18F-Fluorocholine PET and Multiphase CT Integrated in Dual Modality PET/4D-CT for Preoperative Evaluation of Primary Hyperparathyroidism

The present retrospective study evaluates the diagnostic value of integrated 18F-Fluorocholine positron emission tomography/four-dimensional contrast-enhanced computed tomography (18F-FCH PET/4D-CT) as second-line imaging in preoperative work-up of primary hyperparathyroidism (pHPT), and compares 18F-FCH PET with 4D-CT. Patients with pHPT and negative/discordant first-line imaging addressed for integrated 18F-FCH PET/4D-CT were retrospectively selected. Sensitivity and detection rate (DR%) of 18F-FCH PET/CT, 4D-CT, and PET/4D-CT were calculated according to the per patient and per lesion analyses, and afterwards compared. Histology associated with a decrease more than 50% of perioperative parathyroid hormone (PTH) blood level was used as a gold standard. Persistent high serum PTH and calcium levels during a 6-month follow-up was considered as presence of pHPT in both operated and non-operated patients. 50 patients (55 glands) were included. 44/50 patients (88%) were surgically treated. On a per patient analysis, sensitivity was 93%, 80%, and 95%, and DR% was 82%, 68%, and 84%, respectively for PET/CT, 4D-CT, and PET/4D-CT. PET/CT was more sensitive than 4D-CT (p = 0.046). PET/4D-CT performed better than 4D-CT (p = 0.013) but was equivalent to PET/CT alone. On a per gland analysis, sensitivity PET/CT, 4D-CT, and PET/4D-CT was 88%, 66%, and 92%, and DR% was 79%, 57%, and 83%, respectively. PET/CT and PET/4D-CT were more sensitive than 4D-CT alone (p = 0.01, p < 0.001, respectively). However, PET/CT and PET/4D-CT performed similarly. In conclusion, 18F-FCH PET provides better identification of hyperfunctioning parathyroids than 4D-CT and the combination of both did not significantly improve diagnostic sensitivity. Further investigations involving larger populations are necessary to define the role of 18F-FCH PET/4D-CT as a “one-stop shop” second-line imaging in preoperative work-up of pHPT, especially considering the additional patient radiation exposure due to multi-phase CT.


Introduction
Primary hyperparathyroidism (pHPT) is a common endocrine disorder due to excessive secretion of parathyroid hormone (PTH) from one or more hyperfunctioning parathyroid glands. Solitary technique including neck and mediastinum anterior planar scan (Symbia T2/T6, Siemens Healthcare, Erlangen, Germany, low energy high resolution (LEHR) collimator, 256 × 256 matrix, 300 s per frame) and single photon emission tomography/computed tomography (SPECT/CT) acquisition (LEHR collimator, 128 × 128 matrix, 36 projections of 50 s; helical CT, 140 kV tension, 2.5 mA intensity, without injection of contrast agent). Scintigraphic images were obtained with a simultaneous double window recording, around 2 photopeaks of 5% centered over the 140 keV of 99 mTc and 159 keV of 123 I [22].
Integrated 18 F-FCH PET/4D-CT were performed by a combined PET/CT device equipped with time of flight measurement capacity and a 128 detector-row CT scanner (Biograph128 mCT, Siemens Healthcare, Erlangen, Germany). Imaging protocol integrated: Dynamic four-phase CT scan, including non-enhanced CT (140 kV, 115m A, 1s per rotation and pitch 0.8, slice thickness of 1 mm) followed by arterial phase (10-15 s after injection, aortic arch threshold >80 HU), venous phase (45 s after injection), and late-venous phase (70 s after injection). 75 mL of iodine contrast agent (Iomeron 400 mg/mL) was intravenously injected with a 2.5-3 ml/s flow rate followed by a saline chaser. CT parameters of arterial and venous phases were: 120 kV, 1 and 15 mAs, 1 s rotation time, pitch 0.8, slice thickness of 1 mm. CT CARE Dose 4D combined with sinogram-affirmed iterative reconstruction (SAFIRE) was used. Diabetic patients withdrew metformin treatment for 2 days after the PET/4D-CT, and abundant hydration was recommended. 18 F-FCH PET scan. Patients fasted for at least 6 h before 2 MBq/kg of 18 F-FCH intravenous injection. 10-min step PET acquisition from the mandible to the carina was acquired 60 min after 18 F-FCH administration in the supine position with arms along the body and headrest. PET datasets were reconstructed iteratively ordered subset expectation maximization (OSEM) algorithm, 2-iterations, 21-subsets) using no contrast-enhanced CT for attenuation correction.

