ith the advent of tumor-associated monoclonalantibodies brought about by Kohler and Milstein'sdiscovery of hybridoma technology in 1975 (1), diagnostic imaging for a variety of tumors using antibodiesradiolabeled with either iodine- 13 1 (‘@‘I),iodine- 123(1231) indium-l 1 1 (â€â€˜â€˜In),and perhaps technetium-99m

appears to be a reality (2-5). An equal but somewhatless than optimistic view has been expressed in thefeasibility ofperforming radioimmunotherapy with particulate radiation from radionuclides such as ‘@‘I,yttrium-90 (90Y),copper-67 (67Cu),rhenium-l86 (@Re),astatine-2 11 (21‘At)and others (6—9).Radioimmunotherapy modeling calculations based on MIRD type dosimetry (10,1 1) gives reasonable first-order estimates ofgamma-emitting radionuclides in comparing the absorbed doses from organ to organ (S values) in humans,but fails to be predictive when examining particulateradiation in the subcentimeter range at tumor boundaries, for tumor heterogeneities, and at organ interfaces.In the MIRD formalism, particulate radiation is simplylabeled “np―for nonpenetrating. Since it is envisioned

Received Aug. 19, 1985; revision accepted Mar. 6, 1986.For reprints contact: BarryW. Wessels,PhD, Div. of Radiation

Oncology & Biophysics, The George Washington University Mcdical Center, 901 23rd St., N.W., Washington, DC 20037.

that antibody therapy will be performed predominatelyby particulate radiation, heterogeneities in antibodydeposition become a major factor in dose depositionand ultimately in the outcome of the tumor dose response to therapy. Consequently, substantial uncertainty in computing organ doses for radioimmunotherapy in animals or humans results from this methodof dosimetry calculation which is highly dependent onbasically unknown time dependent biodistribution dataand internal organ geometry.

We have developed a method for the direct measurement of absorbed radiation dose through the use ofteflon-imbedded, Teledyne CaSO4:Dy thermoluminescent dosimeter(s) (TLD) which have been modified tofit inside a 20-gauge needle. Thermoluminescent dosimetry has been shown to be a reliable and accuratemethod of assessing radiation dose for a variety ofhealth physics (12) and radiation therapy applications(13). As early as 1967, Kastner (14) recommendedexperimental animal biology based on Loevinger's for

malism (15). It was shown that the light output fromthe thermoluminescent dosimeter is dependent only onthe total absorbed energy and is not a function ofspecific ionization for high-energy beta and gammaradiation. This provides for a convenient dosimeter thatmay be calibrated for any internal beta emitter with a

1 308 Weasels and Griffith TheJournalof NuclearMedicine

MiniatureThermoluminescent DosimeterAbsorbedDose Measurementsin Tumor Phantom ModelsBarry W. Wessels and M. Howard Griffith

Department ofRadiology, The George Washington University, Washington, DC

Miniatureteflon-imbeddedCaSO4:Dythermoluminescentdosimeter(s)(TLD)havebeensizedandcut to fit insidea syringeneedle.Thesedosimetershavebeenshownto be linearinresponse to beta and high energy gamma radiation. This allows for their direct implantationintotumor-bearinganimalsundergoingradioimmunotherapyandsubsequentmeasurementofdosedepositionon a perorganbasis.Inorderto performtheseradiolabeledantibodydosemeasurementswith sufficientaccuracy,staticcalibrationdata mustfirst begenerated.Consequently, phantom models were constructed with artificial tumors of diameters rangingfrom 3-30 mm contained in a surrounding tissue equivalent medium. The TLD were used tocharacterizedosedistributionsin a radialdirectionfromthe centerof the cylindricaltumorvolumescontaining1311@P,or @°Yradionudides.Absorbeddosemeasurementsin theboundary region between tumor and outer medium were found to be dependent on the: (a)tumor specific activity, (b) average range of the beta radiation, and (c) radial tumordimensions.

