Disseminated iodine-avid lung metastases in differentiated thyroid cancer: a challenge to 124I PET

Molecular Imaging and Therapy of Differentiated Thyroid Carcinoma in Adults

ABSTRACT

Differentiated thyroid carcinoma (DTC) has been increasing in incidence in the United States over the last several decades, although mortality rates have remained low. Radioactive iodine therapy (RAI-T) has been a mainstay of treatment for DTC since the 1940s. Imaging of DTC before and after RAI-T primarily focuses on molecular imaging of the sodium iodide symporter. The expanding understanding of the molecular profile of DTC has increased available treatment options. Incorporation of risk stratification to treatment approaches has led to deintensification of both surgical and nonsurgical treatments, leading to decreased morbidity without compromising disease control.

[18F]FAPI-42 PET/CT in differentiated thyroid cancer: diagnostic performance, uptake values, and comparison with 2-[18F]FDG PET/CT

PURPOSE

This study aimed to assess the diagnostic performance of [¹⁸F]FAPI-42 PET/CT and compare it with that of 2-[¹⁸F]FDG PET/CT in patients with differentiated thyroid cancer (DTC) with biochemical elevations in Tg or anti-Tg antibodies.

METHODS

A total of 42 patients with DTC with biochemical elevations in Tg or anti-Tg antibodies underwent [¹⁸F]FAPI-42 PET/CT as part of this study; of which, 11 additionally underwent 2-[¹⁸F]FDG PET/CT within 7 days. Images were semi-quantitatively and visually interpreted, and the quantity, location, and uptake values of lesions were noted. The diagnostic capacity of [¹⁸F]FAPI-42 PET/CT and biomarkers affecting the uptake of [¹⁸F]FAPI-42 were evaluated. In addition, the diagnostic performance and uptake of [¹⁸F]FAPI-42 and 2-[¹⁸F]FDG were compared, and the correlation between lesion diameter and quantitative parameters was investigated.

RESULTS

A total of 161 lesions were detected in 27 (64%) patients on [¹⁸F]FAPI-42 PET/CT. FAPI-positive local recurrence showed the highest uptake intensity, followed by lymphatic, other site-associated (bone and pleura), and pulmonary lesions (mean SUV_max, 4.7 versus 3.7 versus 3.0 versus 2.2, respectively; P < 0.0001). The levels of TSH, Tg, and Tg-Ab did not affect the uptake value of lesions (median SUV_max: 2.4 versus 3.2, P = 0.56; 2.9 versus 2.4, P = 0.0935; 2.8 versus 2.6, P = 0.0525, respectively). A total of 90 positive lesions were detected in 7 patients using both modalities. All positive lesions showed statistically higher uptake of 2-[¹⁸F]FDG than that of [¹⁸F]FAPI-42 (SUV_max, 2.6 versus 2.1; P = 0.026). However, the SUV_max of [¹⁸F]FAPI-42 was higher than that of 2-[¹⁸F]FDG in local recurrences and lymphatic lesions (SUV_max, 4.2 versus 2.9 and 3.9 versus 3.4, respectively; P > 0.05).

CONCLUSION

[¹⁸F]FAPI-42 can be used for detecting lesions and reflecting FAP expression during local recurrence and metastasis in patients with DTC with biochemical elevations in Tg or anti-Tg antibodies. The diagnostic performance of [¹⁸F]FAPI-42 PET/CT is comparable with that of 2-[¹⁸F]FDG PET/CT in such patients.

Dosimetry in Clinical Radiopharmaceutical Therapy of Cancer: Practicality Versus Perfection in Current Practice

The use of radiopharmaceutical therapies (RPTs) in the treatment of cancers is growing rapidly, with more agents becoming available for clinical use in last few years and many new RPTs being in development. Dosimetry assessment is critical for personalized RPT, insofar as administered activity should be assessed and optimized in order to maximize tumor-absorbed dose while keeping normal organs within defined safe dosages. However, many current clinical RPTs do not require patient-specific dosimetry based on current Food and Drug Administration-labeled approvals, and overall, dosimetry for RPT in clinical practice and trials is highly varied and underutilized. Several factors impede rigorous use of dosimetry, as compared with the more convenient and less resource-intensive practice of empiric dosing. We review various approaches to applying dosimetry for the assessment of activity in RPT and key clinical trials, the extent of dosimetry use, the relative pros and cons of dosimetry-based versus fixed activity, and practical limiting factors pertaining to current clinical practice.

