ORIGINAL ARTICLE

124I PET/CT in the pretherapeutic staging of differentiatedthyroid carcinoma: comparison with posttherapy131I SPECT/CT

Cecile de Pont & Servais Halders & Jan Bucerius &

Felix Mottaghy & Boudewijn Brans

Received: 8 August 2012 /Accepted: 18 December 2012 /Published online: 23 January 2013# Springer-Verlag Berlin Heidelberg 2013

AbstractPurpose To compare pretherapy 124I PET/CTand posttherapy131I SPECT/CT in the identification of pathological lesions andthe staging of patients with differentiated thyroid carcinoma.Methods 124I SPECT with low-dose CT in addition to astandard whole-body scan was performed 5 days following131I therapy with the administration of 1,110–7,728 MBq.Pretherapy 124I PET/CT was done 24 h and 96 h after oralingestion of 20–28 MBq, including a noncontrast high-doseCT scan. Scans were evaluated by two independent experi-enced nuclear physicians. In addition to the total number oflesions found, patient-based analyses and lesion-based anal-yses were performed to ascertain the discrepancies betweenthe findings of the two scanning techniques, as well as toevaluate the clinical impact of the findings.Results A group of 20 consecutive patients were analysed.In the lesion-based analysis, a total of 62 foci were foundwith all modalities together. Of these, 124I PET/CT found 57(92 %), 131I SPECT/CT 50 (81 %) and planar imaging 39(63 %). In the patient-based analysis, in 50 % of patientscomplete concordance between the findings of 124I PET and131I SPECT was seen, in 5 % complete discordance and inthe remaining 45 % partial discordance, i.e. a focus or somefoci seen with both modalities but another or others seen

more or less with one or other modality. In 5 of the 20patients (25 %), tumour stage was changed according tothe findings of one of the modalities. In 60 % of thesepatients this was only with the findings of 124I PET/CT.Conclusion This study showed that 124I PET/CT is preferredover 131I imaging for staging differentiated thyroid carcinoma.

Keywords Differentiated thyroid carcinoma . Radioiodine .

SPECT/CT . PET/CT . Iodine-124 . Iodine-131

Introduction

Hybrid tomographic scanning techniques in nuclear medi-cine have been proven to be of substantial incrementaldiagnostic value in the imaging of various diseases, includ-ing differentiated thyroid carcinoma [1]. In this regard, 131ISPECT/CT has been shown to be an effective adjunct toplanar imaging. Several imaging protocols have been eval-uated, i.e. pretherapy and posttherapy 131I SPECT/CT inpatients scheduled for remnant ablation, in patients withmetastatic disease or in mixed patient groups [2–6].

With the addition of posttherapy SPECT/CT to planarwhole-body scans (PTWBS) following 131I remnant ablationtherapy, nodal stage was changed from N0 or Nx to N1 in14 % of patients, with a resulting change of risk stratificationin 25 % of patients [2]. However, that study showed potentialunderestimation of the incidence of microscopic disease incervical lymph node metastases with WBS as well as 131ISPECT/CT. In patients with metastatic disease [3, 4], theaddition of posttherapy SPECT/CT to PTWBS substantiallyimproved the specificity by reducing false-positives by bettercharacterization, localization of thyroid tissue and differenti-ating it from physiological uptake. However, in the study byOh et al. [4], the diagnostic sensitivity determined in a lesion-

C. de Pont : S. Halders : J. Bucerius : F. Mottaghy : B. Brans (*)Department of Nuclear Medicine, Maastricht University MedicalCenter, Postbox 5800, 6202 AZ Maastricht, The Netherlandse-mail: b.brans@mumc.nl

J. Bucerius : F. MottaghyDepartment of Nuclear Medicine, University Hospital RWTHAachen, Aachen, Germany

J. BuceriusCardiovascular Research Institute Maastricht (CARIM), MaastrichtUniversity Medical Center, Maastricht, The Netherlands

Eur J Nucl Med Mol Imaging (2013) 40:693–700DOI 10.1007/s00259-012-2331-5

based analysis was only slightly better (63 % versus 55 % forPTWBS) but determined in a patient-based analysis was thesame (both 65 %).