J Nuci Med 27:1308—1314, 1986

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beta emission energy of at least 20 keY. TheseCaSO4:Dy TLD were chosen over other available crystaltypes since they have been shown to be among the leastenergy dependent and the most sensitive when used inthickness which is substantially less than the radionuclide average beta range (16). The CaSO4:Dy TLDmay be directly implanted into a variety of animal orhuman organs in which dosimetric information is desired and subsequently recovered for read-out. In thiswork, we will discuss the fabrication and calibration ofthese dosimeters in a tumor phantom model and describe their relative accuracy and the reproducibility ofdosimetry data for three radionuclides (‘@‘I,phosphorus-32 (32P), and @°Y)which are of interest to theradioimmunotherapy community.

MATERIALSAND METhODS

FIGURE 1Tip of 20-gaugeneedlewith miniatureTLD in place forphantom implantationprocedure.Second dosimeter isshownbelowneedlefor overalllengthcomparison(5 mm)

Radiac wash upon removal from the solution. TheseTLD were read on the TLD reader in the same mannerused for the 4 MV calibration experiment. Each datapoint was taken in threefold redundancy. No unusualTLD browning or discoloration due to exposure to theliquid suspension was noted after readout. The MIRDformalism was used in calculating the absorbed dosefor these homogeneous, cylindrical phantoms in whichthe specific activities were known to ±5%.Resultantcalibration factors of 0.99, 1.05, and 1.01 were calcubated for ‘@‘I,90Y, and 32P relative to 4 MV x-rayradiation and were determined to an overall uncertaintyof ±8%.

In addition, tumor phantom models were constructed from three different diameters of polystyrenetest tubes or vials (Tube 1, d = 3.5 mm; Tube 2, d =16 mm; and Tube 3, d = 30 mm) machined to uniformwall thickness of 0.3 mm in the region of TLD placement as shown in Fig. 6. Volumes of activity of tumorcomponents were 0.8, 4.5, and 25 ml for tubes T,, T2,T3, respectively. Inner tumor phantom componentswere placed in a larger outer box made of clear acrylicplastic of dimensions 10 cm x 10 cm x 15 cm3 andfilled with water to a volume of900 ml.

For these experiments, each test tube tumor was

The TLD used in the following phantom tumormodel experiments were fabricated from 400-si thick,12-mm diameter CaSO4:Dy teflon matrix disks'. Our

experimental design required a sturdy, rod-shaped dosimeter that was designed to fit inside a syringe needle.Hence, we imbedded the disk dosimeter in a 2 X 2 cm2paraffin block for slicing the TLD with a tissue sectionmicrotome to a thickness of 200 @.This TLD slab wasthen cut and accurately sized under a 20 power dissecting microscope to a length of 5 mm. The final dimensions of the dosimeter were 0.2 x 0.4 x 5.0 mm3. Asshown in Fig. 1, the dosimeter conveniently fits insidea 20-gauge needle.

Each dosimeter was weighed (Mettler balance) anddimensions measured by micrometer to insure an initialoverall uniformity of ±3%.These TLD were first crosscalibrated with 4 MV x-rays from a Varian (linac 4linear accelerator at 20—1,500 cOy output under fullbuildup conditions (Fig. 2) as measured with a Farmertype ionization chamber. This ionization chamber calibration is traceable to the National Bureau of Standards. After reading under dry nitrogen, the TLD werereannealed ( 1 hr at 270°C)and counted for backgroundlight output. The TLD glow curve peak was integratedover 50 sec with T, = I 15°Cand T2 = 275°C.Temperature ramping was 3.6°C/secwith prereadout annealingcycle of 5 sec. The major integration peak occurs at220—240°Cwith a minor peak (5%) at 120°Cfor thismaterial. In order to calibrate the dosimeters relative toa combined gamma and beta ray source, TLD rodswere also immersed in a 5 ml, 1,020 @Ci‘31I-NaVwatercylindrical phantom (d = 1 cm) for times ranging from20 mm to 24 hr (Fig. 3). A similar calibration experi

ment was performed with @°Yin a 40-ml phantom overseveral hours (Fig. 4) and in a 25-ml 32Pphantom for asimilar time period (Fig. 5). The dosimeters were keptin a fixed position and each was rinsed thoroughly with

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I 0'

0LI

UiU)

20

Cl)00

101

I 0'@

1.5

10'

5.0

2.5

10'

5.0

2.5

10'

2.5 5.0@ 1.510'

2.5 5'O10' 10'

lolUGHT OUTPUT (nC)