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.

PET/CT in the management of differentiated thyroid cancer

The standard treatment of differentiated thyroid cancer (DTC) consists of surgery followed by iodine-131 (¹³¹I) administration. Although the majority of DTC has a very good prognosis, more aggressive histologic subtypes convey a worse prognosis. Follow-up consists of periodically measurements of serum thyroglobulin, thyroglobulin antibodies and neck ultrasound and ¹²³I/¹³¹I whole-body scan. However, undifferentiated thyroid tumors have a lower avidity for radioiodine and the ability of DTC to concentrate ¹³¹I may be lost in metastatic disease. Positron emission tomography (PET)/computed tomography (CT) has been introduced in the evaluation of patients with thyroid tumors and the 2-[18F]-fluoro-2-deoxyd-glucose (¹⁸F-FDG) has been largely validated as marker of cell's metabolism. According to the 2015 American Thyroid Association guidelines, ¹⁸F-FDG PET/CT is recommended in the follow-up of high-risk patients with elevated serum thyroglobulin and negative ¹³¹I imaging, in the assessment of metastatic patients, for lesion detection and risk stratification and in predicting the response to therapy. It should be considered that well-differentiated iodine avid lesions could not concentrate ¹⁸F-FDG, and a reciprocal pattern of iodine and ¹⁸F-FDG uptake has been observed. Beyond ¹⁸F-FDG, other tracers are available for PET imaging of thyroid tumors, such as Iodine-124 (¹²⁴I), ¹⁸F-tetrafluoroborate and Gallium-68 prostate-specific membrane antigen. Moreover, the recent introduction of PET/MRI, offers now several opportunities in the field of patients with DTC. This review summarizes the evidences on the role of PET/CT in management of patients with DTC, focusing on potential applications and on elucidating some still debating points.

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.

Multimodal imaging of thyroid cancer

PURPOSE OF REVIEW

Thyroid cancer is the most common endocrine cancer in adults with rising incidence. Challenges in imaging thyroid cancer are twofold: distinguishing thyroid cancer from benign thyroid nodules, which occur in 50% of the population over 50 years; and correct staging of thyroid cancer to facilitate appropriate radical surgery in a single session. The clinical management of thyroid cancer patients has been covered in detail by the 2015 guidelines of the American Thyroid Association (ATA). The purpose of this review is to state the principles underlying optimal multimodal imaging of thyroid cancer and aid clinicians in avoiding important pitfalls.

RECENT FINDINGS

Recent additions to the literature include assessment of ultrasound-based scoring systems to improve selection of nodules for fine needle biopsy (FNB) and the evaluation of new radioactive tracers for imaging thyroid cancer.

SUMMARY

The mainstay of diagnosing thyroid cancer is thyroid ultrasound with ultrasound-guided FNB. Contrast-enhanced computed tomography and PET with [F]-fluorodeoxyglucose (FDG) and MRI are reserved for advanced and/or recurrent cases of differentiated thyroid cancer and anaplastic thyroid cancer, while [F]FDOPA and [Ga]DOTATOC are the preferred tracers for medullary thyroid cancer.

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.

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.

Analysis of radioiodine therapy and prognostic factors of differentiated thyroid cancer patients with pulmonary metastasis: An 8-year retrospective study