Pretherapy (low-activity) 131I SPECT/CT in the follow-up of patients with suspicion of persistent locoregionaland/or metastatic disease provided an incremental value inthe characterization of foci (i.e. better localization, identifi-cation and differentiation) in 68 % of patients [5], and led toa change in therapeutic management (i.e. indication for 131Ior other therapy, 131I activity dosing) in 36–58 % of patients[5, 6]. However, again the effect on diagnostic sensitivity ofthe addition of SPECT/CTwas not as significant as found inthe study by Spanu et al. [5], which showed 131I SPECT/CTdiscordantly positive for metastatic disease with a negativeWBS in only 7 of 117 patients (6 %). On the other hand, 124IPET/CT has emerged as a powerful diagnostic and dosim-etry tool providing quantitative images with superior spatialresolution and image sensitivity [7]. Several studies haveshown its superiority over pretherapy [8, 9] as well asposttherapy [10, 11] planar 131I WBS. However, until now,no studies have addressed its diagnostic value in comparisonto that of 131I SPECT/CT.

Materials and methods

Patients

A total of 20 consecutive patients were included in this retro-spective analysis. These patients consisted of those who hadreceived both posttherapy 131I SPECT/CT and pretherapy 124IPET/CT, the former performed as a standard part of postther-apy 131I scanning in our institution and the latter performed inthose with larger thyroid remnants and/or suspected metastaticdisease (introduced as an additional diagnostic approach inour department with the aim of improving patient care, i.e. toavoid unnecessary high doses of 131I). Secondary inclusioncriteria were: (1) preparation by thyroid hormone withdrawal(THW) or by administration of recombinant human thyroid-stimulating hormone (rhTSH, thyrotropin alfa), (2) a maxi-mum of 8 weeks between 124I PET/CT and 131I SPECT/CT,and (3) availability of correlative imaging to determine thenature of positive 124I/131I PET/SPECT findings. 131I therapywas typically administered orally according to a fixed activitydosing protocol employing 2,775 MBq (75 mCi) for simplethyroid remnant ablation and 5,555 MBq (150 mCi) for re-gional nodal disease or distant metastasis.

Image acquisitions

Patients who received THW preparation, were withdrawnfrom tetraiodothyroxine (T4) medication for 4–6 weeks, orremained without medication postoperatively. In two patients,

triiodothyroxine (T3) was administered between the 124IPET/CT scan and the 131I therapy/SPECT/CT scan for logis-tical reasons resulting in a longer time between 124I PET/CTand 131I SPECT/CT (7 and 8 weeks, respectively). In the otherpatients, the time between 124I PET/CT and 131I SPECT/CTwas 1–2 weeks. At the time of oral administration of 124I,serum TSH levels were determined to assess the level ofendogenous stimulation. 124I PET/CT was performed after24 h and 96 h (Fig. 1). In patients who received rhTSHpreparation, 0.9 mg rhTSH (Thyrogen; Genzyme, Haverhill,UK) was injected intramuscularly on days 1 and 2. 124I wasthen administered orally on day 4, and 124I PET/CT wasperformed on day 5 (24 h after 124I administration) and day8 (96 h after 124I administration) (Fig. 1).With subsequent 131Itherapy, rhTSH was reinjected on days 1 and 2, and then 131Iwas administered on day 3, and 131I PTWBS and 131ISPECT/CT were performed on day 8 (120 h after injection).In one patient, scanning and therapy were done within 1 week.In the other patients, the time between scanning and therapywas 4–8 weeks.

124I PET/CT was performed using a PET cameraequipped with time-of-flight (Gemini TF PET/64-slice CTscanner; Philips, Best, The Netherlands). PET scans wereacquired from the head, neck and thorax using four or fivebed positions of 4 min each. In selected cases imaging wascontinued as far as the pelvis. The standard energy windowof 439–664 keV was set. Reconstruction was performedusing a standard protocol with a matrix size of 288, resultingin a voxel size of 2.0×2.0×2.0 mm, with a standard kerneltruncation of 14.1. A standard CT scan was performed inlow-dose mode (120 keV, 30 mAs, 4-mm slice thickness,4.0 increment) with a supplementary high-dose CT scanwithout administration of contrast agent (120 keV, 150mAs, slice thickness 2 mm, increment 1.8 and matrix512×512 for the neck; 120 keV, 175 mAs, slice thickness5 mm, increment 4.0 and matrix 512×512 for the thorax).

131I SPECT/CT was performed using a standardSPECT/CT camera (Precedence SPECT/6-slice CT scanner;Philips, Best, The Netherlands) equipped with dual 1.6-cm γ-detectors equipped with high-energy general-purpose collima-tors. SPECT data were obtained using a noncircular orbit, a128×128×16 matrix (voxel size 4.7×4.7×4.7 mm) and 32angles over 180° and 45 s per stop, using a 364-keV photopeak with a 10 % window. Reconstruction space and widthwas 3mmusing the Philips Astonish algorithm. TheWBSwasacquired from head to toe using a table speed of 10 cm/min.SPECT/CTwas performed immediately after planar imaging.