FIGURE 2Log —log plot of TLD light output in nC of photomultiplierintegratedchargecolleCtiOnvs. absorbeddose in cGy asmeasured by Farmer type chamber under full buildup andscatter conditions from 10 x 10 cm2 4 MV isocenthc 80cm irradiation field in tissue equivalent Temex phantom.Eachdata point was taken in threefoldredundancy.Forthese measurements, average conversion factor over doserangewasi cGy=1.13±0.O4nC

10'

TIME IN SOLUTION (MINI

FIGURE 3Log—logplotof absorbeddoseincGyvs. timein minutesof TLDfully immersedina 1,020 @Ciof 13115 mlcylindricalwater phantom. Carrier Nal (2 mg) was added to ensurehomogeneous dispersion of 1311-NaI.Decay corrected 1311activities yield calibration factor of 1.12 ±0.08 for cGy tonCTLDreaderoutputbasedon MIRDcalculationsfor thisuniform geometry

placed midphantom in conjunction with the large outerbox such that the central portion of the inner tumorwas equidistant from the outer walls ofthe surroundingbox. The inner test tube tumors were filled with activities of ‘@‘I,32P, and @°Ywhich were 10—200times thespecific concentration of activity relative to the outerphantom box. Activity measurements and manufacturer quotations were confirmed by gamma well orliquid scintillation counting methods as appropriate.Phosphorus-32 decay-corrected manufacturer's activityspecifications were in agreement (±5%) with zeroquenched serial diluted liquid scintillation counter reading (Beckman Model LS7500). Serial dilution volumeswere l00-@deach, suspended in 10 ml of Scintiverse IIscintillator cocktail. Yttrium-90 available privately@wascalibrated by similar serial dilutions using the liquidscintillation counter. Scintillation counter values wereused to determine activity in each ofthese experimentssince the values showed ±20%variation from activitiesquoted by supplier. Knox gelatin was mixed with thewater solution of activity (20 g/l,000 ml) before TLDimplantation such that the resultant cooled mediawould physically stabilize the TLD position in thephantom.

The placement of the TLD was performed with theaid ofan alignmentjig and 20-gauge spinal needle (Fig.6). The alignment jig was a machined 12 x 12 cm2,2-cm-thick bidwhich covered the top ofthe outer phantom. Forty holes were drilled and spaced from the centerof the lid outward on a central line 1 mm apart. Each

5.0

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TIME IN SOLUTION (MIN.)

5.010'

FIGURE 4Log—logplotof absorbeddoseincGyvs. timeinminutesof TLD fully immersedin 5,620 @Ciof @°V-IabeIedprotein40 ml cylindricalphantom.Carrierprotein(humanserumalbumin —2 mg) was added to ensure homogeneousdispersion of @°Y-labeiedprotein. Decay corrected @°Yactivities yield calibration factor of 1.19 ±0.08 for cGy to nCTLD reader output based on MIRD calculations for thisuniformgeometry

10°

131 0 Weasels and Griffith TheJournalof NuclearMedicine

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the tumor/outer phantom combination. TLD placement was over a range of radial distances starting at theinner tumor center and extending 4.5 cm outward.Nonuniform concentration of TLD placement washighest near the tumor wall and lowest in the regionsgreater than 1.5 cm from the inner tumor wall in orderto characterize rapid dose variation near the interfaceregion.

RESULTS

A total of nine phantom experiments were performed: three tumor sizes for each radionuclide, ‘@‘I,32P, and @°Y.For maximum utilization of these datawithout undue redundancy, results ofthese experimentsare shown as plots of dose versus radial distance (Figs.7—11). Each data point in these figures is the result of asingle measurement with an associated standard errorof ±15%.The calibration factors which were derivedfrom the measurements described in the methods seetion were used to convert TLD light output (nC) todose (cOy) for each radionuclide phantom experiment.Phosphorus-32 experiments are shown for all threetumor sizes in Figs. 7—9,and ‘@‘Iand @°Yplots are

FIGURE 7Dose measurements vs. distance from tumor center gel ofTLD imbedded in tumor and outer phantoms containing32Pin specificacitivityratioof 73:1, respectively.Initialspecificconcentrationof innertumorwas22 @zCi/ml.TotalTLDsuspensiontimewas 16 hr.Halfwidthmeasuredfromwall to 1/20th maximum peak height of curve drawnthrough data points is 0.7 ±0.4 mm for tumor diameter of3.5 mm