To assess the efficacy of radioiodine therapy (RIT) and investigate the prognostic factors for patients with pulmonary metastasis secondary to differentiated thyroid carcinoma (DTC) through a retrospective study. A total of 80 patients with radioactive iodine-131 (I)-avid pulmonary metastasis from DTC treated with I from 2007 to 2014 at our institution entered the study. Treatment response was mainly measured by two parameters: serum thyroglobulin (Tg) levels and post-therapeutic I whole-body scan (WBS). Treatment variables were assessed for statistical significance using the univariate and multivariate analyses. A receiver-operating characteristic (ROC) curve was also plotted to verify the accuracy of predictors. Of these 80 patients, the overall effective rate was 72.5% (58/80), the rates for complete response (CR), partial response (PR), and no response (NR) were 20.0%, 52.5%, and 27.5%, respectively. Univariate analysis showed that gender, pulmonary nodule size, absence or presence of extrapulmonary distant metastases, age, and Tg level at diagnosis were significantly associated with I therapy efficacy. Binary logistic regression analysis revealed that older patients (odds ratio [OR]:1.481, 95% confidence interval [CI]: 1.457-2.091, P = .020), subjects with higher Tg levels at diagnosis (OR: 1.046, 95% CI: 1.016-1.119, P = .014), and those with extrapulmonary distant metastases (OR: 1.185, 95%CI: 1.025-1.463, P = .020) had a higher probability of poor prognosis. The optimal cutoffs for age and Tg level to predict I therapy efficacy for DTC with lung metastases were 46 years old and 55.50 ng/mL, respectively, based on ROC analysis. This study indicated that most DTC patients with pulmonary metastases can obtain partial or complete remission after RIT, while older patients with higher Tg levels at diagnosis and extrapulmonary distant metastases more likely show poor prognosis.

ENDOCRINE TUMOURS: Imaging in the follow-up of differentiated thyroid cancer: current evidence and future perspectives for a risk-adapted approach

The clinical and epidemiological profiles of differentiated thyroid cancers (DTCs) have changed in the last three decades. Today's DTCs are more likely to be small, localized, asymptomatic papillary forms. Current practice is, though, moving toward more conservative approaches (e.g. lobectomy instead of total thyroidectomy, selective use of radioiodine). This evolution has been paralleled and partly driven by rapid technological advances in the field of diagnostic imaging. The challenge of contemporary DTCs follow-up is to tailor a risk-of-recurrence-based management, taking into account the dynamic nature of these risks, which evolve over time, spontaneously and in response to treatments. This review provides a closer look at the evolving evidence-based views on the use and utility of imaging technology in the post-treatment staging and the short- and long-term surveillance of patients with DTCs. The studies considered range from cervical US with Doppler flow analysis to an expanding palette of increasingly sophisticated second-line studies (cross-sectional, functional, combined-modality approaches), which can be used to detect disease that has spread beyond the neck and, in some cases, shed light on its probable outcome.

Quantitative Comparison of 124I PET/CT and 131I SPECT/CT Detectability

Radioiodine therapy with (131)I is used for treatment of suspected recurrence of differentiated thyroid carcinoma. Pretherapeutic (124)I PET/CT with a low activity (~1% of (131)I activity) can be performed to determine whether uptake of (131)I, and thereby the desired therapeutic effect, may be expected. However, false-negative (124)I PET/CT results as compared with posttherapeutic (131)I SPECT/CT have been reported by several groups. The purpose of this study was to investigate whether the reported discrepancies may be ascribed to a difference in lesion detectability between (124)I PET/CT and (131)I SPECT/CT and, hence, whether the administered (124)I activity is sufficient to achieve equal detectability.

METHODS

Phantom measurements were performed using the National Electrical Manufacturers Association 2007 image-quality phantom. As a measure of detectability, the contrast-to-noise ratio was calculated. The (124)I activity was expressed as the percentage of (131)I activity required to achieve the same contrast-to-noise ratio. This metric was defined as the detectability equivalence percentage (DEP).

RESULTS

Because lower DEPs were obtained for smaller spheres, a relatively low (124)I activity was sufficient to achieve similar lesion detectability between (124)I PET/CT and (131)I SPECT/CT. DEP was 1.5%, 1.9%, 1.9%, 4.4%, 9.0%, and 16.2% for spheres with diameters of 10, 13, 17, 18, 25, and 37 mm, respectively, for attenuation- and scatter-corrected SPECT versus point-spread function (PSF) model-based and time-of-flight (TOF) PET. For no-PSF no-TOF PET, DEP was 3.6%, 2.1%, 3.5%, 7.8%, 15.1%, and 23.3%, respectively.