Data analysis

The 131I PTWBS SPECT/CT and 124I PET/CT scans wereanalysed separately and independently by two experiencednuclear medicine physicians aware of the clinical findings,

694 Eur J Nucl Med Mol Imaging (2013) 40:693–700

other diagnostic imaging data and definite histopathologicaldiagnosis. Discrepancies were resolved by consensus. The 124IPET/CT findings were compared to the 131I WBS and 131ISPECT/CT findings. All results were correlated with the high-dose CT scan findings. Additionally, ultrasonography withcytology of abnormalities suspicious for malignancy, MRIand/or 18F-FDG PET/CT were done to confirm positive, dis-crepant, unexpected and/or unresolved findings, if necessary.

Abnormal findings on 124I PET/CT, 131I SPECT/CT andplanar imaging were defined as areas of focally increaseduptake not attributable to physiological activity. Foci werelocalized as (1) paratracheal, being immediately adjacent tothe tracheal space, classified as thyroid remnants; (2) thyroidbed, as identifiable on the CT scan, to be correlated with otherimaging modalities; (3) centrally positioned foci above thethyroid bed, classified as thyroid remnants; (4) centrally posi-tioned foci below the thyroid bed, to be correlated with otherimaging modalities; (5) laterally localized foci, classified aslymph node metastases; and (6) distantly localized foci, clas-sified as distant metastases. To obtain the total number of foci,all foci were separately scored except in one patient withdiffuse micronodular metastases of both lungs which werearbitrarily scored as two foci. Foci were regarded positive onPET if they were visible on the scan on day 1 as well as day 4.

Statistics

Differences in discordance between the THW and rhTSHgroups were assessed using an independent samples Mann-Whitney U test. P values <0.05 were considered statisticallysignificant.

Results

Patient characteristics

Table 1 shows the patient characteristics. The group con-sisted of 12 men and 8 women aged between 17 and 87 years(mean 53, median 58 years). Most (14 of 20) patients who

received THW preparation. The use of rhTSH preparationwas restricted to three patients with (very) low-risk disease(patients 15, 16 and 19) and to three patients with metastaticdisease because of poor tolerability (patients 17, 18 and 20).

Patient-based analysis

Table 2 shows the results of 124I PET, 131I SPECT and planarimaging. Comparing 124I PET and 131I SPECT, overall in 10of the 20 patients (50 %) completely concordant results wereseen. In one patient, the results were completely discordant.The remaining 9 patients (45 %) showed partially discordantresults, with a focus or some foci seen with both modalitiesbut another or others seen more or less with one or othermodality. Of the 10 patients with partial or complete discor-dance, 4 (40 %) received rhTSH preparation and six (60 %)THW preparation. In other words, 4 of 6 (67 %) whoreceived rhTSH preparation showed discordance versus 6of 14 patients (43 %) who received THW preparation. Thisdifference was not statistically significant (p=0.342, inde-pendent samples Mann-Whitney U test).

In 5 of the 20 patients (25 %), tumour stage was changedas a result of discrepant findings between the modalities(Table 2). In 3 of these patients (patients 2, 12, 16; 3 of20, 15 %) this was only by 124I PET/CT: nodal statusupstaged to a N1a or N1b in comparison to the othermodalities (patients 2 and 16) and a solitary lung metastasisupstaged to M1 (patient 12). In one patient (patient 3), 124IPET/CT and 131I SPECT/CT findings led to the sameupstaging of nodal status as compared to planar imaging.In one patient (patient 14), M1 disease was only seen byplanar imaging, and not by 124I PET or 131I SPECT.

Lesion-based analysis

A total of 62 foci were found by all modalities together.Of these, 124I PET/CT found 57 of 62 (92 %), 131ISPECT/CT found 50 (81 %) and planar imaging found39 (63 %). Thus, 124I PET found more foci than either131I PTWBS or SPECT.