Imaging Analyses
18 F-FCH PET/CT and 4D-CT from integrated 18 F-FCH PET/4D-CT were independently interpreted on a dedicated workstation (Syngo.via VB30; Siemens Healthcare, Erlangen, Germany) by a single endocrine radiologist and two nuclear medicine physicians. A third nuclear medicine reviewer was required to reach a consensus when necessary. Referring physicians were aware of patients' clinical data, biological results, and first-line parathyroid imaging, but were blinded to the results of either PET or 4D-CT scan. Finally, 18 F-FCH PET/4D-CT studies were interpreted jointly by one nuclear medicine physician and one radiologist. 18 F-FCH PET and 4D-CT were qualitatively interpreted as positive or negative. Focal non-physiological uptake or tissular nodule (with contrast media enhancement and wash-out of >20 HU at late-venous phase) corresponding to any cervical or thoracic abnormalities discriminable from thyroid tissue and positioned in typical parathyroid sites or in ectopic areas was considered positive. For each positive gland, maximum standardized uptake value (SUVmax), metabolic tumoral volume (MTV), and Hounsfield (HU) value were assessed. The number of pathological findings on 18 F-FCH PET/CT, 4D-CT, and 18 F-FCH PET/4D-CT, their topography in reference to the midline and the thyroid gland, ectopic location, and embryologic origin of the parathyroid (superior vs. inferior gland) were recorded.

Gold Standard
Histological proof [23], associated with a decrease of more than 50% of perioperative PTH blood level, as well as PTH and serum calcium values obtained on 6-months follow-up were used as gold standard. In non-operated patients, imaging results were compared to 6-month clinical and biological follow-up after PET/4D-CT, including PTH and calcemia measurements. Persistent high serum PTH and calcium levels were considered to indicate a presence of pHPT.

Statistical Analyses
Results for continuous data were expressed as mean ± standard deviation or median and range as appropriate. Categorical variables were presented as numbers and percentages. Differences between groups were assessed by the Student t-test for continuous variables. Sensitivity and detection rate (DR%) of 18 F-FCH PET/CT, 4D-CT, and PET/4D-CT were calculated according to per-patient and per-lesion analyses. Sensitivity was evaluated on surgically treated patients, while DR% was assessed on the overall population. Moreover, diagnostic performances were determined separately for both identification on the correct side (left/right) and the gland embryological origin using surgical findings as the standard of reference. Patient-to-patient and lesion-to-lesion analyses were performed using the McNemar or Fisher's exact test as appropriate. The receiver operating characteristic (ROC) curves were used to define the SUVmax threshold for diagnostic purpose. Correlations between variables were assessed using the Pearson correlation coefficient. Two-sided p values <0.05 were considered significant. Statistical analyses were performed using open access statistical software (biostatgv.sentiweb.fr, Institut Pierre Louis d'Epidémiologie et de Santé Publique, UMR S 1136, INSERM -Sorbonne Université, Paris).

Patient Population
In the study period, 90 patients with pHPT and negative/inconclusive first-line imaging were addressed for preoperative evaluation. Contrast media agent injection was contraindicated in 11 patients. An additional 29 cases were excluded because of postponed/refused surgery and missing follow-up data. Thus, 50 patients were finally enrolled (Table 1). Population median age was 64 years (range 28-79 years), with a female to male ratio of 3:1. 21 (42%) patients were symptomatic. The preoperative mean serum calcium and PTH levels were 2.69 mmol/L and 136 ng/L, respectively. 10 (18%) patients presented with recurrent pHPT. All patients had at least one preoperative imaging modality. 37 (74%), 45 (90%), and 33 (66%) patients had US, parathyroid scintigraphy, or both. 44/50 patients (88%) were surgically treated (n = 28, minimally invasive parathyroidectomy; n = 15, exploratory cervicotomy including sternotomy in one case; n = 1, thoracoscopy) for a total of 51 excised parathyroid glands. Pathology revealed adenoma and hyperplasia in 37 and 13 cases, respectively. One gland was normal ( Table 2).  parathyroid scintigraphy, or both. 44/50 patients (88%) were surgically treated (n = 28, minimally invasive parathyroidectomy; n = 15, exploratory cervicotomy including sternotomy in one case; n = 1, thoracoscopy) for a total of 51 excised parathyroid glands. Pathology revealed adenoma and hyperplasia in 37 and 13 cases, respectively. One gland was normal ( Table 2).   Thirty-nine and five operated patients showed single (SGD) and multiple glandular disease (MGD), respectively. In the remaining six patients (12%) parathyroidectomy was not carried out Thirty-nine and five operated patients showed single (SGD) and multiple glandular disease (MGD), respectively. In the remaining six patients (12%) parathyroidectomy was not carried out because of patient refusal, comorbidities, or surgical contraindications. Among them, two revealed biological normalization during six months of follow-up (true negative patient), and four showed persistent pHPT. Considering the prevalence of solitary adenoma in patients with pHPT, we have considered these cases as SGD. Thus, a total of 55 glands in 50 patients have been studied.

