124I PET/CT to Predict the Outcome of Blind 131I Treatment in Patients with Biochemical Recurrence of Differentiated Thyroid Cancer: Results of a Multicenter Diagnostic Cohort Study (THYROPET)

Zirconium- 89 Labeled Antibody K1-70 for PET Imaging of Thyroid-stimulating Hormone Receptor Expression in Thyroid Cancer

PURPOSE

Thyroid-stimulating hormone receptor (TSHR) is a G-protein coupled receptor that is highly expressed on benign and malignant thyroid tissues. TSHR binding and activation has long been a component of thyroid cancer molecular imaging and radiotherapy, by promoting expression of the sodium-iodide symporter (NIS) and incorporation of I-131 into thyroid hormones. Here, we report the radiosynthesis and preclinical evaluation of a Zirconium-89 (89Zr) labeled TSHR antibody to serve as a positron emission tomography (PET) diagnostic correlate for therapeutic agents targeting TSHR without reliance on NIS.

PROCEDURES

TSHR human monoclonal antibody K1-70 was conjugated to chelator desferrioxamine-p-benzyl-isothiocyanate, followed by labeling with Zr-89, yielding the radiotracer 89Zr-DFO-TSHR-Ab. The in vitro cellar uptake and binding affinity of 89Zr-DFO-TSHR-Ab were analyzed in three new TSHR stable overexpressing tumor cell lines and their corresponding wild types (WT) with low or no TSHR expression. 89Zr-DFO-TSHR-Ab PET/CT imaging of TSHR expression was evaluated in tumor mouse models bearing one TSHR-positive tumor and other negative control with or without the coinjection of antibody K1-70, and then verified by radiotracer biodistribution study and tumor immunohistochemistry (IHC).

RESULTS

The conjugate DFO-TSHR-Ab was labeled with Zr-89 at 37 °C for 60 min and purified by PD-10 column in radiochemical yields of 68.8 ± 9.9%, radiochemical purities of 98.7 ± 0.8%, and specific activities of 19.1 ± 2.7 mCi/mg (n = 5). In vitro cell studies showed 89Zr-DFO-TSHR-Ab had significantly high uptake on TSHR expressing tumor cells with nanomolar affinity and high potency. Preclinical PET/CT imaging revealed that 89Zr-DFO-TSHR-Ab selectively detected TSHR expressing thyroid tumors and displayed improved in vivo performance with the coinjection of unlabeled TSHR antibody K1-70 leading to higher uptake in TSHR expressing tumors than parental WT tumors and physiologic tissues; this observation was confirmed by the biodistribution and immunostaining analyses.

CONCLUSIONS

We synthesized 89Zr-labeled antibody K1-70 as a new radiopharmaceutical for PET imaging of TSHR. 89Zr-DFO-TSHR-Ab has high radioactive uptake and retention in TSHR expressing tumors and cleared quickly from most background tissues in mouse models. Our study demonstrated that 89Zr-DFO-TSHR-Ab has the potential for PET imaging of TSHR-positive thyroid cancer and monitoring TSHR-targeted therapy.

The use of single-timepoint images to link administered radioiodine activity (MBq) to a prescribed lesion radiation-absorbed dose (cGy): a regression-based prediction interval tool for the management of well-differentiated thyroid cancer patients

PURPOSE

To introduce a biomarker-based dosimetry method for the rational selection of a treatment activity for patients undergoing radioactive iodine 131I therapy (RAI) for metastatic differentiated thyroid cancer (mDTC) based on single-timepoint imaging of individual lesion uptake by 124I PET.

METHODS

Patients referred for RAI therapy of mDTC were enrolled in institutionally approved protocols. A total of 208 mDTC lesions (in 21 patients) with SUVmax > 1 underwent quantitative PET scans at 24, 48, 72, and 120 h post-administration of 222 MBq of theranostic NaI-124I to determine the individual lesion radiation-absorbed dose. Using a general estimating equation, a prediction curve for biomarker development was generated in the form of a best-fit regression line and 95% prediction interval, correlating individual predicted lesion radiation dose metrics, with candidate biomarkers ("predictors") such as SUVmax and activity in microcurie per gram, from a single imaging timepoint.

RESULTS

In the 169 lesions (in 15 patients) that received 131I therapy, individual lesion cGy varied over 3 logs with a median of 22,000 cGy, confirming wide heterogeneity of lesion radiation dose. Initial findings from the prediction curve on all 208 lesions confirmed that a 48-h SUVmax was the best predictor of lesion radiation dose and permitted calculation of the 131I activity required to achieve a lesional threshold radiation dose (2000 cGy) within defined confidence intervals.

CONCLUSIONS

Based on MIRD lesion-absorbed dose estimates and regression statistics, we report on the feasibility of a new single-timepoint 124I-PET-based dosimetry biomarker for RAI in patients with mDTC. The approach provides clinicians with a tool to select personalized (precision) therapeutic administration of radioactivity (MBq) to achieve a desired target lesion-absorbed dose (cGy) for selected index lesions based on a single 48-h measurement 124I-PET image, provided the selected activity does not exceed the maximum tolerated activity (MTA) of < 2 Gy to blood, as is standard of care at Memorial Sloan Kettering Cancer Center.

