To the best of our knowledge, this study is the first to evaluate 68Ga-Trivehexin PET/CT in the parathyroid adenoma population and compare it to the MIBI scan. It shows that 68Ga-Trivehexin PET/CT exhibits significant tracer uptake across all but one parathyroid adenoma. Notably, comparative analysis unveiled the detection of 7 parathyroid lesions overlooked by the MIBI scan, while also providing a high lesion-to-background ratio and improved delineation of inconclusive lesions identified on the MIBI scan.

Parathyroid scintigraphy, using [99mTc]Tc-MIBI is regarded as the primary radionuclide imaging for localization of hyperfunctioning parathyroid tissue in cases of PHPT. Dual-phase imaging, acquiring images at both early and delayed time points, has been found to be more accurate than single-phase acquisition. Moreover, comparative studies have demonstrated the superiority of SPECT/CT with [99mTc]Tc-MIBI over planar or stand-alone SPECT imaging. A recent meta-analysis including 1236 patients reported a patient-based and lesion-based pooled detection rate of 88% for [99mTc]Tc-MIBI SPECT/CT highlighting it as the standard method for detecting hyperfunctioning parathyroid glands. Similarly, EANM guidelines recommend utilizing at least one SPECT/CT study covering the anatomical region from the skull base to the heart base, while employing dual-phase acquisition [17]. This comprehensive approach maximizes the chances of precisely localizing hyperfunctioning parathyroid glands and helps appropriate clinical management and surgical planning for patients with pHPT. Considering this proven diagnostic accuracy, it was surprising that in our cohort 7 lesions out of 17 were negative on MIBI scan but positive on 68Ga-Trivehexin PET/CT, although these lesions were in the field of view of both dual-phase and SPECT/CT images. [99mTc]Tc-MIBI SPECT/CT and dual-phase planar imaging have a reported combined sensitivity ranging from 87–97% [18]. However, several factors may affect the sensitivity of a MIBI scan. The size and location of the adenoma as well as the presence of concurrent thyroid pathologies or potential interference from high thyroid activity are reasons that merit consideration in this context. In this regard, in our cohort, multiple thyroid nodules of millimetric dimensions were noted. However, while two thyroid nodules showed uptake on 99mTechnetium thyroid scintigraphy; and one of these nodules surprisingly demonstrated 68Ga-Trivehexin uptake on PET/CT, none of them showed uptake on the MIBI scan. Therefore, interfering thyroid nodules does not explain the negativity of the MIBI scan. In addition, it should be noted that except for one, thyroid nodules did not exhibit significant 68Ga-Trivehexin. On the other hand, in our cohort, comprising 17 detected lesions, it was observed that only three exceeded a diameter of 1 cm when assessed on 2D axial cross-sections. When examining lesions that were negative for MIBI but positive for 68Ga-Trivehexin, all seven lesions were of millimetric dimensions, with the largest measuring 9 × 3 mm. Moreover, of the 9 lesions that were positive for both [99mTc]Tc-MIBI and 68Ga-Trivehexin, PET/CT provided superior delineation for 4 of these lesions. In this regard, the superiority and enhanced clarity of 68Ga-Trivehexin PET/CT can be attributed to the inherent characteristics of PET, particularly its superior spatial resolution compared to planar and [19] SPECT modalities.

MIBI scan using either dual planar or SPECT/CT or a combination of conventional methods is the imaging of choice in cases where USG fails to detect suspected adenoma. However, studies are also highlighting the suboptimal sensitivity of MIBI scans. For instance, in a study involving 658 patients, the detection rate of [11C]-methionine or [11C]-choline PET was notably higher compared to MIBI scans (79.4% vs. 25.4%). Notably, this study did not encompass the entirety of its patient cohort undergoing SPECT/CT, which warrants caution. Moreover, a recent meta-analysis has emphasized the superiority of [99mTc]Tc-MIBI SPECT/CT over traditional planar imaging methods, demonstrating a pooled sensitivity of 86%. Considering these findings, it is essential to reassess the role of MIBI scans and explore alternative imaging modalities in the diagnosis of suspected parathyroid adenomas. In line with this context, EANM guidelines stated that it can be considered an alternative first-line imaging method. The utilization of [18F]fluorocholine PET/CT in the identification of hyperfunctioning parathyroid glands is presently acknowledged as an emerging area of interest [19]. Furthermore, several evidence-based data highlight the excellent diagnostic performance of this imaging method in detecting hyperfunctioning parathyroid glands with superior diagnostic performance surpassing all other currently accessible imaging modalities currently available. Notably, its reported sensitivity ranges from 90 to 97%, indicative of superior diagnostic performance [20,21,22,23]. Given the fact that not all patients underwent [18F]fluorocholine PET/CT in our cohort definitive conclusions regarding the diagnostic efficacy of 68Ga-Trivehexin PET/CT compared to [18F]fluorocholine PET/CT cannot be drawn. However, the impressive detection rate of 94.1% observed with 68Ga-Trivehexin PET/CT in our study cohort underscores its potential as a novel imaging tool. Given this promising rate which align with [18F]fluorocholine PET/CT related data in the literature, further studies comparing 68Ga-Trivehexin and [18F]fluorocholine PET/CT are essential to fully explore its clinical utility.