D1 D3D2 D4 D5 D8D7D6

(0.9 mg rhTSH)

(0.9 mg rhTSH)

1110-7728 MBq131I

20-28 MBq124I

124I PET/CT

124I PET/CT

131I WBS + SPECT/CT

(0.9 mg rhTSH)

(0.9 mg rhTSH)

Fig. 1 Flow diagram of thediagnostic and therapeuticprocedures

Eur J Nucl Med Mol Imaging (2013) 40:693–700 695

Tab

le1

Patient

characteristics

Patient

Gender

Age

(years)

Pathologicalstage

Typ

ePrior

131I(M

Bq)

Indicatio

nPreparatio

nAdm

inisteredactiv

ity(M

Bq)

Interval(w

eeks)

TSH(m

U/l)

a

124I

131I

1M

57pT

1mN0

Follicular

No

Ablation

THW

242,77

58

111

2F

42PT1N

xPapillary

No

Ablation

THW

252,77

52

71.7

3M

27pT

2Nx

Papillary

No

Ablation

THW

272,77

52

135

4M

27pT

2Nx

Papillary

No

Ablation

THW

232,77

52

140

5M

58pT

3N0

Papillary

No

Ablation

THW

232,77

52

110

6M

82pT

3N0

Follicular

No

Ablation

THW

222,77

51

27.4

7F

39pT

3N1a

Papillary

Yes

(2,775

)Residualdisease

THW

245,55

07

78.1

8F

26pT

3N1b

Papillary

No

Ablation

THW

262,77

51

n.d.

9M

33pT

3Nx

Follicular

No

Ablation

THW

222,77

52

58.9

10M

87pT

3Nx

Papillary

No

Ablation

THW

242,77

52

n.d.

11M

68pT

4aN0

Papillary

No

Ablation

THW

222,77

51

72.6

12F

82pT

4aN0

Papillary

No

Ablation

THW

2877

281

61.5

13F

72pT

4aN1a

Papillary

No

Ablation

THW

262,77

52

68.7

14M

72pT

4aN1b

Papillary

No

Ablation

THW

242,77

52

n.d.

15M

63pT

1N0

Follicular

Yes

(2,775

)Residual

rhTSH

201,110

642

.8

16F

17pT

1Nx

Papillary

No

Ablation

rhTSH

222,77

57

94.5

17F

44pT

1N1b

Papillary

Yes

(2,775

)Recurrence

rhTSH

232,77

54

n.d.

18M

67pT

2N1b

Papillary

Yes

(13,87

5)Recurrence

rhTSH

205,55

08

177

19M

24pT

2N0

Follicular

No

Ablation

rhTSH

222,77

55

91.8

20F

72pT

xNx

Follicular

Yes

(14,98

5)Residualdisease

rhTSH

225,55

01

7.8

n.d.

nodata.

aSerum

TSH

levelsin

(mU/l)

atthedayof

124Iadministration.

696 Eur J Nucl Med Mol Imaging (2013) 40:693–700

Regarding paratracheal foci, 124I PET/CT found the highestnumber of foci, but in only one patient (patient 18) was thyroidremnant demonstrated by PET but not by the other modalities.Regarding foci in the thyroid bed, 124I PET and 131I SPECTwere highly concordant; only in one patient (patient 16) a focuswas seen only on 124I PET/CT, and this focus was confirmedby 18F-FDG PET/CT. Regarding foci in the upper centralcervical region, the findings were quite similar; only in onepatient (patient 11) was a focus seen only on 124I PET/CT (andplanar imaging). In the lower central cervical region, only onefocus was found by 131I SPECT/CT but not by 124I PET/CT,and this focus was confirmed as lymph node metastasis byMRI (patient 14). In four patients (patients 2, 3, 12 and 14)lateral lymph nodes were found and confirmed as metastases;the findings were often discrepant between modalities, but the

overall detection rate was highest for 124I PET/CT, as shown inFig. 2. Finally, 124I PET/CTwas clearly most sensitive in termsof the number of detected distant metastases (patients 12 and20), with the exception of one patient (patient 14) with micro-nodular lung metastases that were only seen on planar imaging.

Day 1 and day 4 124I PET/CT images were generallysimilar. Positive lesions were usually clearly visible on bothdays. However, the target-to-background ratio improvedover this time, making lesion detection easier.