CONCLUSION

A relatively low (124)I activity of 74 MBq (~1% of (131)I activity) is sufficient to achieve similar lesion detectability between (124)I PSF TOF PET/CT and (131)I SPECT/CT for small spheres (≤10 mm), since the reported DEPs are close to 1%. False-negative (124)I PET/CT results as compared with posttherapeutic (131)I SPECT/CT may be ascribed to differences in detectability for large lesions (>10 mm) and for no-PSF no-TOF PET, since DEPs are greater than 1%. On the basis of DEPs of 3.5% for lesion diameters of up to 17 mm on no-PSF no-TOF PET, (124)I activities as high as 170 MBq may be warranted to obtain equal detectability.

Pretherapeutic dosimetry in patients affected by metastatic thyroid cancer using 124I PET/CT sequential scans for 131I treatment planning

PURPOSE

This study evaluates the use of sequential I PET/CT for predicting absorbed doses to metastatic lesions in patients with differentiated thyroid cancer undergoing I therapy.

METHODS

From July 2011 until July 2013, 30 patients with metastatic differentiated thyroid cancer were enrolled. Each participant underwent PET/CT at 4, 24, 48, and 72 hours with 74 MBq of I. Blood samples and whole-body exposure measurements were obtained to calculate blood and red marrow doses. Activity concentrations and lesion volumes obtained from PET/CT were used to evaluate tumor doses with medical internal radiation dose formalism and spheres modeling. Mean administered I therapeutic dose was 5994 MBq (range, 1953-11,455 MBq).

RESULTS

I PET/CT demonstrated all lesions detected by posttherapy I whole-body scans. Mean dose rates for blood, red marrow, and lesions were as follows: 0.07 ± 0.02 mGy/MBq, 0.05 ± 0.02 mGy/MBq, and 46.5 ± 117 mGy/MBq, respectively. Despite the high level of thyroid-stimulating hormone and CT detectable lesions, 15 of 30 patients did not show any abnormal I uptake.

CONCLUSIONS

The quantitative value of I PET/CT allows simple and accurate evaluation of lesion dosimetry following medical internal radiation dose formalism. Negative I PET/CT predicts absence of iodine avidity, potentially allowing avoidance of therapeutically ineffective I administration.

Do negative 124I pretherapy positron emission tomography scans in patients with elevated serum thyroglobulin levels predict negative 131I posttherapy scans?

BACKGROUND

The management of patients with differentiated thyroid cancer (DTC) who have elevated serum thyroglobulin (Tg) levels and negative (131)I or (123)I scans is problematic, and the decision regarding whether or not to administer (131)I therapy (a "blind" therapy) is also problematic. While (124)I positron emission tomography (PET) imaging has been shown to detect more foci of residual thyroid tissue and/or metastases secondary to DTC than planar (131)I images, the utility of a negative (124)I PET scan in deciding whether or not to consider performing blind (131)I therapy is unknown. The objective of this study was to determine whether a negative (124)I pretherapy PET scan in patients with elevated serum Tg levels and negative (131)I or (123)I scans predicts a negative (131)I posttherapy scan.

METHODS

Several prospective studies have been performed to compare the radiopharmacokinetics of (124)I PET versus (131)I planar imaging in patients who 1) had histologically proven DTC, 2) were suspected to have metastatic DTC (e.g., elevated Tg, positive recent fine-needle aspiration cytology, suspicious enlarging mass), and 3) had (131)I planar and (124)I PET imaging performed. Using these criteria, we retrospectively identified patients who had an elevated Tg, a negative diagnostic (131)I/(123)I scan, a negative diagnostic (124)I PET scan, therapy with (131)I, a post-therapy (131)I scan, and a prior (131)I therapy with a subsequent positive post-(131)I therapy scan. For each scan, two readers categorized every focus of (131)I and (124)I uptake as positive for thyroid tissue/metastases or physiological.

RESULTS

Twelve patients met the above criteria. Ten of these 12 patients (83%) had positive foci on (131)I posttherapy scan.