TRIAL REGISTRATION

NCT04462471, Registered July 8, 2020. NCT03647358, Registered Aug 27, 2018.

Towards an era of precise diagnosis and treatment: Role of novel molecular modification-based imaging and therapy for dedifferentiated thyroid cancer

Dedifferentiated thyroid cancer is the major cause of mortality in thyroid cancer and is difficult to treat. Hence, the essential molecular mechanisms involved in dedifferentiation should be thoroughly investigated. Several studies have explored the biomolecular modifications of dedifferentiated thyroid cancer such as DNA methylation, protein phosphorylation, acetylation, ubiquitination, and glycosylation and the new targets for radiological imaging and therapy in recent years. Novel radionuclide tracers and drugs have shown attractive potential in the early diagnosis and treatment of dedifferentiated thyroid cancer. We summarized the updated molecular mechanisms of dedifferentiation combined with early detection by molecular modification-based imaging to provide more accurate diagnosis and novel therapeutics in the management of dedifferentiated thyroid cancer.

Positron Emission Tomography Radiopharmaceuticals in Differentiated Thyroid Cancer

Differentiated thyroid cancer (DTC), arising from thyroid follicular epithelial cells, is the most common type of thyroid cancer. Despite the well-known utilization of radioiodine treatment in DTC, i.e., iodine-131, radioiodine imaging in DTC is typically performed with iodine-123 and iodine-131, with the current hybrid scanner performing single photon emission tomography/computed tomography (SPECT/CT). Positron emission tomography/computed tomography (PET/CT) provides superior visualization and quantification of functions at the molecular level; thus, lesion assessment can be improved compared to that of SPECT/CT. Various types of cancer, including radioiodine-refractory DTC, can be detected by 2-[¹⁸F]fluoro-2-deoxy-D-glucose ([¹⁸F]FDG), the most well-known and widely used PET radiopharmaceutical. Several other PET radiopharmaceuticals have been developed, although some are limited in availability despite their potential clinical utilizations. This article aims to summarize PET radiopharmaceuticals in DTC, focusing on molecular pathways and applications.

FDG PET/CT in differentiated thyroid cancer patients with low thyroglobulin levels

OBJECTIVE

To evaluate the usefulness of [18F]fluorodeoxyglucose (FDG) positron emissive tomography (PET)/CT in patients with low detectable thyroglobulin levels suspicious for persistent or recurrent differentiated thyroid cancer (DTC).

METHODS

A retrospective case series study evaluating FDG PET/CT in patients with detectable thyroglobulin (Tg) levels (≥0.20 and <10.00 ng/mL) after initial treatment with total thyroidectomy and I-131 thyroid remnant ablation for pT1-3aN0-1bM0 DTC. Sensitivity, specificity, positive (PPV) and negative predictive value (NPV) of FDG PET/CT were calculated.

RESULTS

Twenty-seven patients underwent FDG PET/CT. Median Tg level at FDG PET/CT was 2.00 ng/mL (range 0.30-9.00). FDG PET/CT was positive in 14 patients (51.9%): lesions suspicious for lymph node metastases were depicted in 12 patients, and lung metastases in 2. DTC was confirmed in 13/14 FDG PET/CT-positive patients. In 9/13 patients with a negative FDG PET/CT, DTC was confirmed ≤3 months after FDG PET/CT. The sensitivity, PPV, specificity and NPV were 59.1, 92.9, 80.0 and 30.8%, respectively.

CONCLUSIONS

This case series shows that FDG PET/CT might be useful to detect persistent or recurrent DTC in patients with low detectable Tg. However, when FDG PET/CT is negative, this does not rule out DTC and further investigations are necessary.

Implications of physics, chemistry and biology for dosimetry calculations using theranostic pairs

Theranostics is an emerging paradigm that combines imaging and therapy in order to personalize patient treatment. In nuclear medicine, this is achieved by using radiopharmaceuticals that target identical molecular targets for both imaging (using emitted gamma rays) and radiopharmaceutical therapy (using emitted beta, alpha or Auger-electron particles) for the treatment of various diseases, such as cancer. If the therapeutic radiopharmaceutical cannot be imaged quantitatively, a “theranostic pair” imaging surrogate can be used to predict the absorbed radiation doses from the therapeutic radiopharmaceutical. However, theranostic dosimetry assumes that the pharmacokinetics and biodistributions of both radiopharmaceuticals in the pair are identical or very similar, an assumption that still requires further validation for many theranostic pairs. In this review, we consider both same-element and different-element theranostic pairs and attempt to determine if factors exist which may cause inaccurate dose extrapolations in theranostic dosimetry, either intrinsic (e.g. chemical differences) or extrinsic (e.g. injecting different amounts of each radiopharmaceutical) to the radiopharmaceuticals. We discuss the basis behind theranostic dosimetry and present common theranostic pairs and their therapeutic applications in oncology. We investigate general factors that could create alterations in the behavior of the radiopharmaceuticals or the quantitative accuracy of imaging them. Finally, we attempt to determine if there is evidence showing some specific pairs as suitable for theranostic dosimetry. We show that there are a variety of intrinsic and extrinsic factors which can significantly alter the behavior among pairs of radiopharmaceuticals, even if they belong to the same chemical element. More research is needed to determine the impact of these factors on theranostic dosimetry estimates and on patient outcomes, and how to correctly account for them.

