Potential Applications of 68Ga-PSMA-11 PET/CT in the Evaluation of Salivary Gland Uptake Function: Preliminary Observations and Comparison with 99mTcO4− Salivary Gland Scintigraphy

Purpose To preliminarily evaluate the feasibility and potential of using 68Ga-PSMA-11 PET/CT in evaluating the function of salivary glands and lacrimal glands in comparison with 99mTc-pertechnetate (99mTcO4−) salivary gland scintigraphy (SGS). Methods A retrospective study was performed in 15 patients with different degrees of xerostomia and suspected salivary gland dysfunction. Each patient underwent 68Ga-PSMA-11 PET/CT first and SGS the next day, and the findings of both scans were compared. Results The results of 68Ga-PSMA-11 PET/CT and SGS were consistent in 12/15 patients (80%) and were inconsistent in the remaining patients (20%). For 5 (33.3%) of 15 patients, 68Ga-PSMA-11 PET/CT provided more information than did SGS. Additionally, 68Ga-PSMA-11 PET/CT corrected the misdiagnosis by SGS for 1 patient. Conclusions 68Ga-PSMA-11 PET/CT is a potentially useful imaging tool for evaluating the function of salivary glands and lacrimal glands. 68Ga-PSMA-11 PET/CT can be a promising supplement to SGS, and its clinical value deserves further study.


Introduction
e salivary glands consist of three pairs of major salivary glands (parotid glands, submandibular glands, and sublingual glands) and multiple minor salivary glands that are widely distributed in the oral mucosa. Glandular function impairment occurs in approximately 60% of patients with xerostomia [1]. Dry mouth is frequently observed in salivary gland dysplasia, in obstructive diseases, in autoimmune syndrome, and in space-occupying lesions, with the help of radiotherapy and chemotherapy for head and neck tumors and with large-dose 131 I treatment, and is most commonly observed in Sjogren's syndrome. According to the American-European Consensus Group (AECG) criteria, the diagnosis of salivary and lacrimal gland dysfunction is mainly based on the objective and subjective evaluations of patients [2]. However, most of the current studies use objective indicators as a more convincing means of evaluation because the subjective symptoms of patients do not necessarily reflect the dysfunction of salivary glands [3,4]. e objective evaluation methods of clinical salivary gland function mainly include saliva analysis, radiography of the parotids, and 99m Tc-pertechnetate ( 99m TcO − 4 ) salivary gland scintigraphy (SGS). e first two methods are affected by physiological factors, and their application is complicated; therefore, they are not ideal for clinical use. SGS allows a noninvasive inspection of the condition and has the advantages of simplicity, safety, and relatively low cost [5]. SGS is a commonly used method for the clinical evaluation of salivary gland function and has high specificity and sensitivity [6]. SGS has been generally proposed to assess salivary gland function in different indications [7,8]. SGS can be used to directly observe the morphology and function of the bilateral parotid and submandibular glands. Image analysis for SGS depends on the observer and lacks uniform standards [9,10].
Over the past few years, prostate-specific membrane antigen-(PSMA-) targeted radiotracers have been widely used in the diagnosis ( 68 Ga-PSMA) and treatment ( 177 Lu-PSMA) of prostate cancer. Among them, PSMA-targeted positron emission tomography (PET), especially 68 Ga-PSMA-11, has been widely used for the detection, staging, posttreatment efficacy evaluation, and recurrence assessment of prostate cancer [11][12][13]. However, the binding of PSMA ligands is not limited to prostate cancer cells, and their physiological distribution includes the lacrimal glands, salivary glands, kidneys, duodenum, and small intestine [14]. Salivary glands and lacrimal glands have high uptake levels [14][15][16][17]. 68 Ga-PSMA PET/CT can enable the visualization of head and neck lacrimal glands, primary and secondary salivary glands, and seromucous glands. All normal glands have increased tracer uptake [16]. A recent study by Rupp found that the normal submandibular gland had high 68 Ga-PSMA-11 aggregation, but uptake of 68 Ga-PSMA-11 in chronic inflammatory atrophy of the submandibular gland was significantly lower [18]. e purpose of this study was to explore the value of 68 Ga-PSMA-11 PET/CT in evaluating salivary and lacrimal gland function and whether 68 Ga-PSMA-11 PET/CT has a complementary or alternative role to SGS.

