Monte Carlo simulations were carried out to study the response of a thyroid monitor for measuring intake activities of 125I and 131I. The aim of the study was 3-fold: to cross-validate the Monte Carlo simulation programs, to study the response of the detector using different phantoms and to study the effects of anatomical variations. Simulations were performed using the Swiss reference phantom and several voxelised phantoms. Determining the position of the thyroid is crucial for an accurate determination of radiological risks. The detector response using the Swiss reference phantom was in fairly good agreement with the response obtained using adult voxelised phantoms for 131I, but should be revised for a better calibration for 125I and for any measurements taken on paediatric patient

Baechler, S.

Bailat, C.

Bochud, F. O.

Damet, J.

Laedermann, J. P.

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VARIABILITY OF RADIOIODINE MEASUREMENTS IN THETHYROIDJ. Damet*, F. O. Bochud, C. Bailat , J. P. Laedermann and S. BaechlerUniversity Institute for Radiation Physics, University Hospital of Lausanne, 1007 Lausanne, Switzerland*Corresponding author: jerome.damet@chuv.chMonte Carlo simulations were carried out to study the response of a thyroid monitor for measuring intake activities of 125Iand 131I. The aim of the study was 3-fold: to cross-validate the Monte Carlo simulation programs, to study the response ofthe detector using different phantoms and to study the effects of anatomical variations. Simulations were performed using theSwiss reference phantom and several voxelised phantoms. Determining the position of the thyroid is crucial for an accuratedetermination of radiological risks. The detector response using the Swiss reference phantom was in fairly good agreementwith the response obtained using adult voxelised phantoms for 131I, but should be revised for a better calibration for 125I andfor any measurements taken on paediatric patients.INTRODUCTIONWhen radioactive iodine intake occurs, in vivomeasurements of radioactivity in the thyroid arerequired to assess the committed effective dose.These thyroid measurements are performed with aspectrometer that has been accordingly calibratedusing a standard neck phantom. However, measure-ments of the radioiodine activity may fluctuate dras-tically since the depth, size, shape and position of thethyroid gland varies widely from one individual tothe other. The goal of the present study was to quan-tify the variability of measurements of radioiodine inthe thyroid based on Monte Carlo simulations usingboth geometrical and voxelised phantoms. Separateevaluations were performed for the contributions ofthe size and the position of the thyroid, the size ofthe neck and the positioning of the detector.MATERIALS AND METHODSInstrumentationThe thyroid monitor used in the study was based ona 2 in.2 in. NaI crystal detector mounted on ashielded lead/iron collimator. The crystal was placedat 9 cm from the edge of the collimator. The Swissstandard neck phantom used for the calibration wasa 14-cm diameter cylinder with a height of 16 cm,made of polyethylene. The thyroid was simulated bytwo 20-ml vials placed inside the cylinder at 1 cmfrom the edge. This neck phantom is recommendedby the Swiss national authorities and is herebyreferred to as the GEDI phantom (Figure 1).Simulations were performed with the GEDIphantom and various voxelised phantoms: theZubal VOXELMAN(1), MAX(2) and FAX(3), revisedversions MAX-06 and FAX-06(4), male and femaleICRP 110 adult reference computationalphantoms(5) and the paediatric phantom of the UF-B series(6, 7). Two iodine isotopes were considered inthe study: 125I [12.3, 45.6] keV and 131I [311.6,419.3] keV. The intervals correspond to the energyranges that were used for identifying the isotope anddetermining intake activity.Monte Carlo simulationsA cross-validation of the Monte Carlo simulationwas first performed using the GEANT4 andMCNP5 codes with the GEDI phantom. Thesecond step focused on studying the response of thedetector in units of count per second (cps) perBecquerel of iodine and compared the respectiveresponse of the monitor using the GEDI and voxe-lised phantoms. The third step focused on studyingthe variability of the response as a function of ana-tomical parameters.The volume containing the thyroid monitor wasset as close as possible to the thyroid organ.Practically, it had to be inserted into the voxelisedvolume of the phantom by removing outer voxels.Schematic views are shown on Figures 2 and 3.Anatomical variations were also simulated. Theinitial volume of the thyroid was then modified bysubstituting outer voxels associated with the thyroidglands by voxels of surrounding organs, or by chan-ging surrounding voxels by thyroid voxels.Furthermore, in addition to the reference neck, twophantoms with 19 and 76 cm circumference (i.e.factors 0.5 and 2 from the reference) were generatedby homothetic transformations of the three dimen-sions of each voxel. These extreme values are repre-sentative of the span of the size of regular necks. In afirst approximation, one can assume that individualsof different build can be modelled by a simple homo-thetic transformation of the standard human body.# The Author 2010. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.comRadiation Protection Dosimetry (2011), Vol. 144, No. 1–4, pp. 326–329 doi:10.1093/rpd/ncq312Advance Access publication 19 October 2010This correction can be directly related to the neck cir-cumference. A correction factor could thus be definedas a function of the size of the neck and normalisedto a reference phantom. The average circumference ofthe neck of children aged from 0 to 3 months is 21cm, and from 3 to 18 months is 23.5 cm(8).Concerning horizontal (i.e. depth off-set) and ver-tical (i.e. up/downwards shifts) displacements, allvoxels associated with the thyroid glands wereextracted and repositioned along the correspondingaxis, i.e. the corresponding voxels were displacedfrom the initial position by +2 and +2/+4 cm,respectively.RESULTSComparison of Monte Carlo codesThe GEDI phantom was implemented in bothGEANT4 and MCNP codes and the respectiveresponses of the detector were determined. Theresponses are comparable for 131I and 125I (Table 1).Comparison of phantomsThe size of the thyroid gland is similar in all adultvoxelised phantoms, but twice as large in the GEDIphantom, i.e. 40.6 cm3 to be compared with anaverage of 18 cm3. Due to the different anatomicalpositions in the phantoms, the distance from thethyroid to the skin surface varied from 2 cm foradult ICRP 110 phantoms to 3.6 cm for MAX. Thedetector was placed as close as possible to the skinsurface for each phantom. The distance from thethyroid to the crystal is not fixed, but close to 14 cm.A rather large discrepancy was observed whencomparing the response using different phantoms.Results are summarised in Figure 4. The differencewas more apparent for low-energy gamma emitters,typically for 125I. The response using the Voxelmanphantom was 40 % lower than those obtained withthe GEDI phantom for 131I and even 65 % lower for131I. The same effect, although less accentuated, wasobserved with other voxel phantoms.Study of the anatomical variability effectsResults are summarised in Table 2. The study of theeffects of the anatomical variations was made usingthe MAX phantom exclusively.Figure 1. The GEDI phantom.Figure 2. Schematic chart of the position of the thyroidmonitor on the GEDI phantom.Figure 3. Schematic chart of the position of the detectoron the ICRP 100 male phantom. Thyroid glands appear inlight colour.Table 1. Response of the detector in units of cps perBecquerel using two Monte Carlo codes, MCNP resultsbeing the reference to calculate the ratio and compare thetwo codes.Code 125I 131Icps Bq21 Ratio (%) cps Bq21 Ratio (%)MCNP 0.00894 100 0.00322 100GEANT 0.00901 101 0.00327 102VARIABILITY OF RADIOIODINE MEASUREMENTS IN THE THYROID327The maximum deviation from the response withthe reference phantom was observed for the smallestthyroid (2.6 g, 2.4 cm3, i.e. 1/8 of the reference size)and found to be 17 % for 125I and 10 % for 131I.The size of the neck is a key parameter. Its vari-ation has noticeable effects, in particular, for low-energy gamma emitters, i.e. 125I. Homothetictransformations of a factor 2 changed the normal-ised response of the detector by 50 % for 131I andby 70 % for 125I.The closer the thyroid was to the skin surface; thehigher the response of the detector. This effect wasmost noticeable for 125I. The detector responseincreased by 2 when the thyroid glands were moved2 cm closer to the skin surface.The variation of the vertical position was relevantonly for 125I. The effect was not observed when thethyroid was displaced upwards, but was clearlyobserved when the organ was displaced downwards.A vertical shift of 2 cm reduced the estimated detec-tor response by a factor 2.The distance between the skin surface and thecrystal is an important factor; the detector responsewas reduced by 10 % if the crystal was 1 cm furtherback and 40 % if displaced by 5 cm.DISCUSSIONSThe slight difference between the two Monte Carlocodes can be explained by the fact that the MCNPcode does not take into account the complete decayscheme of the radionuclide and does not take intoaccount corrections for summation effects.Measurements performed with 125I, a low-energygamma emitter, are generally more affected by geo-metrical changes, when compared with measure-ments of higher energy gamma from 131I.Geometrical effects explain the discrepancyobserved in the response of the detector. Indeed, thevials in the GEDI phantom are close to the surfacecompared with voxel phantoms and have a shorterpath length through matter. In Switzerland, thyroidmonitors are calibrated using the GEDI phantom.The responses of the detector obtained with voxelphantoms were up to 60 % lower than those esti-mated with the geometrical GEDI phantom for 125I,and 25 % lower for 131I. In consequence, thyroidmonitors that have been calibrated with the GEDIphantom clearly underestimate the intake activityand the calculated committed effective dose. Thisraises the question of the reliability of the monitors’calibration procedure and the possibility or need tointroduce a correction factor for determining theintake activity measured with thyroid monitors cali-brated using the GEDI phantom.Better agreements (higher than 90 % for 131I) werefound when smaller necks were considered, i.e.women, teenagers and children. The difference ismore apparent with low-energy gamma emitters like125I.Varying the volume of the thyroid glands does notsignificantly alter the detector’s response. Previousstudies have shown similar results, even goingfurther and