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8

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@ 4@

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2

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FIGURE 5Log —log plot of absorbed dose in cGy vs. time in minutesof fully immersed TLD. Data were taken from three experiments which were conducted with a 25-mI cylindricalphantomcontaining421 @Ciof ‘@P.Decaycorrected @Pactivitiesyieldcalibrationfactorof 1.14 ±0.08 for cGytonCTLDreaderoutputbasedon MIRDcalculationsfor thisuniformgeometry

hole was oversize by 0.01 mm with the outer diameterof the 20-gauge spinal needle to facilitate tight slidingfit. The alignment jig afforded precise three-dimensional localization (±0.3mm) ofTLD radially outwardfrom the center of the tumor to the wall region of theouter large phantom.

Between 10-1 5 TLD per experimental run were usedto determine the dose profile in a radial direction for

FIGURE 6Tumor/normaltissue equivalentphantom.This is a thinwall test-tube ‘tumor―imbedded in gel suspensionofhomogeneousradionudide activity. Activity inside testtube is alsoin gel form rangingin specificactMty 10—200times that of specific activity of the outer media. TLDs areinjected and suspended in gel for times ranging from 12—36 hr using alignmentjig and spinal needlewith innerstylus. All TLD are removed and recovered from gel simultaneously

TumorCenter

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0(I

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I cmDISTANCE (mm)

1311Volume27 eNumber8 •August1986

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shown in Figs. 10 and 11, respectively, for test tubetumor number 3 only. Half-width, one-twentieth, orone-tenth maximum peak values (1'2oor F,0) are givenfor each dose profile in order to characterize the effectof activity, range of particulate radiation, and tumorgeometry. The halfwidth ofthe curve is measured fromthe curve to the tumor wall. Quoted uncertainties associated with r were determined by graphic analysisusing the single point data. A similar analysis is frequently used to characterize peak shapes obtained innuclear spectroscopy studies (1 7). Curves were drawnas a best fit by visual examination.

DISCUSSION

By closely examining the data shown in Figs. 7—11,three immediate observations can be made with respectto dose deposition in and surrounding the tumor volume. First, specific activity or percent dose per g (volume) remains the first order or primary indicator of the

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DISTANCE (mm)

FIGURE 8Dose measurementsvs. distancefrom tumor center ofTLDimbeddedingeltumorandouterphantomscontaining32Pin specificactivity ratio of 200:1, respectively.Initialspecificconcentrationof innertumorwas 80 @Ci/ml.TotalTLDsuspensiontimewas 16 hr.Halfwidthmeasuredfromwall to 1/20th maximumof curve drawn through datapointsis 1.0 ±0.4 mmfor tumordiameterof 16 mm

magnitude of dose deposition. Secondly, beta particlerange (Rmax 1.5, 8, 11 mm for ‘@‘I,32P, and @°Y,respectively) is the determinate factor to dose deliverednear the tumor wall. The peak half-width, one-tenthmaximum (F,0) for ‘‘i,32P,and @°Yare 0.8 ±0.5 mm(Fig. 10), 1.3 ±0.5 mm (Fig. 9), and 2.5 ±0.6 mm(Fig. 1 1), respectively, for the large tumor geometry(test tube number 3). It is important to note that these“effectivedose ranges―are far below the quoted maximum ranges of the beta particles and are generallysomewhat less than the average quoted beta ranges.This effect naturally leads into the third point of discussion: geometric size of the tumor. Examination ofthe 32Pdata for different tumor data yields a r20 of 0.7±0.4, 1.0 ±0.4, and 2.0 ±0.5 mm for progressivelylarger tumors, d = 3.5, 16, and 30 mm, respectively(Figs. 7—9).

