Tumor Burden of Iodine-Avid Bone Metastatic Thyroid Cancer Identified via ¹⁸F-Sodium Fluoride PET/CT Imaging

Abstract

Background: Patients with differentiated thyroid cancer (DTC) are referred to radioactive ¹³¹I (RAI) therapy and post-therapy ¹³¹I whole-body scintigraphy (WBS) to identify local and/or remote metastases. Positron emission tomography (PET)/computed tomography (CT) imaging with ¹⁸F-fluoro-D-glucose (FDG) or ¹⁸F-sodium fluoride (NaF) may also be used with these patients for the evaluation of bone metastases. We compared the role of ¹⁸F-NaF PET/CT and ¹⁸F-FDG-PET/CT in patients with DTC and documented bone metastases at post-therapy WBS. Methods: Ten consecutive DTC patients with iodine avid bone metastasis at post-therapy WBS referred to ¹⁸F-NaF PET/CT and ¹⁸F-FDG PET/CT were studied. The findings of the three imaging procedures were compared for abnormal detection rates and concordance. Results: At post-therapy ¹³¹I WBS, all patients had skeletal involvement with a total of 21 bone iodine avid lesions. At ¹⁸F-FDG PET/TC, 19 bone lesions demonstrated increased tracer uptake and CT pathological alterations, while 2 lesions did not show any pathological finding. At ¹⁸F-NaF PET/CT, the 19 bone lesions detected at ¹⁸F-FDG PET/TC also demonstrated abnormal tracer uptake, and the other 2 bone iodine avid foci did not show any pathological finding. Conclusions: In patients with DTC, ¹⁸F-NaF PET/CT did not obtain more information on the metastatic skeletal involvement than post-therapy ¹³¹I WBS and ¹⁸F-FDG PET/CT.

1. Introduction

Differentiated thyroid cancer (DTC) is the most frequent endocrine malignancy, and its rates are raising all around the world [1]. It is believed that this increase is mainly due to better screening procedures, such as the introduction of neck ultrasounds in daily practices as part of the basic assessment of thyroid diseases. According to histological staging, after surgery, patients with DTC can be led to radioactive (RAI) therapy with ¹³¹I for ablation purposes in protected hospitalization. After 5–9 days since RAI, a ¹³¹I whole-body scintigraphy (WBS) is performed to identify local and/or distant metastases greedy for iodine. Follow-ups consist of periodical instrumental and biochemical analyses of patients in suppressive therapy with thyroid hormones [1]. As part of the monitoring of DTC patients undergoing RAI, the surveillance program includes the evaluation of thyroglobulin (Tg) levels, anti-Tg antibodies and neck ultrasounds for the early detection of loco-regional recurrence [2]. Once the Tg blood level rises and remote metastases are suspected, a positron emission tomography (PET)/computerized tomography (CT) with 2-deoxy-2-[18F]-fluoro-D-glucose (¹⁸F-FDG) is performed to identify any metastatic foci. In addition to cervical lymph nodes, the most frequent localizations of repetitive DTC lesions are the lung and skeleton. Although PET/CT with ¹⁸F-FDG has high sensitivity in identifying lesions with high metabolic turnover, an earlier marker of bone involvement would allow more effective and less invasive therapeutic approaches before bone damage inexorably affects the quality of life [3]. The positron-emitting radiopharmaceutical ¹⁸F-NaF was introduced decades ago for skeletal imaging [4]. Currently, ¹⁸F-NaF PET/CT is used for the functional imaging of pathologies with high osteogenic metabolism. This diagnostic procedure is used for the detection and localization of bone metastases in cancer patients [5,6,7,8]. Prior studies suggest that ¹⁸F-NaF PET/CT imaging may provide high sensitivity and specificity in the detection of bone metastases [9,10,11]. A study carried out in 52 oncological patients demonstrated the superiority of PET/CT with ¹⁸F-NaF over bone scintigraphy with ^99mTc methyl-diphosphonate (MDP) in terms of image quality and a more accurate evaluation of the extent of skeletal disease [12]. Schirrmeister et al. [13] confirmed the better performance of ¹⁸F-NaF PET/CT over ^99mTc-MDP bone scintigraphy in DTC patients with skeletal metastases. Yet, only a few data are available on the comparison between ¹⁸F-FDG PET/CT and ¹⁸F-NaF PET/CT in oncological patients [14,15]. The aim of the present pilot investigation was to assess the role of ¹⁸F-NaF PET/CT in evaluating patients with DTC and known bone metastases at post-therapy ¹³¹I WBS undergoing ¹⁸F-FDG-PET/CT.