Patient Selection.
A total of 15 patients (10 men, 5 women; aged 19-75 years [56.9 ± 15.4 years]) with xerostomia and suspected salivary gland dysfunction were recruited in this study. e study included 4 patients with Sjogren's syndrome, 10 patients with head and neck tumors after radiotherapy and chemotherapy, and 1 patient with surgery for parotid space-occupying lesions. All patients underwent 68 Ga-PSMA PET/CT first, followed by SGS the next day. e inclusion criteria were as follows: clinical diagnosis of Sjogren's syndrome; radiotherapy and chemotherapy for head and neck tumors; high-dose 131 I therapy; salivary gland surgery; salivary gland space-occupying lesions; and salivary gland dysplasia. e exclusion criteria were as follows: impairment of liver and kidney function, low white blood cell count, or pregnancy or lactation. e study followed the 1964 Helsinki Declaration and its later amendments or comparable ethical standards.

Image Acquisition
2.2.1. 68 Ga-PSMA-11 PET/CT. Images were acquired from the skull vertex to the root of neck using a PHILIPS Gemini TF PET/CT 16-tier system approximately 60 minutes after intravenous injection of 1.85MBq/kg 68 Ga-PSMA-11. Patients did not need specific preparation before PET/CT imaging. A low-dose CT scan (voltage: 120 kV, current: 100 mAs, 5 mm layer, 512 × 512 matrix, and 60 cm FOV) was performed first. en, PET was performed (9-10 beds, 3 minutes/bed). After reconstruction, image analysis was performed using PHILIPS postprocessing fusion software.

99m TcO 4
− Salivary Gland Scintigraphy. On the day after 68 Ga-PSMA-11 PET/CT, the patients underwent SGS. e patients were injected with 185 MBq of 99m TcO − 4 , and dynamic images were acquired using a dual-head gamma camera (GE Healthcare, USA) equipped with a low-energy, high-resolution collimator (128 × 128 matrix, 140 KeV energy peak, 20% window width, 2 times amplification). e anterior image was collected with a probe, and the field of vision covered the entire thyroid and salivary glands. e blood perfusion phase image was collected immediately after intravenous injection of 99m TcO − 4 (2 s/frame, 30 frames in total). en, functional imaging was performed for 30 minutes at 40 s/frame. Vitamin C was given to stimulate salivary secretion 5 minutes before completion of the imaging. Images were analyzed on a Xeleris Workstation (GE Healthcare).

68 Ga-PSMA-11 PET/CT Visual analysis.
ree experienced nuclear physicians independently read the PET/CT images. When opinions differed, the majority opinion prevailed. Homogeneous, symmetrical, and strong uptake of 68 Ga-PSMA-11 in the bilateral parotid and submandibular glands was defined as normal. Abnormal uptake of 68 Ga-PSMA-11 in salivary glands included one of the following conditions. (a) e level of 68 Ga-PSMA-11 uptake in salivary glands was similar to that in background tissues (e.g., subcutaneous soft tissue of the head and neck); and (b) 68 Ga-PSMA-11 uptake into the glands presents a visible decrease compared to the contralateral side. Quantitative analysis. To quantify 68 Ga-PSMA-11 uptake, the volumes of interest (VOIs) of the bilateral lacrimal glands, parotid glands, submandibular glands, and thyroid glands were mapped on serial images. e maximum standardized uptake value (SUVmax) in the VOI of each gland was obtained by the software.

99m TcO 4
− Salivary Gland Scintigraphy. Visual evaluation: the images were independently and visually assessed by three experienced nuclear physicians. When the opinions differed, the majority opinion prevailed. e physicians were unaware of the results of other clinical studies. Classification of salivary gland function was based on the criteria used in the study by Kim et al. [19]. Salivary gland uptake is considered normal when it is similar to thyroid uptake. Normal excretion is defined as salivary gland activity similar to background activity after stimulation. Salivary gland function was graded on a scale from 1 to 3: 1, normal; 2, mild-tomoderate; and 3, severe. Grade 1 indicated that the uptake and excretion of salivary glands were normal; grade 2 indicated low uptake and/or delayed excretion; and grade 3 was defined as severe dysfunction with a complete absence of radioactivity in the salivary glands. Semiquantitative analysis: semiquantitative analysis was performed using the region of interest (ROI) technique. e ROIs of the bilateral parotid gland and submandibular gland were depicted, and the bilateral temporal region was used as a background reference. e computer automatically plotted the timeactivity curves. From the time-activity curves, the maximum value before vitamin C administration, the minimum value after vitamin C administration, and the background uptake value were obtained. e uptake ratio (UR) and excretion fraction (EF) of each salivary gland were calculated according to the following equation: UR � (maximumbackground uptake)/background uptake; EF � (maximumminimum)/(maximum-background uptake) × 100% [20]. Decreased uptake function for the respective glands was defined as follows: UR (parotid gland) <2.28 and UR (submandibular gland) <1.60 [19]. Abnormality was indicated if the criteria were met in either visual analysis or semiquantitative analysis.