DISTANCE (mm)

FIGURE 9Dose measurementsvs. distancefrom tumor center ofTLDimbeddedingeltumorandouterphantomscontaining32Pin specific activity ratio of 89:1, respectively.Initialspecificconcentrationof innertumorwas 21 MCi/mI.TotalTLD suspensiontime was 18.5 hr. Half width measuredfromwallto 1/20thmaximumof curvedrawnthroughdatapoints is 2.0 ±0.5 mm for tumor diameter of 30 mm

Well

Tumor Center

Wall

1(

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10 20 30 40 50 6020 30 40 50 60

1312 WesselsandGriffith TheJournalof NuclearMedicine

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FIGURE 11Dose measurementsvs. distancefrom tumor center ofTLDimbeddedingeltumorandouterphantomscontaining90vin specificactivityratioof 17:1, respectively.Estimatedinitialspecificconcentrationof innertumor was 6 ,@Ci/mlbased on post-calibration data with liquid scintillationcountingandTLD comparisonmeasurements.Total TLDsuspensiontimewas 12 hr. Halfwidthmeasuredfromwallto 1/10th of curvedrawnthroughdatapointsis 2.5 ±0.6mmfor maximumtumordiameterof 30 mm

that CaSO4:Dy TLD exhibit an over response in sensitivity for low-energy x-ray irradiation (20), no correction was necessary to compensate for bremsstrahlungradiation produced from the beta emitters used in theseexperiments for the quoted accuracy of the measurements. Care must be exercised when applying thesecalibration methods to 1251and ‘‘‘Inradionuclides dueto this over response in sensitivity for low-energyx-rays.

It is clear from the above boundary effect data thatthese direct measurements add a level of detailed description of dose profile at tumor/nontumor interfacesonly obtained by complex point source functions orMonte Carlo techniques and are not readily accessibleby the MIRD formalism. The presentation of the capabilities of these TLD as applied to activity interfaceproblems for relatively simple phantom geometry hasbeen aimed at providing a firm basis to more complexgeometries encountered in vivo. Experiments are now

10

DISTANCE (mm)

FIGURE 10Dose measurementsvs. distancefrom tumor center ofTLDimbeddedingeltumorandouterphantomscontaining1311 in specific activity ratio of 70:1 , respectively. Initial

specificconcentrationof innertumorwas72 @Ci/ml.TotalTLDsuspensiontimewas20hr.Halfwidthmeasuredfromwall to 1/10th maximumof curve drawn through datapointsis 0.8 ±0.5 mmfor tumordiameterof 30 mm

The dose falboffregion for these results appears to besimilar to the idealized infinite sheet case calculated for32p as described by Leichner et al. (18). The overalltumor volume effect and beta range measurements hereare also in reasonable agreement with calculations madeby Kwok et al. (19) for spherical volumes. A subtler,but theoretically predicted, dose effect evident at thetumor boundary is that of the dose build-down in theregion which occurs just inside the tumor boundary.The absorbed dose measured by the TLD in this regiontends to decrease before the activity profile has startedto decrease as shown in all doses vs. radial distanceplots (Figs. 7—11).

The comparison of the linear accelerator calibrationcurve to the higher energy beta calibration data (@°Y,32P) shows good agreement with MIRD prediction forthese simple geometries@.Although it has been reported

1313Volume27 •Number8 •August1986

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7. Wessels BW, Rogus RP: Radionuclide selection andmodel absorbed dose calculations for radiolabeled tumor associated antibodies. Med Phys I 1:638—645,1984

8. DeNardo S, Erickson K, Benjamin E, et al: Monoclonal antibodies for radiation therapy of melanoma.Proceedings ofthe Third World Congress of NuclearMedicine and Biology, Paris, Pergamon Press, 1982,pp 182—185

9. Quadri SN, Wessels BW, Dekazos D, et al: Radiolabeled biomolecules with Re-l86: Potential for radioimmunotherapy. Symposium on Chemical Aspectsofthe Production and Use ofProspective TherapeuticRadionuclides, ACS Abstracts, Miami, FL, 1985

10. Loevinger R and Berman M: A Revised Schema forCalculating the Absorbed Dosefrom Biologically Distributed Radionuclides, MIRD Pamphlet No 1, Revised, New York, The Society of Nuclear Medicine,1976

11. Snyder WS, Ford MR, Warner GG, et al: “S'@AbsorbedDoseper Unit AccumulatedActivityfor SelectedRadionuclides and Organs, MIRD Pamphlet No 11,New York, The Society of Nuclear Medicine, 1975