2. Methods

2.1. Study Popolation

Ten consecutive patients were recruited prospectively from June 2020 to November 2022. The inclusion criteria were as follows: (a) age older than 18 years; (b) DTC previously treated with total thyroidectomy and post-operative ¹³¹I administration; (c) presence of iodine-avid bone metastases at post-therapy WBS with indication for ¹⁸F-NaF PET/CT to evaluate further skeletal involvement; and (d) suspicion of non-iodine avid metastases with indication to conduct a ¹⁸F-FDG PET/CT scan according to serum Tg and Tg antibody levels. Exclusion criteria were (a) pregnancy, (b) blood glucose levels greater than 140 mg/dL (7.77 mmol/L), (c) inability to tolerate the scan due to claustrophobia or pain and (d) previous external beam treatment on bone metastatic lesions. The protocol was approved by the Ethics Committee of the National Cancer Institute of Naples (Protocol numbers 2–11) and was performed in accordance with the guidelines of the Helsinki II declaration. All participants signed an informed consent form before being included in the study.

2.2. Imaging

Post-therapy WBS was conducted 7 days after the administration of a therapeutic dose of ¹³¹I (1850–5550 MBq). All patients received serum thyroid-stimulating hormone (TSH) concentrations of 30 µIU/mL or more at the time of ¹³¹I administration. Planar images were obtained using a dual-head γ-camera (E.CAM, Siemens Medical Systems, Hoffman Estates, IL, USA) equipped with a high-energy collimator. Data analysis was performed by two nuclear medicine physicians on a dedicated workstation. The WBS results were considered positive when at least one abnormal focus of ¹³¹I uptake was found. All pathological foci were recorded and noted. For the purpose of the study, only patients with iodine-avid bone metastases were referred to further imaging investigations. ¹⁸F-FDG PET/CT whole-body scans were performed 45 to 60 min after tracer administration (3.7 MBq/kg). All patients fasted for at least 6 h prior to imaging, and blood glucose levels were <180 mg/dL at the time of tracer injection. Images were obtained using a PET/CT scanner (Gemini TF 64 scanner, Philips Healthcare, Best, The Netherlands). A CT scan for attenuation correction (average parameters 80 kV, 40 mA) was acquired before PET for a total imaging time of 20 min. A ¹⁸F-NaF PET/CT whole-body scan was obtained 90 min after tracer administration (2.6 ± 1.0 MBq/kg). Images were obtained using a PET/CT system (Discovery IQ, GE Healthcare Discovery IQ, GE Healthcare, Chicago, IL, USA). A diagnostic CT scan for fusion was obtained using a standard protocol without intravenous contrast (120 kV; Auto mA range, 30–250 mA, thickness 3.75 mm). All scans were performed in 3-dimensional mode. PET data were reconstructed with and without attenuation correction into transverse, sagittal and coronal images with a standard iterative algorithm using software provided by the equipment manufacturers that considered attenuation, detector efficiency, scatter and random coincidence corrections. PET images were classified as follows: areas of focally increased tracer uptake corresponding to pathological CT findings including those showing mixed or lytic patterns, with or without sclerotic margins or sclerotic CT features with benign patterns or absent CT alterations.

2.3. Statistical Analysis

Data were expressed as mean ± standard deviation or as percentages. Weighted kappa values (Cohen’s coefficient) were calculated to measure the degree of agreement between imaging methods.

3. Results

3.1. Clinical Characteristics

The clinical characteristics of the study population are shown in

Table 1. Clinical characteristics of the study population.