Silicon-photomultiplier-based PET/CT reduces the minimum detectable activity of iodine-124

The radioiodine isotope pair 124I/131I is used in a theranostic approach for patient-specific treatment of differentiated thyroid cancer. Lesion detectability is notably higher for 124I PET (positron emission tomography) than for 131I gamma camera imaging but can be limited for small and low uptake lesions. The recently introduced silicon-photomultiplier-based (SiPM-based) PET/CT (computed tomography) systems outperform previous-generation systems in detector sensitivity, coincidence time resolution, and spatial resolution. Hence, SiPM-based PET/CT shows an improved detectability, particularly for small lesions. In this study, we compare the size-dependant minimum detectable 124I activity (MDA) between the SiPM-based Biograph Vision and the previous-generation Biograph mCT PET/CT systems and we attempt to predict the response to 131I radioiodine therapy of lesions additionally identified on the SiPM-based system. A tumour phantom mimicking challenging conditions (derived from published patient data) was used; i.e., 6 small spheres (diameter of 3.7-9.7 mm), 9 low activity concentrations (0.25-25 kBq/mL), and a very low signal-to-background ratio (20:1). List-mode emission data (single-bed position) were divided into frames of 4, 8, 16, and 30 min. Images were reconstructed with ordinary Poisson ordered-subsets expectation maximization (OSEM), additional time-of-flight (OSEM-TOF) or TOF and point spread function modelling (OSEM-TOF+PSF). The signal-to-noise ratio and the MDA were determined. Absorbed dose estimations were performed to assess possible treatment response to high-activity 131I radioiodine therapy. The signal-to-noise ratio and the MDA were improved from the mCT to the Vision, from OSEM to OSEM-TOF and from OSEM-TOF to OSEM-TOF+PSF reconstructed images, and from shorter to longer emission times. The overall mean MDA ratio of the Vision to the mCT was 0.52 ± 0.18. The absorbed dose estimations indicate that lesions ≥ 6.5 mm with expected response to radioiodine therapy would be detectable on both systems at 4-min emission time. Additional smaller lesions of therapeutic relevance could be detected when using a SiPM-based PET system at clinically reasonable emission times. This study demonstrates that additional lesions with predicted response to 131I radioiodine therapy can be detected. Further clinical evaluation is warranted to evaluate if negative 124I PET scans on a SiPM-based system can be sufficient to preclude patients from blind radioiodine therapy.

Predictive Factors for RAI-Refractory Disease and Short Overall Survival in PDTC

BACKGROUND

The clinical phenotype of poorly differentiated thyroid cancer (PDTC) can vary substantially. We aim to evaluate risk factors for radioiodine refractory (RAI-R) disease and reduced overall survival (OS).

METHODS

We retrospectively screened our institutional database for PDTC patients. For the assessment of RAI-R disease, we included patients who underwent dual imaging with ¹⁸F-FDG-PET and ¹²⁴I-PET/¹³¹I scintigraphy that met the internal standard of care. We tested primary size, extrathyroidal extension (ETE), and age >55 years as risk factors for RAI-R disease at initial diagnosis and during the disease course using uni- and multivariate analyses. We tested metabolic tumor volume (MTV), total lesion glycolysis (TLG) on ¹⁸F-FDG-PET, and the progression of stimulated thyroglobulin within 4-6 months of initial radioiodine therapy as prognostic markers for OS.

RESULTS

Size of primary >40 mm and ETE were significant predictors of RAI-R disease in the course of disease in univariate (81% vs. 27%, p = 0.001; 89% vs. 33%, p < 0.001) and multivariate analyses. Primary tumor size was an excellent predictor of RAI-R disease (AUC = 0.90). TLG/MTV > upper quartile and early thyroglobulin progression were significantly associated with shorter median OS (29.0 months vs. 56.9 months, p < 0.05; 57.8 months vs. not reached p < 0.005, respectively).

DISCUSSION

PDTC patients, especially those with additional risk factors, should be assessed for RAI-R disease at initial diagnosis and in the course of disease, allowing for early implementation of multimodal treatment. Primary tumor size >40 mm, ETE, and age >55 are significant risk factors for RAI-R disease. High MTV/TLG is a significant risk factor for premature death and can help identify patients requiring intervention.

Optimal Time for 124I PET/CT Imaging in Metastatic Differentiated Thyroid Cancer

BACKGROUND

The objective of this study was to determine the optimal time for 124I PET/CT imaging to maximize the detection of locoregional and/or distant metastases of differentiated thyroid cancer.