12. Thermoluminescent Dosimetry, Cameron JR, Suntharalingam N, Kennedy 0(3, eds. Madison, WI, University of Wisconsin Press, 1968

13. Applied Thermoluminescence Dosimetry, OberhoferM,ScharmannA,eds.Bristol,England,AdamHilger,1981

14. Kastner J, Hukko R, Oltman BG, et al: Thermoluminescent internal beta-ray dosimetry. Rad Res32:625—640,1967

15. Loevinger R, Japha EM, Brownell GL: Discrete radioisotope sources. In Radiation Dosimetry, Hine GJ,Brownell GL, eds. New York, Academic Press, 1956,pp 693—799

16. Piesch E: Application ofTLD to personnel dosimetry.In Applied Thermoluminescence Dosimetry, Oberhofor M, Scharmann A, eds., Bristol, England, AdamHilger,1981,pp 167—195

17. Segre E: Nuclei and Particles, New York, W.A. Benjamin, 1965, pp 442—467

18. Leichner PK, Cash SA, Backx C, et al: Effects ofinjection volume on the tissue dose, dose rate, andtherapeutic potential of intraperitoneal P-32. Radio!ogy141:193—199,1981

19. Kwok CS, Prestwich WV, Wilson BC: Calculation ofradiation doses for non-uniformly distributed beta andgamma radionuclides in soft tissue. MedPhys 12:405—412,1985

20. Driscoll CMH: Fundamental aspects of TLD materials. In PracticalAspects ofTherrnoluminescence Dosimetry, CRS-43, London, England, The HospitalPhysicists' Association, 1984,pp 5—11

21. Wessels BW, Bacha P. Quadri SN: MicrodosimetricTLD measurements for tumor associated antibodytherapy. J NuclMed 25:39, 1984

ongoing in which TLD dose information is being obtamed from various organs and tumors in animalswhich are undergoing radioimmunotherapy with ‘@‘Ior @°Y-labeledtumor-associated monocbonal antibodies(21).

FOOTNOTES

. Teledyne, Inc., NJ.

$ DuPont NEN Medical Products, No. Billerica, MA.

* Oak Ridge @°Srfl°Ygenerator, Hybritech Corp., La Jolla,

CA.* For example, using the interval average decay-corrected

activity (pCi) multiplied by time in hours or cumulated activity for the full buildup phantom shown in Fig. 9, the dosevalue for TLD absorbed dose may be computed by theexpression

MTI

for 32Pat the center of the tumor. Hence:

DT 20.8 @Ci/g. I 8.5 hr. 1.48 g. rad/@tCi . hr = 570 rad.

This is to be compared with a TLD reading of 530 rad where1 md = 1 cOy. For other phantom experiments (Figs. 7—11),the tumor dose may be computed in a similar manner for fullbuildup conditions.

REFERENCES

1. Kohler G, Milstein C: Continuous cultures of fusedcells secreting antibody of predefined specificity. Nature256:495—497,1975

2. Larson SM, Brown JP, Wright PW, et al: Imaging ofmelanoma with 1-131-labeledmonocbonalantibodies.JNuclMed24:123—129,1983

3. Epenetos AA, Shepherd J, Britton KE, et al: Radioimmunodiagnosis ofovarian cancer using 123-I-labelled,tumor-associated monocbonal antibodies. Cancer Deted Prevent 7:45—49,1984

4. Halpern SE, Hagan PL, Garver PR, et al: Stability,characterization, and kinetics of 111-In-labeledmonocbonal antitumor antibodies in normal animals andnude mouse-human tumor models. Cancer Res43:5347—5355,1983

5. Paik CH, Saham MS. Hong JJ, et al: Preparation of astable Tc-99m complex of(Fab'>@-DTPA and its biodistribution in mice. J Nucl Med 25:128, 1984

6. Goldenberg DM, Gaffar SA, Bennett SJ, et al: Experimental radioimmunotherapy ofa xenografted humancobonictumor (GW-39) producing carcinoembryonicantigen. Cancer Res 41:4354—4360,1981

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1986;27:1308-1314.J Nucl Med.   Barry W. Wessels and M. Howard Griffith  Phantom ModelsMiniature Thermoluminescent Dosimeter Absorbed Dose Measurements in Tumor

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