Discussion

This study showed superior results with pretherapy 124IPET/CT than with posttherapy 131I planar imaging and

Table 2 Findings of 124I PET/CT, 131I SPECT/CT and 131I planar imaging in thyroid carcinoma

Patient Thyroglobulin(ng/ml)

Number and localization of foci

Paratracheal Thyroid bed Upper centralcervical

Lower central cervical Lateral neck Distant

PET/SPECT/planar scans

PET/SPECT/planar scans

Interpretation PET/SPECT/planar scans

PET/SPECT/planar scans

Interpretation PET/SPECT/planar scans

PET/SPECT/planar scans

1 <0.25 1/1/1 Remnant, byUS/cytology

2a 0.7 1/1/1 Remnant, byUS/cytology

1/1/1 1/0/0

3a 68 2/1/1 1/1/1 1/1/0

4 8.2 Negative

5 1.3 1/1/1 1/1/1 Remnant, by CT

6 4,028 3/2/1 1/1/1 (skull)

7 <0.25 1/1/1 Lymph nodemetastasis, byCT

8 12.2 1/1/1

9 6.6 2/2/2

10 <0.25 2/2/2 Remnant, by CTand US/cytology

11 44 1/1/1 Remnant, by CT 1/0/1

12a 803 2/2/2 2/1/0 1/0/0 (lung)

13 2.6 3/3/3

14a Not performed 2/2/1 0/1/0 Lymph nodemetastasis, byMRI

0/2/0 0/0/2 (lung)

15 0.9 1/2/1 1/1/0 Remnant, byUS/cytology

1/1/1

16a <0.25 1/1/1 1/0/0 Lymph nodemetastasis, byFDG PET

17 <0.25 Negative

18 7.8 1/0/0

19 1.2 3/3/2 1/1/0 Lymph nodemetastasis, byCT

20 36,790 14/11/9

Total 21/19/15 10/9/7 5/4/5 0/1/0 4/4/0 16/12/12

a Patients in whom discrepant findings resulted in a change in tumour stage.

Eur J Nucl Med Mol Imaging (2013) 40:693–700 697

SPECT/CT, resulting in a clinically relevant upstaging in15% of the included, consecutive and clinically representativepatient group. We believe this difference was related to false-negative findings with SPECT/CT as a result of the inferiorspatial resolution and camera sensitivity, despite much higheradministered activities, and not to false-positive findings withPET/CT, as positive findings were verified by correlativeimaging and/or cytology. The most important advantage ofpre- rather than posttherapy staging is that it provides optimalrisk stratification before therapy, disclosure of unexpectedsites influencing specific therapy (surgery, radioiodine, localtherapy), prognostic information, and the opportunity for le-sion dosimetry [6]. Although high-activity radioiodine admin-istration as empirical therapy, together with posttherapydiagnostic scintigraphy and 131I SPECT/CT (because of thelow sensitivity of planar 131I imaging and the risk of stunning),is still a widely accepted practice, the long-term side effects ofthis practice have increasingly been emphasized [12, 13].Presently, there are no studies in the literature that havecompared pre- and posttherapy 131I SPECT/CT scanning.

Theoretically, the sensitivity of radioiodine imagingdepends on the administered radioactivity, implying thatlow-activity pretherapy planar 131I scanning would have alower detection rate than a high-activity posttherapy scan[14]. Therefore, although our study population was small,

extrapolation of this would imply a higher detection rate with124I PET/CT than with posttherapy as well as pretherapy 131ISPECT/CT. Several authors have indicated the usefulness ofpretherapy 123I imaging [15, 16]. Indeed, high-quality imagescan be obtained and SPECT/CT performed. Unfortunately, upto now no studies have compared pretherapy 124I PET/CTwith pretherapy 123I SPECT/CT. However, the big downsideof 123I is its short half-life of 12.3 h (as compared to 4.2 daysfor 124I) which is insufficient to accurately model the washoutphase of radioiodine from the target lesion beyond 24 h,during which most of the radiation dose is delivered.Because the resulting effective half-life of radioiodine variesbetween patients and even between lesions, accurate doseestimates can only be derived from individual calculations ofthis washout phase [17].

An important question in this study is whether confound-ing factors other than those related to differences in perfor-mance of the imaging techniques could have influenced theobserved differences. For logistic reasons, there were slightdifferences between the imaging schedules. Patients re-ceived 124I 1 day later than 131I following rhTSH adminis-tration on 2 days. However, as can be seen from Table 1,TSH values at the time of 124I administration generallyindicated a maximum stimulatory effect. Second, scanningwas done on day 5 for 131I as compared to day 4 for 124I, and

Fig. 2 Comparison betweenpretherapy 124I PET/CT andposttherapy 131I SPECT/CTimages. On the 124I PET/CTMIP image (top left) and on thefused PET/CT image (topright), a lateral neck focus isclearly seen (arrows). On theCT image (centre), this focuscorresponds to a small lymphnode (arrow). On thecorresponding 131I SPECT/CTimages (bottom), noabnormality is seen in this area