METHODS

Differentiated thyroid cancer patients suspected of having metastatic disease were prepared with low-iodine diet and appropriate thyroid-stimulating hormone stimulation. 124I PET and low-dose localization CT were performed over 4 days after oral administration of 31.5 or 62.9 MBq (0.85 or 1.7 mCi) of 124I. Each scan was independently reviewed by 2 nuclear medicine physicians. All foci of activity were categorized, and the visual intensity of uptake was scored by a semiquantitative 3-point grading system (1: mild uptake, 2: moderate uptake, 3: intense uptake). Lesion volumes were determined on the CT image or on the PET images. Background (bkg) was also measured for each lesion and on each individual PET image. For each lesion, the mean activity concentration rate per unit administered activity (ACRmean/AA) and lesion-to-bkg ratios were compared across the 5 different time points. The semiquantitative grade and the quantitative measurements were compared.

RESULTS

A total of 45 124I PET/CT scans were reviewed for 9 patients. In the visual assessment, a total of 31 foci suggestive for or highly suggestive of metastasis were identified on 124I PET/CT. Of these, 6 were seen on the 2-h, 18 on the 24-h, 27 on the 48-h, 24 on the 72-h, and 20 on the 96-h scan. There was a significant difference between the 24- and 48-h scans in the total number of foci (ie, locoregional and distant metastasis) (P < 0.05) and in the number of distant metastases (P < 0.05). The 24-, 48-, and 72-h scans identified the same number of locoregional foci. The 48-h scan visualized more of the distant metastases than any other time point. 124I PET/CT with dual-time-point imaging was superior to single-time-point imaging (97% vs 87%). In the quantitative analysis, the median ACRmean/AA was highest at 24 and 48 h, and the median lesion-to-bkg ratio was variable for different lesion locations. For lung metastases, the highest median lesion-to-bkg ratio was at 72 and 96 h.

CONCLUSIONS

124I PET/CT with dual-time-point imaging was superior to any single-time-point imaging (P < 0.10). Based on the visual assessment, dual time points at 48 + 72 h or 48 + 96 h yielded the highest lesion detection rate, whereas for single-time-point imaging, the 48-h images had the highest lesion detection rate. If the 48-h scan is completely negative or has negative 124I uptake in the region of interest, then a 72- or 96-h scan may be valuable. If lung metastases are suspected, then one should consider additional imaging at 72 or 96 h.

Radioiodine in Differentiated Thyroid Carcinoma: Do We Need Diagnostic Pre-Ablation Iodine-123 Scintigraphy to Optimize Treatment?

Changing insights regarding radioiodine (I-131) administration in differentiated thyroid carcinoma (DTC) stir up discussions on the utility of pre-ablation diagnostic scintigraphy (DxWBS). Our retrospective study qualitatively and semi-quantitatively assessed posttherapy I-131 whole-body scintigraphy (TxWBS) data for thyroid remnant size and metastasis. Findings were associated with initial treatment success after nine months, as well as clinical, histopathological, and surgical parameters. Possible management changes were addressed. A thyroid remnant was reported in 89 of 97 (92%) patients, suspicion of lymph node metastasis in 26 (27%) and distant metastasis in 6 (6%). Surgery with oncological intent and surgery by two dedicated thyroid surgeons were independently associated with a smaller remnant. Surgery at a community hospital, aggressive tumor histopathology, histopathological lymph node metastasis (pN1) and suspicion of new lymph node metastasis on TxWBS were independently associated with an unsuccessful treatment. Thyroid remnant size was unrelated to treatment success. All 13 pN1 patients with suspected in situ lymph node metastases on TxWBS had an unsuccessful treatment, opposite 19/31 (61%) pN1 patients without (p = 0.009). Pre-ablative knowledge of these TxWBS findings had likely influenced management in 48 (50%) patients. Additional pre-ablative diagnostics could optimize patient-tailored I-131 administration. DxWBS should be considered, especially in patients with pN1 stage or suspected in situ lymph node metastasis. Dependent on local surgical expertise, DxWBS is not recommended to evaluate thyroid remnant size.

Comparing lesion detection efficacy and image quality across different PET system generations to optimize the iodine-124 PET protocol for recurrent thyroid cancer

Background

In recurrent differentiated thyroid cancer patients, detectability in ¹²⁴I PET is limited for lesions with low radioiodine uptake. We assess the improvements in lesion detectability and image quality between three generations of PET scanners with different detector technologies. The results are used to suggest an optimized protocol.

Methods

Datasets of 10 patients with low increasing thyroglobulin or thyroglobulin antibody levels after total thyroidectomy and radioiodine therapies were included. PET data were acquired and reconstructed on a Biograph mCT PET/CT (whole-body, 4-min acquisition time per bed position; OSEM, OSEM-TOF, OSEM-TOF+PSF), a non-TOF Biograph mMR PET/MR (neck region, 4 min and 20 min; OSEM), and a new generation Biograph Vision PET/CT (whole-body, 4 min; OSEM, OSEM-TOF, OSEM-TOF+PSF). The 20-min image on the mMR was used as reference to calculate the detection efficacy in the neck region. Image quality was rated on a 5-point scale.