Discussion
Herein, we have retrospectively evaluated the diagnostic performance of combined stand-alone 18 F-FCH PET/4D-CT versus both 4D-CT and non-contrast-enhanced PET/CT, as second line imaging for preoperative detection of hyperfunctioning parathyroid glands in 50 patients with pHPT. The main conclusions that can be drawn comprise the following. Firstly, 18 F-FCH PET/CT was more sensitive than 4D-CT in both per patient (93% vs. 80%, p = 0.046) and per gland analysis (88% vs. 66%, p = 0.01), confirming the high diagnostic performance of 18 F-FCH PET/CT as second line imaging in preoperative work-up of pHPT. Secondly, the integration of dynamic 4D-CT and 18 F-FCH PET in one single PET/4D-CT examination does not seem to offer a significant diagnostic benefit rather than PET alone as shown by a per patient and per gland analysis. Regarding the comparison between 18 F-FCH PET/CT and 4D-CT, our results are in agreement with the recent study by Piccardo et al. [19], which shows the superiority of PET/CT compared to 4D-CT. However, on a gland-based analysis, PET/CT sensitivity was higher than that reported in Piccardo's study (80%) and closer to the sensitivity value

Discussion
Herein, we have retrospectively evaluated the diagnostic performance of combined stand-alone 18 F-FCH PET/4D-CT versus both 4D-CT and non-contrast-enhanced PET/CT, as second line imaging for preoperative detection of hyperfunctioning parathyroid glands in 50 patients with pHPT. The main conclusions that can be drawn comprise the following. Firstly, 18 F-FCH PET/CT was more sensitive than 4D-CT in both per patient (93% vs. 80%, p = 0.046) and per gland analysis (88% vs. 66%, p = 0.01), confirming the high diagnostic performance of 18 F-FCH PET/CT as second line imaging in preoperative work-up of pHPT. Secondly, the integration of dynamic 4D-CT and 18 F-FCH PET in one single PET/4D-CT examination does not seem to offer a significant diagnostic benefit rather than PET alone as shown by a per patient and per gland analysis. Regarding the comparison between 18 F-FCH PET/CT and 4D-CT, our results are in agreement with the recent study by Piccardo et al. [19], which shows the superiority of PET/CT compared to 4D-CT. However, on a gland-based analysis, PET/CT sensitivity was higher than that reported in Piccardo's study (80%) and closer to the sensitivity value stated in a recent meta-analysis on the subject (92%) [14]. The design of our study probably contributes to the explanation of these findings. Indeed, 74% of our patients underwent US, 90% had parathyroid scintigraphy, and only 66% had both. This approach could have significantly increased the number of false-negative or inconclusive first-line imaging results, potentially overrating PET/CT sensitivity. Moreover, the potentially different PET/CT devices and related acquisition protocols between our study and those of Piccardo should be considered. In the same study, 4D-CT was more sensitive than was found in our population (74% vs. 66%). The presence of MGD (compared to exclusive SGD) might justify such results [24]. More importantly, and in contrast to Piccardo's study [19], sensitivities of PET/CT and PET/4D-CT were not statistically different, and the integrated 4D-CT corrected the diagnosis for only two PET-negative hyperfunctioning parathyroids (left lower adenoma of 10 × 5 × 3 mm and 0.1 g, and right superior hyperplasia of 10 × 7 × 1 mm and <0.1 g) in two patients with pHPT and serum PTH values of 94 et 74 pg/mL, respectively. In these two cases, no pathological features contributed to explain the negative PET results, which are probably related to both the size and weight of the gland.
Our investigation does not allow for a "head-to-head" comparison between 18 F-FCH PET/CT and 123 I/ 99m Tc-sestamibi SPECT/CT. At this proposal, a recent study underlines the superiority of radiolabeled-choline PET/CT over conventional first-line imaging methods in patients with pHPT [25]. Although some reports suggest superiority of 4D-CT compared to scintigraphy for initial hyperfunctioning gland identification in pHPT [26], 4D-CT is mainly used as a second-line imaging modality in patients with mild hypercalcemia or recurrent hyperparathyroidism (HPT). Comparison between 4D-CT and 18 F-FCH PET/CT for recurrent pHPT management is available in only one study investigating a population of 20 patients with prior neck surgery for pHPT or unrelated reasons [18]. In this study, a 4D-CT sensitivity was lower than that of 18 F-FCH PET/CT (63% vs. 96% on a gland-based analysis), suggesting only a confirmatory role for 4D-CT in this challenging population. Our results seem to confirm this observation showing a per gland analysis sensitivity of 55% and 100% for 4D-CT and PET/CT, though the results are not statistically significant (p = 0.07), likely because of the small sample size (10 patients, 11 glands). Seven of 50 examined patients had a previously history of thyroidectomy. In these cases, 18 F-FCH PET showed a sensitivity of 100% for hyperfunctioning parathyroid detection suggesting that previous thyroid surgery has limited consequences on gland detectability. Conversely, a thyroid remnant closer to the surgical clips could have a moderate absorption of 18 F-FCH resulting in a false positive PET study. In this situation, SUVmax seems useful to discriminate hyperfunctioning parathyroids from other causes of 18 F-FCH uptake. More studies on larger population are still necessary to confirm the value of (semi)quantitative diagnostic approach.