698 Eur J Nucl Med Mol Imaging (2013) 40:693–700

with much higher, posttherapeutic activities. 131I SPECTcould also have been more sensitive because of more intensestimulation by TSH, either endogenous in the THW groupbecause of a slightly longer T4 withdrawal time before 131ISPECT as compared 124I PET, or exogenous in the rhTSHgroup after a second pair of rhTSH injections as comparedto a single pair before 124I. In the rhTSH group, there was ahigher, although not statistically significantly higher, num-ber of patients with discordance between the PET/CT andSPECT/CT findings, than in the THW group. Concern hasbeen expressed that rhTSH preparation may not be equal toTHW preparation [18]. Our patient sample was too small toallow a comparison between THW and rhTSH preparationThe higher discordance between the PET/CT andSPECT/CT findings in patients who received rhTSH prep-aration could be explained by the fact that it is more chal-lenging to achieve the same degree of TSH stimulation in arhTSH diagnostic/therapeutic protocol than with THW be-cause of double versus single rhTSH stimulation, and/or avariable degree of TSH suppression by the administration ofT4/T3 in the time between diagnostic scanning and therapy.Further studies in larger groups will allow differences be-tween THW and rhTSH to be evaluated, and this is also animportant factor to consider in dosimetry oriented trials.

In one patient, diffuse micronodular lung metastases werevisible on planar imaging, but were false-negative on 124IPET/CT as well as on 131I SPECT/CT. This has previouslybeen reported by others [7]. In our patient, a subtle increasein lung activity on planar imaging caused by a very moder-ate uptake of 131I by the metastases apparently faded awayin the background activity after PET as well as after SPECTreconstruction. In such patients, a quantitative index hasbeen suggested for use as an aid in 124I PET/CT [7].Careful inspection of the CT portion is absolutely necessaryto pick up apparently low or absent uptake in lung metasta-ses in order to avoid this problem.

The clinical heterogeneity in our population in terms oftumour stages I–IVand indications for 131I (ablation, or resid-ual or recurrent disease) may be viewed as a limitation of thisstudy whose aim was to include a clinically representativepatient population. Although a total of 62 evaluated lesions isa reasonable number, results in this relatively small patientsample should be confirmed by others. We did not study theclinical relevance of a pretherapy PET/CT or pre- or post-therapy SPECT/CT staging procedure as such, but merely theperformance of these modalities. Although it is generallyaccepted that pretherapy staging using nuclear medicine tech-niques is useful in patients with high-risk disease, this may notbe the case for patients with low-risk disease. In our institu-tion, patients eligible for 124I PET/CT represent a higherpercentage of patients with high-risk disease as compared toother patient populations with differentiated thyroid cancer,and thus a higher pretest probability of abnormal findings was

expected in this population. We did not use histological con-firmation of lesions as the gold standard, as this is not feasiblein an average patient group in whom neck surgery has beenperformed and in whom sampling of distant sites carries asubstantial risk of complications. We relied on correlativeimaging using ultrasonography, CT, MRI, 18F-FDG PET/CTand cytological sampling, as indicated, to verify positive anddiscrepant findings, for example for the distinction betweenlocal remnants and local lymph node metastases, or differen-tiation between distant metastases and physiological activity.The purpose of our study was to compare 124I PET/CT with131I SPECT/CT, not to compare with these other modalities. Inthis series, high-dose diagnostic CTwithout administration ofcontrast agent was used to aid the identification of the ana-tomical substrate at sites of small thyroid remnants and/orlymph node metastases in the neck, as well as the diagnosisof (micronodular) pulmonary metastases. Recently, combined124I-PET/MRI has provided a further advance in the field,enhancing diagnostic sensitivity for lesions <10 mm. andimproving pretherapeutic lesion dosimetry [19].

Conclusion

This study showed that 124I PET/CT is preferred over 131Iimaging for staging differentiated thyroid carcinoma. Furtherstudies should be done to confirm these results, to investigatepossible differences between withdrawal or rhTSH prepara-tion and to study the impact of pretherapy 124I imaging onoutcome variables such as patient survival and quality of lifeand its cost effectiveness. The unique capabilities of 124IPET/CT and in the future PET/MRI may allow optimal do-simetry in small and large lesions, allowing individual radio-therapy planning.

Acknowledgments The technical and logistic support of Florence deVreede-Lacroix and Renee Franssen is highly appreciated.

Conflicts of interest None.

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