Results

All detected lesions were in the neck region. Detection efficacy was 8/9 (Vision OSEM-TOF and OSEM-TOF+PSF), 4/9 (Vision OSEM), 3/9 (mMR OSEM and mCT OSEM-TOF+PSF), and 2/9 (mCT OSEM and OSEM-TOF). Median image quality was 4 (Vision OSEM-TOF and OSEM-TOF+PSF), 3 (Vision OSEM, mCT OSEM-TOF+PSF, and mMR OSEM 20-min), 2 (mCT OSEM-TOF), 1.5 (mCT OSEM), and 1 (mMR OSEM 4 min).

Conclusion

At a clinical standard acquisition time of 4 min per bed position, the new generation Biograph Vision using a TOF-based image reconstruction demonstrated the highest detectability and image quality and should, if available, be preferably used for imaging of low-uptake lesions. A prolonged acquisition time for the mostly affected neck region can be useful.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40658-021-00361-y.

30 mCi exploratory scan for two-step dosimetric 131I therapy in differentiated thyroid cancer patients: A novel approach and case report

Differentiated thyroid cancer patients with significantly elevated or rapidly rising serum thyroglobulin (Tg) levels and negative diagnostic radioiodine scans (DxScan) often present a therapeutic dilemma in deciding whether or not to administer an ¹³¹I treatment. In this report, we describe a novel two-step approach of a 30 mCi ¹³¹I exploratory scan before a dosimetric ¹³¹I therapy to help “un-blind” the treating physician of the benefit/risk ratio of a further “blind” ¹³¹I treatment. A 51-year-old man presented with rising Tg levels, a negative DxScan, and a history of widely metastatic follicular thyroid cancer. He had undergone total thyroidectomy, remnant ablation with 3.8 GBq (103.5 mCi) of ¹³¹I, Gammaknife^®, and treatment with 12.1 GBq (326 mCi) of ¹³¹I for multiple metastases. However, at 19 months after the treatments, his Tg levels continued to rise, and scans demonstrated no evidence of radioiodine-avid metastatic disease. In anticipation of a “blind” ¹³¹I treatment, the medical team and the patient opted for a 30 mCi exploratory scan. The total dosimetrically guided prescribed activity (DGPA) was decided based on the whole-body dosimetry. The patient was first given 30 mCi of ¹³¹I, and the exploratory scan was performed 22 h later, which demonstrated ¹³¹I uptake in the left lung, left humeral head, T10, and right proximal thigh muscle. Based on the positive exploratory scan, the remainder of the DGPA was administered within several hours after the scan. On the post-DGPA treatment scan performed at 5–7 days, the lesions seen on the ~ 22 h exploratory scan were confirmed, and an additional lesion was observed in the left kidney. The 30 mCi exploratory scan suggested the potential for a response in the radioiodine-avid lesions despite a negative diagnostic scan. This method allows ¹³¹I treatment to be administered to patients who may have a greater potential for a therapeutic response while avoiding unwarranted side effects in those patients with nonavid disease.

Updated Review of Nuclear Molecular Imaging of Thyroid Cancers

OBJECTIVES

We initiate this comprehensive review to update the advances in this field by objectively elucidating the efficacies of promising radiopharmaceuticals.

METHODS

We performed a comprehensive PUBMED search using the combined terms of "thyroid cancer" and "radiopharmaceuticals" or "nuclear medicine", yielding 3273 and 11026 articles prior to December 31, 2020, respectively.

RESULTS

Based on the mechanism of molecular metabolism, the evaluation of differentiated thyroid cancer and dedifferentiated thyroid cancer is largely centered around radioiodine and fluorine 18 (¹⁸F)-fludeoxyglucose, respectively. Further, ¹⁸F-L-dihydroxyphenylalanine and gallium 68 DOTATATE are the preferred tracers for medullary thyroid cancer. In dedifferentiated medullary thyroid cancer and anaplastic thyroid cancer, ¹⁸F-fludeoxyglucose is superior.

CONCLUSIONS

The future lies in advances in molecular biology, novel radiopharmaceuticals and imaging devices, paving ways to the development of personalized medication for thyroid cancer patients.

Non-FDG PET/CT

The major applications for molecular imaging with PET in clinical practice concern cancer imaging. Undoubtedly, 18F-FDG represents the backbone of nuclear oncology as it remains so far the most widely employed positron emitter compound. The acquired knowledge on cancer features, however, allowed the recognition in the last decades of multiple metabolic or pathogenic pathways within the cancer cells, which stimulated the development of novel radiopharmaceuticals. An endless list of PET tracers, substantially covering all hallmarks of cancer, has entered clinical routine or is being investigated in diagnostic trials. Some of them guard significant clinical applications, whereas others mostly bear a huge potential. This chapter summarizes a selected list of non-FDG PET tracers, described based on their introduction into and impact on clinical practice.