Four pathologic parathyroids (two patients) were not detected by the three imaging modalities probably due to their small size and weight (less than 10 mm and 0.1 g), an uncommon histological characteristic (one case of oncocytic micro-adenoma), and patient biological profiles (slight PTH increase (83 pg/mL) and subnormal calcemia (2.60 mmol/L) in one case, and moderate PTH increase (102 pg/mL) and vitamin D insufficiency (21 ng/mL) in the other one). Interestingly, some recent evidence points out the potential of a PET dual-time-point acquisition protocol to reduce false negative results in cases of negative/inconclusive standard PET study [27]. As previously reported [28,29], we found a positive and significant correlation between blood PTH level, parathyroid gland weight, and both SUVmax and MTV. Accordingly, PTH blood level could be imagined as a predictor of PET/CT results in preoperative pHPT work-up, but more studies are needed for defining a reliable PTH threshold for clinical use. On the contrary, PET parameters were not related to the kinetic of iodinated contrast-media, as recently reported [19].
We found both SUVmax and MTV significantly higher for adenomas than hyperplasia (p = 0.007 and 0.05, respectively) in influencing PET sensitivity. These findings could be explained by previous metabolomic investigations of hyperfunctioning parathyroids [30]. Indeed, adenomas in patients with pHPT and SGD exhibit a higher concentration of choline, phosphorylcholine, and glycerophosphocholine than hyperplastic glands. These metabolites belong to the structural components of cell membranes and their relative abundance probably represents the biologic substrate, justifying the use of 18 F-FCH PET imaging in pHPT.
4D-CT raises justified concerns for radiation exposure of the patient, which is estimated to be double that of 99m Tc-sestamibi scintigraphy [31]. Because 4D-CT scanners use a multi-phase CT protocol, it delivers a high dose to the thyroid, which is problematic in younger patients. Moreover, it needs the injection of contrast medium in a population with potential impaired renal function. Therefore, the benefit of the use of 4D-CT for identifying a symptomatic parathyroid adenoma, particularly if it is possibly resectable, with minimally invasive parathyroidectomy, should be weighed against the risk associated with increased patient radiation exposure. 2D-CT (arterial and venous only without non-contrast phase) could be a good compromise if combined with high resolution US, and it is recommended by some experts to reduce the patient s radiation exposure while preserving diagnostic accuracy [32].
Recently, PET/MRI hybrid systems have become available for clinical use and may open new research avenues, especially considering the emerging role of magnetic resonance imaging (MRI) in the preoperative identification of hyperfunctioning parathyroid in patients with pHPT [33,34]. However, further studies will be needed to validate and justify the use of this complex imaging approach, including long-term economic considerations. The real challenge to maximize the therapeutic benefit will be to redefine imaging diagnostic protocols according to specific clinical contexts, and to articulate both the strengths and limitations of each diagnostic modality. The optimal combination of imaging technique, imaging sequencing, and the choice of patient needs to be determined to improve the diagnostic accuracy and reduce non-essential patient radiation exposure.
The retrospective character and the lack of pathological gold standard in 6/50 patients represent the main limitations of our study. Second, the relatively small sample size limits a meaningful analysis with potential underestimation of the added value of integrated PET/4D-CT compared to PET alone. Moreover, as previously discussed, the obtained diagnostic performances of 18 F-FCH PET parathyroid imaging could be potentially influenced by a patient selection bias overestimating PET/CT sensitivity. However, despite these limitations, this study is the largest available that compares 18 F-FCH PET and 4D-CT, consolidating the potential role of 18 F-FCH PET as second-line imaging in patients with pHPT. However, the lower availability and higher costs of 18 F-FCH PET compared to usual first line imaging (US, scintigraphy) may negatively impact routinely clinical utilization.

Conclusions
18 F-FCH PET/CT provides better preoperative identification than 4D-CT, and the combination of the two imaging modalities did not significantly improve diagnostic sensitivity in patients with pHPT. However, contrast-enhanced-CT remains useful for operative planning and enables surgical guiding by 3D-virtual neck exploration. Further investigations involving large patient populations are necessary to define the relevance of integrated 18 F-FCH PET/4D-CT as "one-stop shop" second-line imaging in preoperative work-up of pHPT and to consider the additional patient radiation exposure due to multi-phase CT.