for photon beams For electrons,Thermo-Shieldacts as a
good shielding material,with the addedadvantageof being
an effectivelow Z materialreducesthe productionof bremsstrahlungphotons.
dElectronic mait [email protected](E)
lB, R. Paliwal, M. B. PodgorsalcP. M. Harari, P. Haney, and P,
A.
Jursinic, "Bxperimentalevaluationand quality conbol of a 3.D compensalorsystem,"Med. Dosim.f9, 179-185(1994.).
?. A. Iuninic, M. B, Podgorsak,andB. R. Paliwal, ..tmplementrtion of a
3-D dosecompensationsysrcmbasrdotr computcdtomogrsphygencrated
surfacecontoursand tissueinhomogeneities,"Med" Phys.21, 35?-365
(ree4).
ItledlcalPhysics,Vol. 25, No, 4, Aprll 1998
2.0
Elfective Encrgy (MeY)
---+-Ccrrobind
ness.For photons,the linear attenuation
coemcientwas deterrninedfor eachenergyby fitting the datato an exponential
functiongivenby IlIs:exp(-N). whereIllsis the relative
transmissionreading,trr is the linear attenuationcoefficient
and t the thicknessof the thermoplastic
material.The linear
attenuationcoefficientsfor Thermo-Shieldwere then plotted
againsteffective photon beamenergy,alongwith attenuation
coefflcientsfor cerrobend,lead,lucite,polystyrene,and water.
486
of a new ctomponSatormderlal
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