124I Positron Emission Tomography/Computed Tomography Versus Conventional Radioiodine Imaging in Differentiated Thyroid Cancer: A Review

Background: Studies report a wide spectrum of ¹²⁴I positron emission tomography (PET)/computed tomography (CT) sensitivity and specificity in the detection of differentiated thyroid cancer (DTC) lesions. This study reviews the lesion detection rate of pretherapy ¹²⁴I PET/CT in different patient populations and further analyzes the factors necessary for a better detection on ¹²⁴I PET/CT. Methods: A literature search was performed using multiple different databases (MEDLINE, EMBASE, Northern Lights, and handsearching) covering 1996 to April 2018. Two reviewers reviewed and extracted study data for ¹²⁴I, ¹²³I, and ¹³¹I scans in DTC. Results: This review includes 4 retrospective and 10 prospective studies in which 495 DTC patients underwent ¹²⁴I and ¹³¹I imaging; no studies made comparisons with ¹²³I. In the reports that compared ¹²⁴I PET/CT with diagnostic ¹³¹I scans, there were a total of 72 patients in whom 120 lesions were detected on ¹²⁴I imaging, whereas only 52 were detected on diagnostic ¹³¹I scans. In publications that compared ¹²⁴I with post-therapy ¹³¹I scans in 266 patients, 410 lesions were detected with ¹²⁴I PET, whereas 390 were detected on post-therapy ¹³¹I scans. Based on ¹²⁴I PET/CT in six studies, TNM staging was revised in 15-21% of patients, and disease management was altered in 5-29% of patients. Conclusions:¹²⁴I PET/CT is able to identify a greater number of foci compared with diagnostic ¹³¹I scans. ¹²⁴I PET may have better detection compared with post-therapy ¹³¹I scans in patients who are ¹³¹I therapy naive, have less aggressive pathology, or do not have disseminated lung metastases. Additional metastatic lesion detection by ¹²⁴I PET may have a significant clinical impact in the management of patients before ¹³¹I therapy in some patients.

Mitogen-Activated Protein Kinase Pathway Inhibition for Redifferentiation of Radioiodine Refractory Differentiated Thyroid Cancer: An Evolving Protocol

Background: Some patients with metastatic differentiated thyroid cancer (DTC) lack iodine avidity and are therefore unsuitable for radioactive iodine (RAI) therapy. Limited experience suggests that single-agent selective mitogen-activated protein kinase (MAPK) pathway inhibitors can restore expression of the sodium-iodide symporter rendering RAI refractory (RAIR) DTC patients amenable to RAI therapy. The aim of this study was to assess the feasibility of mutation-guided MAPK-pathway blockade combined with thyroid hormone withdrawal (THW) for redifferentiation. Methods: This is a retrospective review of metastatic RAIR DTC and driver mutation in MAPK pathway, treated on a redifferentiation protocol. All patients had metastatic disease that had never been RAI-avid and/or imaging and biochemical progression despite treatment with RAI within the past 12 months. Patients with tumors harboring an NRAS mutation were treated with an MEK inhibitor (trametinib), and tumors with a BRAF^V600E mutation with combined BRAF and MEK inhibition (dabrafenib and trametinib; or vemurafenib and cobimetinib) for four weeks. Thyrotropin stimulation was performed by THW for four weeks. Restoration of RAI uptake was determined by ¹²⁴I positron emission tomography/computed tomography imaging. The response was assessed at least three months post-RAI. Results: From 2015 to 2017, six patients (age 45-70, four females) received redifferentiation therapy. Three patients had an NRAS mutation; two with follicular thyroid carcinoma (FTC) and one with a poorly differentiated thyroid carcinoma (PDTC); and three patients had a BRAF^V600E mutation and papillary thyroid carcinoma (PTC). One NRAS and all BRAF^V600E mutation cases demonstrated restoration of RAI uptake and proceeded to RAI therapy with a median follow-up of 16.6 months (range 13.5-42.3 months). The patient with an NRAS mutation and two of three patients with a BRAF^V600E demonstrated partial imaging response beyond a three-month follow-up. Grade 3 adverse events (acneiform rash) were observed in two patients with NRAS mutations. Conclusions: Mutation-guided MAPK pathway inhibition with MEK inhibitor or a combination of BRAF inhibitor and MEK inhibitor under concurrent THW is a feasible and a promising strategy to redifferentiate RAIR DTC, thereby rendering them suitable for RAI therapy with satisfactory retention following treatment.

2019 European Thyroid Association Guidelines for the Treatment and Follow-Up of Advanced Radioiodine-Refractory Thyroid Cancer

The vast majority of thyroid cancers of follicular origin (TC) have a very favourable outcome, but 5-10% of cases will develop metastatic disease. Around 60-70% of this subset, hence less than 5% of all patients with TC, will become radioiodine refractory (RAI-R), with a significant negative impact on prognosis and a mean life expectancy of 3-5 years. Since no European expert consensus or guidance for this challenging condition is currently available, a task force of TC experts was nominated by the European Thyroid Association (ETA) to prepare this document based on the principles of clinical evidence. The task force started to work in September 2018 and after several revision rounds, prepared a list of recommendations to support the treatment and follow-up of patients with advanced TC. Criteria for advanced RAI-R TC were proposed, and the most appropriate diagnostic tools and the local, systemic and palliative treatments are described. Systemic therapy with multikinase inhibitors is fully discussed, including recommendations on how to start it and at which dosage, on the duration of treatment, and on the management of side effects. The appropriate relationship between the specialist and the patient/family as well as ethical issues are covered. Based on the available studies and on personal experience, the experts provided 39 recommendations aimed to improve the management of advanced RAI-R TCs. Above all of them is the indication to treat and follow these patients in a specialized setting which allows the interaction between several specialists in a multidisciplinary team.