Pt	Age (Years)	Sex	Histology	Stage	Tg (ng/dL)	TSH (30 µIU/mL)	Activity (MBq)	RAI (n)
1	55	F	FV-PTC	pT3NxM1	90	77	3700	4
2	51	M	PTC	pT3bNxM1	147	33	5550	2
3	51	F	FV-PTC	pT3bNxM1	77	52	3700	3
4	58	F	FTC	pT2NxM1	461	32	3700	2
5	65	M	PTC	pT1(m)Nx	2245	36	3700	5
6	65	F	FTC	M1	4306	113	7400	1
7	86	F	FTC	pT3NxMx	199	30	3700	2
8	71	F	FV-PTC	Pt2(m)N0M1	3383	120	5550	4
9	78	M	FV-PTC	pT2NxM1	13,098	75	6785	2
10	81	F	FTC	pT3N0Mx	403	100	3700	1

PTC, Papillary thyroid carcinoma; FTC, Follicular thyroid carcinoma; FV-PTC, Follicular variant of papillary thyroid carcinoma; Tg, Thyroglobulin; TSH, Thyroid-stimulating hormone; RAI, radioactive iodine.

. Nine patients were on LT4 withdrawal at the time of RAI, while one patient underwent the rh-TSH protocol before RAI therapy due to comorbidity. Tg levels at time of RAI therapy ranged from 77 to 13,098 ng/dL with a mean of 2441 ± 4048 ng/dL (mean thyroid-stimulating hormone: 66 ± 36 mUI/mL). Tg antibody levels were unremarkable in all patients. Only two patients received the first RAI dose at the time of investigation, and they had bone metastases diagnosed by a pathologist’s report. The other eight patients had previously received at least one RAI therapy (range 1–5), and ¹³¹I was formerly administered once to four of them. All of these eight patients had biochemically and structurally persistent metastatic disease, with evidence of RAI-avid metastases prior post-therapy WBS. All patients were treated for metastatic disease therapy purposes.

3.2. Imaging Findings

Post-therapy WBS revealed skeletal involvement with a total of 21 iodine-avid bone foci across all patients (

Table 2. Skeletal imaging findings in the study population.

Pt	Lesion Site	Post-Therapy WBS	18F-FDG PET/CT	18F-NaF PET/CT	CT
1	L5	+	+	+	Lytic with sclerotic margins
	Right femur	+	+	+	Lytic with sclerotic margins
2	Sternum	+	+	+	Mixed
3	Right femur	+	+	+	Mixed
4	L3	+	+	+	Lytic
5	Frontal bone	+	+	+	Lytic
6	Sternum	+	+	+	Lytic with sclerotic margins
	L1	+	+	+	Lytic
	L5-S1	+	+	+	Lytic
7	Left ischiopubic branch	+	+	+	Lytic
8	Left iliac wing	+	+	+	Lytic
	Right sacral wing	+	+	+	Lytic with sclerotic margins
9	Right sphenoid	+	+	+	Lytic
	Sternum	+	+	+	Lytic
	III left rib	+	+	+	Lytic
	Left humerus	+	+	+	Lytic
	Right iliac plug	+	+	+	Lytic
	Right pubic bone	+	+	+	Lytic
	Right acetabular cavity	+	+	+	Lytic
10	D12	+	−	−	−
	Right femur	+	−	−	−

). In particular, five patients presented a singular bone lesion; three patients, two bone lesions; and two patients, ≥3 bone metastatic foci. Regarding extra-skeletal observations, only one patient had residual thyroid bed uptake, while two patients had thyroid bed recurrence; three patients, latero-cervical or mediastinal iodine-avid nodes; and five patients, lung uptake (

Table 3. Extra-skeletal imaging findings in the study population.

Pt	Lesion Site	Post-Therapy WBS	18F-FDG PET/CT
1	Lungs	+	+
	Thyroid bed recurrence	+	+
2	Lungs	+	+
	Thyroid bed recurrence	+	+
	Mediastinal lymph nodes	+	+
3	Mediastinal lymph nodes	+	+
4	−	−	−
5	Right lung	+	−
6	−	−	−
7	Right lung	+	+
8	Left lung	+	−
9	Mediastinal lymph nodes	+	+
10	Thyroid residue	+	−

).