Results
e general information and PET and SGS imaging results for all 15 patients are shown in Table 1. is study included 4 patients with Sjogren's syndrome (patients 1-4), 10 patients with head and neck tumors after radiotherapy and chemotherapy (patients [5][6][7][8][9][10][11][12][13][14], and 1 patient with surgery for parotid space-occupying lesions (patient 15). e results of 68 Ga-PSMA-11 PET/CT and SGS were consistent in 12/15 patients (80%; patients 1-12). Four patients with Sjogren's syndrome and 4 patients with head and neck cancer after radiotherapy and chemotherapy showed positive results, whereas four patients with head and neck cancer after radiotherapy and chemotherapy showed negative results. e results of the two examinations were not consistent with those in the other 3 patients (patients 13-15). One patient who underwent surgery for parotid space-occupying lesions was positive only on 68 Ga-PSMA-11 PET/CT, which corrected the misdiagnosis by SGS. Two patients with head and neck cancer after radiotherapy and chemotherapy were positive only on SGS.

Image Findings by
In 2 (13.3%; patients 5 and 15) of 15 patients, 68 Ga-PSMA-11 PET/CT provided more accurate information about the salivary glands than did SGS. is patient was diagnosed with Hashimoto's thyroiditis.
In our study, the results of 68 Ga-PSMA-11 PET/CT and SGS were consistent in 12 patients and inconsistent in the other 3 patients. Moreover, 68 Ga-PSMA-11 PET/CTcorrected the misdiagnosis of patient 15 (Figure 2) by SGS and provided more information for 5 patients (33%) than SGS. 68 Ga-PSMA-11 PET/CT showed promising potential in evaluating the function of the lacrimal gland and salivary gland.
SGS is a planar SPECT imaging technique, while 68 Ga-PSMA-11 PET/CT is a positron emission computed tomography fusion imaging technique. 68 Ga-PSMA-11 PET/ CT can provide functional metabolic and anatomical images with higher resolution than SGS. 68 Ga-PSMA-11 PET/CT can accurately assess the morphology of the gland and the degree of atrophy to evaluate gland function more accurately. For example, in patient 1 ( Figure 1) and patient 5 (Figure 3), 68 Ga-PSMA-11 PET/CT clearly shows the degree of atrophy and the density changes in the parotid and submandibular glands. In addition to the inherent advantages of PET/CT, 68 Ga-PSMA-11 PET/CT has the following unique advantages in the evaluation of lacrimal and salivary gland function. Ga-PSMA-11 in the right submandibular gland (Figure 3(a, c-e) small arrows, SUVmax, 2.6) and differences in the bilateral submandibular glands (SUVmax, right: left � 2.6 : 1.9). It could not be observed on the SGS image (Figure 3(b)). e bilateral  Figure 2). At this time, image analysis based on the thyroid uptake level underestimates the uptake function of salivary glands. No significant 99m TcO − 4 uptake was observed in the thyroid of patient 1 (Figure 1(b)), and visual analysis of salivary gland function relied on the experience of the observer. 68 Ga-PSMA-11 PET/CT images can be directly visualized and quantitatively analyzed without the thyroid as a reference. In patient 1 (Figure 1(f )-1(h)), diffuse uptake of 68 Ga-PSMA-11 in the thyroid was unexpectedly found but without 99m TcO − 4 uptake on the corresponding SGS imaging. e patient was diagnosed with Hashimoto's thyroiditis. A study by Kirchner, J et al. found that diffuse uptake in the thyroid (SUVmax ± SD: 4.5 ± 1.2) of 68 Ga-PSMA-11 was found in 22% (12/55) of patients. However, there were no indications about thyroid malignancies or other serious diseases in their data [14]. Some reports described the uptake of 68 Ga-PSMA in follicular adenoma, differentiated thyroid cancer, and medullary thyroid cancer [21][22][23]. A study by Silver et al. showed no expression of PSMA in the thyroid gland [24]. A previous study has shown that neovascular PSMA expression is common in thyroid cancer but may also rarely be found in benign thyroid diseases, such as follicular adenoma [25].  