CONCLUSION

In our selected patient population, (131)I posttherapy scans are frequently positive in patients with elevated serum Tg levels, a negative diagnostic (131)I or (123)I scan, and a negative (124)I PET scan. Thus, for a patient with elevated serum Tg level, negative diagnostic (131)I planar scan, and a prior post-(131)I therapy scan that was positive, a negative (124)I PET scan will have a low predictive value for a negative post-(131)I therapy scan and should not be used to exclude the option of blind (131)I therapy.

Effectiveness of [(124)I]-PET/CT and [(18)F]-FDG-PET/CT for localizing recurrence in patients with differentiated thyroid carcinoma

Although the prognosis of patients with differentiated thyroid carcinoma (DTC) is generally encouraging, a diagnostic dilemma is posed when an increasing level of serum thyroglobulin (Tg) is noted, without detection of a recurrent tumor using conventional imaging tools such as the iodine-131 whole-body scanning (the [(131)I] scan) or neck ultrasonography (US). The objective of the present study was to evaluate the diagnostic value of [(124)I]-PET/CT and [(18)F]-FDG-PET/CT in terms of accurate detection of both iodine- and non-iodine-avid recurrence, compared with that of conventional imaging such as the [(131)I] scan or neck ultrasonography (US). Between July 2009 and June 2010, we prospectively studied 19 DTC patients with elevated thyroglobulin levels but who do not show pathological lesions when conventional imaging modalities are used. All involved patients had undergone total thyroidectomy and radioiodine (RI) treatment, and who had been followed-up for a mean of 13 months (range, 6-21 months) after the last RI session. Combined [(18)F]-FDG-PET/CT and [(124)I]-PET/CT data were evaluated for detecting recurrent DTC lesions in study patients and compared with those of other radiological and/or cytological investigations. Nine of 19 patients (47.4%) showed pathological [(18)F]-FDG (5/19, 26.3%) or [(124)I]-PET (4/19, 21.1%) uptake, and were classed as true-positives. Among such patients, disease management was modified in six (66.7%) and disease was restaged in seven (77.8%). In particular, the use of the described imaging combination optimized planning of surgical resection to deal with locoregional recurrence in 21.1% (4/19) of patients, who were shown to be disease-free during follow-up after surgery. Our results indicate that combination of [(18)F]-FDG-PET/CT and [(124)I]-PET/CT affords a valuable diagnostic method that can be used to make therapeutic decisions in patients with DTC who are tumor-free on conventional imaging studies but who have high Tg levels.

Three-dimensional radiobiological dosimetry (3D-RD) with 124I PET for 131I therapy of thyroid cancer

Radioiodine therapy of thyroid cancer was the first and remains among the most successful radiopharmaceutical (RPT) treatments of cancer although its clinical use is based on imprecise dosimetry. The positron emitting radioiodine, (124)I, in combination with positron emission tomography (PET)/CT has made it possible to measure the spatial distribution of radioiodine in tumors and normal organs at high resolution and sensitivity. The CT component of PET/CT has made it simpler to match the activity distribution to the corresponding anatomy. These developments have facilitated patient-specific dosimetry (PSD), utilizing software packages such as three-dimensional radiobiological dosimetry (3D-RD), which can account for individual patient differences in pharmacokinetics and anatomy. We highlight specific examples of such calculations and discuss the potential impact of (124)I PET/CT on thyroid cancer therapy.

Synthesis and biological evaluation of [(18)F]tetrafluoroborate: a PET imaging agent for thyroid disease and reporter gene imaging of the sodium/iodide symporter

Purpose

The human sodium/iodide symporter (hNIS) is a well-established target in thyroid disease and reporter gene imaging using gamma emitters ¹²³I-iodide, ¹³¹I-iodide and ^99mTc-pertechnetate. However, no PET imaging agent is routinely available. The aim of this study was to prepare and evaluate ¹⁸F-labelled tetrafluoroborate ([¹⁸F]TFB) for PET imaging of hNIS.