At ¹⁸F-FDG PET/TC, 19 bone lesions in nine patients demonstrated focal tracer uptake and CT abnormalities. Conversely, the two foci with increased ¹³¹I activity at post-therapy WBS in the remaining patient did not show any ¹⁸F-FDG focal pathological uptake. Regarding the extra-skeletal findings, the residual thyroid tissue, demonstrating ¹³¹I uptake at post-therapy WBS, did not show any ¹⁸F-FDG accumulation, while the two thyroid bed recurrences and iodine-avid nodes exhibited significant metabolic activity. Of the five patients with lung involvement, ¹⁸F-FDG uptake was observed in only three of them. However, it is essential to consider that the lung nodules in the other two patients were less than 5 mm in size at CT, rendering them metabolically not assessable. At ¹⁸F-NaF PET/CT, the same 19 bone foci identified with ¹⁸F-FDG PET/TC exhibited abnormal tracer uptake. Similarly, the remaining two bone avid foci did not show any pathological tracer accumulation (Figure 1), and there were no notable CT findings, achieving a 100% agreement (K coefficient, 1) between the two PET/CT methods in assessing bone metastasis. Additionally, another six focal ¹⁸F-NaF bone uptake sites were observed in five patients. However, these findings showed benign patterns at CT, attributed to arthrosis degeneration. Of note, both PET scans were not able to identify bone metastatic lesions in addition to those already identified at post-therapy WBS (K coefficient, 0.95).

Representative examples of imaging findings in two patients are depicted in Figure 2 and Figure 3.

4. Discussion

In our patient population, 21 bone iodine-avid lesions were detected via post-therapy ¹³¹I WBS. The large majority (19) of these lesions also demonstrated ¹⁸F-FDG- and ¹⁸F-NaF-increased uptakes along with CT alterations. The remaining two bone iodine-avid lesions detected in the same patient were not noticed either at the metabolic or morphological level by either scan. However, these findings should be read in the light of the elevated Tg levels in a patient undergoing their first RAI treatment, confirming the strong diagnostic value of high-dose ¹³¹I imaging in DTC in the early identification of metastatic involvement. Additionally, another six focal ¹⁸F-NaF bone uptake sites were observed in five patients with an arthrosis degeneration pattern. The relatively young age of the majority of patients (6 out of 10) is consistent with the observation of only six degenerative lesions with focal ¹⁸F-NaF uptake. Therefore, our results suggest that the use of ¹⁸F-NaF did not obtain more information on the skeletal metastatic burden than post-therapy ¹³¹I WBS and ¹⁸F-NaF PET/CT. From a dosimetry point of view, it should be considered that ¹⁸F-NaF PET/CT implies that a similar amount of ¹⁸F-FDG PET/CT is an effective dose with a cost-effectiveness-increased price. Indeed, while ¹⁸F-NaF imaging may provide information regarding only the bone metastatic load, the use of a ¹⁸F-FDG scan may deliver a more complete evaluation of disease staging [16], including nodes, lung and other extra-skeletal compartments also offering a true complement to post-therapy ¹³¹I WBS for the evaluation of tumor burden involvement [17,18,19]. Moreover, when integrated with magnetic resonance imaging, ¹⁸F-FDG PET may offer a more accurate evaluation of soft tissue compartments with lower radiation exposure [20]. According to prior studies [21,22], the areas of increased ¹⁸F-NaF uptake that were not observed on the ¹³¹I scan demonstrated sclerotic morphological characteristics, confirming that the method features high sensitivity but low specificity. Increased ¹⁸F-NaF uptake in bone may have different etiologies [21,22,23]. Indeed, the level of ¹⁸F-NaF activity is not strictly correlated with the malignant nature of the finding, and benign lesions may demonstrate an even prominent ¹⁸F-NaF accumulation.

A co-registered CT scan may lead to the correct interpretation of the integrated imaging data. CT morphological evaluations are still a benchmark with a sensitivity of 73% and specificity of 95 for oncological applications [24]. Nevertheless, structural alterations became evident only after pathological metabolic changes [25]. Thus, the combined utilization of high-dose ¹³¹I imaging as an early marker of disease advancement and the use of a CT scan to guide morphological assessment may be considered the best method for DTC theragnostic management, and also for the evaluation of bone involvement [26].