showing that the shape of the glandsdoes not have a strong effect(8). Varying the horizon-tal position of the thyroid inside the phantom is alsoimportant. Indeed, in both cases, increasing the dis-tance from the thyroid to the detector and theinduced augmentation of the pathlength throughtissues significantly reduces the detector’s response.The effects of horizontal displacement of thethyroid gland are consistent with the exponentiallaw of attenuation of the radiation in tissue withFigure 4. Relative detector response for 125I and 131I, usingvoxelised phantoms, compare with the response obtainedwith the GEDI phantom.Table 2. Effects of the anatomic variations on the detectorresponse normalised to the response obtained with the GEDIphantom.Variation Scale factor Normalisedresponse MAX/GEDI125I (%) 131I (%)MAX—reference — 42 78Thyroid volume(fraction of MAX)1/8 49 861/2 46 802 43 778 44 80Neck diameter(fraction of MAX)1/2 92 1172 10 37Horizontal displacementof the thyroid (cm)22 92 117þ2 19 57Vertical displacement ofthe thyroid (cm)24 3 6822 27 72þ2 47 76þ4 42 75J. DAMET ETAL.328m ¼ 0.3 cm21 corresponding to the energy range of125I, and m ¼ 0.1 cm21 for 131I.Without correcting for the size of the person’sneck, the iodine activity in the thyroid may then beenclearly overestimated in the order of 50 % (respect-ively, 100 %) in the case of 131I (respectively, 125I).For an adult, neck circumferences vary from 30 to 45cm depending on body mass index and sex.Deviations span from –20 to þ12 % in the case of131I, and from –40 to þ20 % for 125I. A correctionfactor based on the size of an individual’s neck couldbe an effective and simple procedure to respond to amajor accident situation. This correction factorwould lead to a better estimation of the committedeffective dose.Uncertainties due to the position of the thyroidinside the neck could only be corrected after a clini-cal examination (i.e. ultrasound scan). This can bedone for standard measurements or a small acciden-tal intake but becomes a cumbersome procedurewhen used to respond to a major accident situation.Uncertainties due to the vertical position of thethyroid could be reduced by performing a scan todetermine the position of the highest response, butthis additional step in the overall procedure is timeconsuming.Studying the variability of the detector responseas a function of the displacement of the detector isbeyond the scope of this paper and it was assumedthat the centre of gravity of the thyroid glands andthe detector is aligned.The GEDI phantom could be slightly modified toget a closer response to ICRP 110 phantoms whichare used to defined the new committed effective dosecoefficients and therefore provide a better assessmentof the radiological risk. For instance, the position ofthe vials could be adapted and placed further awayfrom the surface. A more realistic thyroid shapecould also be implemented in the GEDI phantom asa means for improvements.CONCLUSIONUsing a thyroid monitor that has been calibratedwith the Swiss reference phantom leads to an under-estimation of the activity, in particular for an adultmale. This underestimation is estimated to be in theorder of 20 % for 131I and of 40–60 % for 125I. Theeffect is less pronounced for individuals with asmaller neck, for female and paediatric patients, forexample. Varying either the size or the mass of thethyroid gland has almost no effect on the measure-ment of the iodine activity(9). Determining the pos-ition of the thyroid is, however, the cornerstone foran accurate measurement, especially in the case of125I. A correction factor depending on the size ofthe neck of the patient could be introduced for abetter estimate of the radiological risks in the case oflarge-scale accidental events. Further uncertaintiesdue to the position of the thyroid inside the neckcould only be corrected after a clinical examination(i.e. ultrasound scan) and could be done in standardmeasurements of radioactive iodine incorporation.REFERENCES1. Zubal, I. G. et al. Computerized three-dimensional seg-mented human anatomy. Med. Phys. 21(2), 299–302(1994).2. Kramer, R. et al. All about MAX: a male adult voxelphantom for Monte Carlo calculations in radiation pro-tection dosimetry. Phys. Med. Biol. 48, 1239–1262(2003).3. Kramer, R. et al. All about FAX: a female adult voXelphantom for Monte Carlo calculation in radiation protec-tion dosimetry. Phys. Med. Biol. 49, 5203–5216 (2004).4. Kramer, R. et al. MAX06 and FAX06: update of twoadult human phantoms for radiation protection dosimetry.Phys. Med. Biol. 51, 3331–3346 (2006).5. International Commission on Radiological Protection.Adult Reference Computational Phantoms ICRPPublication 110. Ann. ICRP (2009).6. Lee, C. et al. The UF series of tomographic compu-tational phantoms of paediatric patients. Med. Phys.32(12), 3537–3548 (2005).7. Lee, C. et al. Whole-body voxel phantoms of paediatricpatients—UF series B. Phys. Med. Biol. 51, 4649–4661(2006).8. The handbook of the child measurements and capabilities.Department of Trade and Industry, DTI/Pub/1732/2k/2.96 AR, URN 95/681, ISBN 0-9522 571-1-4 (1995).9. Kramer, R. et al. The assessment of the effect of thyroidsize and shape on the activity estimate using Monte Carlosimulation. Health Phys. 78(6), 727–738 (2000).VARIABILITY OF RADIOIODINE MEASUREMENTS IN THE THYROID329

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