Identification of Radioactive Iodine Refractory Differentiated Thyroid Cancer

Most differentiated thyroid cancer (DTC) patients have an excellent prognosis. However, about one-third of DTC patients with recurrent or metastatic disease lose the hallmark of specific iodine uptake initially or gradually and acquire radioactive iodine-refractory DTC (RAIR-DTC) with poor prognosis. Due to the potentially severe complications from unnecessarily repeated RAI therapy and encouraging progress of multiple targeted drugs for advanced RAIR-DTC patients, it has become crucial to identify RAIR-DTC early. In this review, we focus on the progress and controversies regarding the defining of RAIR-DTC, further with subsistent approaches and promising molecular nuclear medicine imaging in identifying RAIR-DTC, which may shed light on the proper management methodsof such patients.

PET/CT in thyroid nodule and differentiated thyroid cancer patients. The evidence-based state of the art

A more conservative approach to the clinical management of thyroid nodules and differentiated thyroid cancer has recently been proposed by the 2015 ATA guidelines. In this context, fine-needle aspiration biopsy has been reserved for nodules with particular ultrasound features or dimensions that exclude low-risk thyroid lesions. Accordingly, a less aggressive surgical approach (i.e. lobectomy) has been recommended as the first-choice treatment in nodules with indeterminate cytology or in small cytologically confirmed malignant nodules. At the same time, radioactive remnant ablation has been considered only for DTC patients with concrete risks of disease persistence/relapse after thyroidectomy. In addition, further radioactive iodine therapies (RAI) have been proposed only for patients presenting unresectable and iodine-avid structural relapse. In this complex scenario, which requires attention to each clinical aspect of the patient, the introduction of accurate diagnostic tools is highly warranted. PET/CT is a very sensitive and specific diagnostic procedure that can better characterize the risk of thyroid nodules, identify DTC relapse early and predict the response to RAI. Thus, it seems essential to customize a more conservative approach to thyroid nodules and DTC patients. The aim of this review is to report the principal clinical context in which PET/CT has been used and to evaluate the evidence-based support for each diagnostic indication.

Recent Development of Nuclear Molecular Imaging in Thyroid Cancer

Therapies targeting specific tumor pathways are easy to enter the clinic. To monitor molecular changes, cellular processes, and tumor microenvironment, molecular imaging is becoming the key technology for personalized medicine because of its high efficacy and low side effects. Thyroid cancer is the most common endocrine malignancy, and its theranostic radioiodine has been widely used to diagnose or treat differentiated thyroid cancer. This article summarizes recent development of molecular imaging in thyroid cancer, which may accelerate the development of personalized thyroid cancer therapy.

Utility of I-124 PET/CT in identifying radioiodine avid lesions in differentiated thyroid cancer: a systematic review and meta-analysis

INTRODUCTION

Diagnostic I-123 scans have been shown to underestimate the disease burden in differentiated thyroid cancer (DTC) when compared to I-131 post-treatment scans, especially in children and patients who have had prior radioiodine (RAI) therapy and/or distant metastasis. I-124 PET/CT has been shown to be highly effective in imaging DTC-related metastatic disease.

METHODS

We performed a systematic review and meta-analysis of studies investigating the sensitivity and specificity of I-124 PET/CT in identifying lesions amenable to RAI therapy as confirmed by I-131 post-treatment scanning.

RESULTS

There were 141 patients and 415 lesions of DTC identified altogether. There was significant heterogeneity in the individual studies. The pooled sensitivity of the I-124 PET/CT in detecting lesions of differentiated thyroid cancer amenable to I-131 therapy was 94·2% (91·3-96·4% CI, P < 0·01), and the pooled specificity was 49·0% (34·8-63·4% CI, P < 0·01). The pooled positive likelihood ratio (LR) was 1·43 (1·05-1·94 CI), and the pooled negative LR was 0·28 (0·15-0·53 CI). Overall, the diagnostic odds ratio was 7·90 (3·39-18·48 CI). There were a small but increased number of lesions identified by I-124 PET/CT that was not detected on post-treatment scan.

CONCLUSION

I-124 PET/CT is a sensitive tool to diagnose RAI avid DTC lesions, but also detects some new lesions that are not visualized on the post-treatment I-131 scan. Further, carefully designed dosimetric studies may be required to fully establish the role of I-124 PET CT for identifying potential lesions for I-131 therapy. I-124 PET/CT in patients with DTC may have other applications in specific clinical situations.

A phantom study: Should 124 I-mIBG PET/CT replace 123 I-mIBG SPECT/CT?