While our study demonstrates the promising diagnostic potential of 68Ga-Trivehexin PET/CT in localizing hyperfunctioning parathyroid adenomas, it is crucial to acknowledge that the unknown mechanisms driving integrin receptor overexpression in these adenomas represent the primary limitation of our investigation. Integrins are heterodimeric transmembrane glycoproteins, consisting of one α- and one β-subunit, and have a fundamental role in regulating crucial functions during cell adhesion, proliferation, migration/invasion, survival, and apoptosis. Many of the 24 human integrin subtypes known to date are involved in every step of cancer development which makes them compelling targets for further investigation and therapeutic intervention. In this regard, αvβ3 is a well-established integrin subtype for its role as a promoter of tumor angiogenesis whereas, the dimer αvβ6 targeted by 68Ga-Trivehexin is uniquely expressed by epithelial cells and plays a central role in the complex process of carcinogenesis. However, the mechanisms underlying the presence or overexpression of integrins in parathyroid adenomas remain unknown, posing a significant challenge.

The parathyroid glands are primarily composed of chief cells, responsible for PTH secretion, and oxyphil cells, distinguished by their high mitochondrial content. [99mTc]Tc-MIBI diffuses passively through oxyphilic cell membrane and accumulates in the mitochondria enabling the use of MIBI scan to effectively detect parathyroid lesions. In our cohort, the reference standard for lesions was not based on post-surgical histopathological examination, which is a limitation of our study. Consequently, we cannot report the ratio of chief to oxyphilic cells, which could provide insight into MIBI-negative lesions detected by 68Ga-Trivehexin. The specific pathways underlying integrin overexpression in parathyroid adenomas remain elusive, presenting a significant barrier to fully exploiting the diagnostic implications of integrin receptor targeting in parathyroid imaging. In a previous research [24], it was found that PTH-related Protein (PTHrP) increases cell migration and invasion and upregulates the expression of the integrin α6β4. While PTH regulates serum calcium and phosphate levels, PTHrP, a hormone in the PTH family, has crucial developmental and physiological roles and is a primary cause of malignancy-related hypercalcemia. Additionally, PTH and PTHrP act through a common receptor [25], therefore it is plausible to suggest that PTH might also induce the overexpression of integrin receptors. However, 68Ga-Trivehexin binds specifically to the integrin receptor subtype αvβ6, which is known to be expressed in epithelial cells of the lung, skin, and kidney, but its expression in the parathyroid glands remains undetermined.

We also acknowledge noteworthy findings in our cohort that merit careful consideration, indicating potential variability in the specificity of this novel modality. For instance, a hyperfunctioning thyroid nodule on thyroid scintigraphy showed no 68Ga-Trivehexin or [99mTc]Tc-MIBI uptake. However, a normal functioning thyroid nodule showing mild [99mTc]NaTcO4 uptake surprisingly exhibited mild 68Ga-Trivehexin uptake. Another patient presented heterogeneous mild uptake in the thyroid glands which significantly decreased on the delayed images. 68Ga-Trivehexin uptake in these instances may cause an overlap in imaging characteristics between parathyroid and thyroid tissues underscoring the need for careful interpretation of 68Ga-Trivehexin PET/CT across different thyroid conditions. Nevertheless, the underlying mechanisms driving these uptake patterns remain unclear especially given the scarcity of data in the literature. Future studies should focus on these mechanisms, as understanding the molecular basis of integrin receptor expression could not only enhance the accuracy of imaging techniques but also pave the way for novel targeted therapeutic interventions in PHPT. Furthermore, we acknowledge limitations including the relatively small cohort size, potential patient selection bias, and the controlled study design, which preclude definitive sensitivity, specificity, and predictive value assessments.