Methods

[¹⁸F]TFB was prepared by isotopic exchange of BF₄⁻ with [¹⁸F]fluoride in hot hydrochloric acid and purified using an alumina column. Its identity, purity and stability in serum were determined by HPLC, thin-layer chromatography (TLC) and mass spectrometry. Its interaction with NIS was assessed in vitro using FRTL-5 rat thyroid cells, with and without stimulation by thyroid-stimulating hormone (TSH), in the presence and absence of perchlorate. Biodistribution and PET imaging studies were performed using BALB/c mice, with and without perchlorate inhibition.

Results

[¹⁸F]TFB was readily prepared with specific activity of 10 GBq/mg. It showed rapid accumulation in FRTL-5 cells that was stimulated by TSH and inhibited by perchlorate, and rapid specific accumulation in vivo in thyroid (SUV = 72 after 1 h) and stomach that was inhibited 95% by perchlorate.

Conclusion

[¹⁸F]TFB is an easily prepared PET imaging agent for rodent NIS and should be evaluated for hNIS PET imaging in humans.

Electronic supplementary material

The online version of this article (doi:10.1007/s00259-010-1523-0) contains supplementary material, which is available to authorized users.

124I PET-based 3D-RD dosimetry for a pediatric thyroid cancer patient: real-time treatment planning and methodologic comparison

Patient-specific 3-dimensional radiobiologic dosimetry (3D-RD) was used for (131)I treatment planning for an 11-y-old girl with differentiated papillary thyroid cancer, heavy lung involvement, and cerebral metastases. (124)I PET was used for pharmacokinetics. Calculation of the recommended administered activity, based on lung toxicity constraints, was performed in real time (i.e., during the data-acquisition interval). The results were available to the physician in time to influence treatment; these estimates were compared with conventional dosimetry methodologies. In subsequent, retrospective analyses, the 3D-RD calculations were expanded to include additional tumor dose estimates, and the conventional methodologies were reexamined to reveal the causes of the differences observed. A higher recommended administered activity than by an S-value-based method with a favorable clinical outcome was obtained. This approach permitted more aggressive treatment while adhering to patient-specific lung toxicity constraints. A retrospective analysis of the conventional methodologies with appropriate corrections yielded absorbed dose estimates consistent with 3D-RD.

Imaging in targeted delivery of therapy to cancer

We review the current status of imaging as applied to targeted therapy with particular focus on antibody-based therapeutics. Antibodies have high tumor specificity and can be engineered to optimize delivery to, and retention within, the tumor. Whole antibodies can activate natural immune effector mechanisms and can be conjugated to beta- and alpha-emitting radionuclides, toxins, enzymes, and nanoparticles for enhanced therapeutic effect. Imaging is central to the development of these agents and is used for patient selection, performing dosimetry and assessment of response. gamma- and positron-emitting radionuclides may be used to image the distribution of antibody-targeted therapeutics While some radionuclides such as iodine-131 emit both beta and gamma radiation and are therefore suitable for both imaging and therapy, others are more suited to imaging or therapy alone. Hence for radionuclide therapy of neuroendocrine tumors, patients can be selected for therapy on the basis of gamma-emitting indium-111-octreotide imaging and treated with beta-emitting yttrium-90-octreotate. Positron-emitting radionuclides can give greater sensitivity that gamma-emitters but only a single radionuclide can be imaged at one time and the range of radionuclides is more limited. The multiple options for antibody-based therapeutic molecules, imaging technologies and therapeutic scenarios mean that very large amounts of diverse data are being acquired. This can be most effectively shared and progress accelerated by use of common data standards for imaging, biological, and clinical data.

Highlights of the annual congress of the European Association of Nuclear Medicine, Copenhagen 2007

INTRODUCTION

The Annual Congress of the European Association of Nuclear Medicine took place in Copenhagen on October 13-17, 2007. The event is the major scientific and professional effort in the field of nuclear medicine in Europe. The most important developments in the fields of instrumentation, radionuclide production, radiochemistry, radiotherapy, as well as the clinical imaging fields of neurology, cardiology, oncology, and general sciences were reported.

OBJECTIVE

This paper emphasizes the major findings and trends at this important gathering. This review is, however, only a brief summary of the large amount of data discussed.