PURPOSE

The isotope ¹²³ I is commonly labeled with meta-iodobenzylguanidine (mIBG) for imaging of neuroendocrine tumors, such as pheochromocytomas and neuroblastomas. ¹²³ I-mIBG SPECT/CT imaging is performed for staging, follow-up and selection of patients for treatment with ¹³¹ I mIBG. As an alternative to ¹²³ I, ¹²⁴ I-mIBG PET/CT may be used, potentially taking advantage of the superior PET image quality. The purpose of this study was to investigate whether ¹²⁴ I PET/CT improves image quality as compared with ¹²³ I SPECT/CT for equal patient effective radiation dose (in mSv).

METHODS

Phantom measurements were performed using the NEMA-2007 image quality phantom. SPECT and PET reconstruction settings were used with and without time-of-flight (TOF) and point-spread-function (PSF) modeling. As a measure of image quality, the contrast-to-noise ratio (CNR) was calculated. The ratio of the ¹²³ I to ¹²⁴ I activity concentration was determined at which the contrast-to-noise ratio was equal for both modalities. This metric was defined as the contrast equivalent activity ratio (CEAR).

RESULTS

CEARs of 47.7, 25.6, 23.1, 14.6, 10.0, and 9.1 were obtained for a TOF and PSF modeled ¹²⁴ I reconstruction method and an attenuation and scatter-corrected ¹²³ I reconstruction method for sphere sizes of 10 to 37 mm, respectively. As the effective radiation dose of ¹²⁴ I-mIBG is higher than of ¹²³ I-mIBG (in mSv/MBq), an equal effective dose corresponds to a CEAR of 5 to 10. Therefore, CEARs higher than 5 to 10 indicate that ¹²⁴ I PET/CT outperforms ¹²³ I SPECT/CT in the sense of image quality for equal patient effective radiation dose.

CONCLUSION

The CEAR is much larger than a factor of 5 to 10 (needed for equal patient effective radiation dose) for most of the reconstruction methods and sphere sizes. Therefore, ¹²⁴ I-mIBG PET/CT is expected to improve image quality and/or may be used to reduce effective patient dose as compared with ¹²³ I-mIBG SPECT/CT.

I-124 Imaging and Dosimetry

Although radioactive iodine imaging and therapy are one of the earliest applications of theranostics, there still remain a number of unresolved clinical questions as to the optimization of diagnostic techniques and dosimetry protocols. I-124 as a positron emission tomography (PET) radiotracer has the potential to improve the current clinical practice in the diagnosis and treatment of differentiated thyroid cancer. The higher sensitivity and spatial resolution of PET/computed tomography (CT) compared to standard gamma scintigraphy can aid in the detection of recurrent or metastatic disease and provide more accurate measurements of metabolic tumor volumes. However the complex decay schema of I-124 poses challenges to quantitative PET imaging. More prospective studies are needed to define optimal dosimetry protocols and to improve patient-specific treatment planning strategies, taking into account not only the absorbed dose to tumors but also methods to avoid toxicity to normal organs. A historical perspective of I-124 imaging and dosimetry as well as future concepts are discussed.

Calibration of PET/CT scanners for multicenter studies on differentiated thyroid cancer with (124)I

BACKGROUND

Studies on imaging of differentiated thyroid cancer (DTC) using (124)I often require a multicenter approach, as the prevalence of DTC is low. Calibration of participating scanners is required to obtain comparable quantification. As determination of a well-defined range of recovery coefficients is complicated for various reasons, a simpler approach based on the assumption that the iodine uptake is highly focal with a background that significantly lacks radioactivity might be more efficient. For each scanner, a linear conversion between known and observed activity can be derived, allowing quantification that can be traced to a common source for all scanners within one study-protocol. The aim of this paper is to outline a procedure using this approach in order to set up a multicenter calibration of PET/CT scanners for (124)I.

METHODS

A cylindrical polyethylene phantom contained six 2-ml vials with reference activities of ~2, 10, 20, 100, 400, and 2000 kBq, produced by dilution from a known activity. The phantom was scanned twice on PET/CT scanners of participating centers within 1 week. For each scanner, the best proportional and linear fit between measured and known activities were derived and based on statistical analyses of the results of all scanners; it was determined which fit should be applied. In addition, a Bland-Altman analysis was done on calibrated activities with respect to reference activities to asses the relative precision of the scanners.

RESULTS

Nine Philips (vendor A) and nine Siemens (vendor B) PET/CT scanners were calibrated in a time period of 3 days before and after the reference time. No significant differences were detected between the two subsequent scans on any scanner. Six fitted intercepts of vendor A were significantly different from zero, so the linear model was used. Intercepts ranged from -8 to 26 kBq and slopes ranged from 0.80 to 0.98. Bland-Altman analysis of calibrated and reference activities showed that the relative error of calibrated activities was smaller than that of uncalibrated activities.

CONCLUSIONS

A simplified multicenter calibration procedure for PET/CT scans that show highly focal uptake and negligible background is feasible and results in more precise quantification. Our procedure can be used in multicenter (124)I PET scans focusing on (recurrent) DTC.