To the best of our knowledge, the present investigation is the first to explore the role of ¹⁸F-NaF PET/CT on bone evaluations in patients with DTC undergoing RAI therapy and ¹⁸F-FDG PET/CT with known skeletal involvement as assessed via post-therapy ¹³¹I WBS. The application of the ¹⁸F-NaF PET/CT tool on oncological grounds has been considered in patients with different tumors [6,22,27,28,29]. In comparison with bone scintigraphy, ¹⁸F-NaF imaging had boasted the potential to replace ^99mTc-MDP as a reference bone imaging tool in a phase 3 trial conducted on 290 patients with high-risk prostate or breast cancers [30]. Yet, Piccardo and co-workers [21] found that while ¹⁸F-FDG PET/CT has independent prognostic implications in patients with breast cancer and bone metastases, ¹⁸F-NaF PET/CT does not achieve independent predictive values even with a higher diagnostic accuracy than that of ¹⁸F-FDG PET/CT. Conversely, Ueda et al. [26] observed that in 31 patients with medullary thyroid cancer, ¹⁸F-NaF demonstrated equivalent or higher diagnostic power compared to other imaging modalities in the detection of whole-body skeletal metastases.

It must be pointed out that the current survey considers only patients with DTC and iodine-avid bone metastases. Thus, the role of ¹⁸F-NaF PET/CT in iodine-refractory DTC patients has not been explored yet, and the possibility for ¹⁸F-NaF PET/CT to identify bone metastatic DTC patients before post-therapy ¹³¹I WBS should be further investigated. This issue still represents an unmet need. On the other hand, between the RAI-avid and non-avid patients’ categories, there is a population with both ¹³¹I and ¹⁸F-FDG uptakes in the same lesion or in different lesions representing an assorted group of patients with tumor heterogeneity, and similar prognosis compared to the only ¹⁸F-FDG uptake group [31,32]. However, from the present investigation, it can be reasonable that ¹⁸F-NaF PET/CT may not add any information for the identification of skeletal involvement over WBS and ¹⁸F-FDG, even in the heterogeneous disease group. A potential opportunity in this context could be provided by ⁶⁸Ga-DOTA-FAPI-04. A recent study demonstrated that this radiopharmaceutical is a promising molecule for diagnosing, opening up the way to radioligand therapy in iodine-refractory DTC patients [33].

Of note, it should be highlighted that patients with previous external beam treatment on bone metastatic lesions were not enrolled in this study. Radiation therapy determines bone hypoxia, hypocellularity and the reduction of vascularization bringing down the osteoblastic and osteoclastic metabolisms [34]. All these effects, taken together, may lead to a decrease in radiopharmaceuticals bone uptake including ¹⁸F-NaF. Other limitations should also be considered. Our investigation refers to a small population size. Moreover, the sample was also heterogeneous in terms of treatment, as different doses were applied as well as different numbers of doses received, and all this can influence the number of lesions that can be seen and vice versa as there was no control group to compare with. On the other hand, the presented preliminary data are not encouraging to go on recruitment with additional radiation dose administration to perform further ¹⁸F-NaF PET-CT in this category of patients without the potential for a significant improvement in clinical management. In addition, follow-up data are not available yet. Thus, the prognostic value of the ¹⁸F-NaF method still needs to be assessed. Finally, only planar ¹³¹I imaging was available at the time of the study, and hybrid ¹³¹I scintigraphy integrated with CT data would have improved the accuracy of the current work.

5. Conclusions

The results of this study demonstrate that ¹⁸F-NaF PET/CT does not obtain additional information on metastatic skeletal involvement in DTC patients undergoing post-therapy ¹³¹I WBS and ¹⁸F-FDG PET/CT. Furthermore, post-therapy WBS and PET/CT with ¹⁸F-FDG allowed the evaluation of other potential disease compartments such as the thyroid bed, lymph nodes and lungs, confirming the higher disease detection rates of these approaches compared to other imaging modalities. However, the potential use of ¹⁸F-NaF PET/CT in patients with iodine-refractory DTC suggests that further investigations are needed to better understand its role in DTC patients with bone metastatic disease.