glands. is is not possible on SGS. ree patients showed decreased uptake of 68 Ga-PSMA-11 in their lacrimal glands. For example, in patient 1 (Figure 1), the uptake of lacrimal gland 68 Ga-PSMA-11 on the right side was lower than that on the left side (SUVmax, right:left � 2.0 : 4.2). Combined with the patient's symptom of right eye dryness and the diagnosis of Sjogren's syndrome, the uptake function of the right lacrimal gland was considered to be reduced. SGS can only evaluate the morphology and function of the parotid and submandibular glands, but it is useful for imaging and observing small glands, such as the lacrimal gland and sublingual gland. Our study shows that 68 Ga-PSMA-11 PET/CT can be used to visualize the lacrimal gland and evaluate its function. Figure 1: A 55-year-old woman (patient 1) with dry mouth and a dry right eye for half a year was initially diagnosed with sjogren's syndrome. 68 Ga-PSMA-11 PET/CT MIP (a) and tomographic images (c-e) showed that the left parotid gland (SUVmax, 1.7) was smaller in volume and lower in density without 68 Ga-PSMA-11 uptake than the right parotid gland (SUVmax, 9.2). SGS (b) confirmed that the uptake function of the left parotid gland was severely decreased (UR left parotid , 2.5; UR right parotid , 10.0). In addition, it was found that the 68 Ga-PSMA-11 uptake of the right lacrimal gland was lower than that of the left side (SUVmax right lacrimal gland , 2.0; SUVmax left lacrimal gland , 4.2). Combined with 68 Ga-PSMA-11 PET/CTand the patient's medical history, the uptake function of the right eye was considered to be reduced. It was also found that the thyroid (f-h) density decreased with 68 Ga-PSMA-11 diffuse uptake but without 99m TcO − 4 uptake. e patient with a history of taking supplements had a 7-year history of Hashimoto's thyroiditis.
However, 68 Ga-PSMA-11 PET/CT imaging also has the following limitations in assessing salivary gland function. (a) 68 Ga-PSMA-11 PET/CTcannot evaluate the excretion function of salivary glands. ere is no effective method to reduce PSMA ligand uptake in salivary glands [26]. Further research is warranted. (b) Visual evaluation of 68 Ga-PSMA-11 PET/CT may have limitations in patients with mild to moderate reduction of salivary gland uptake on SGS. In patient 14 (Figure 4), SGS showed a mild-to-moderate decrease in bilateral parotid and submandibular gland uptake. However, there was no significant abnormality on 68 Ga-PSMA PET/CT imaging.
is may be a defect in the visual assessment of the 68 Ga-PSMA-11 PET/CT, which may be compensated by more accurate quantitative analysis criteria.
Salivary gland analysis in 68 Ga-PSMA-11 PET/CT still has some problems that need to be solved. (a) e uptake mechanism of 68 Ga-PSMA-11 in salivary glands is unclear. Tracer uptake in prostate cancer is based on the expression of PSMA receptors in cells. However, the previous literature shows that the mechanism of the intense accumulation of PSMA radioligands in salivary glands is unclear [18]. Normal salivary glands and lacrimal glands have high PSMA ligand uptake. It was previously reported that PSMA is physiologically expressed in normal salivary glands but at much lower levels than those in prostate cancer tissue [18,27].
ere are also studies showing that there is no PSMA expression in salivary glands [24,28]. e intense uptake of PSMA ligands in salivary glands is not correlated with the expression level of PSMA. Some studies have shown that the uptake of PSMA-targeted radioligands in salivary gland tissues may be caused by both nonspecific and PSMAspecific uptake. However, the exact proportion of each form of uptake is still uncertain, and the mechanism of the nonspecific uptake is still unclear [29][30][31][32]. Recently, it has been reported that the accumulation of PSMA-targeted radioligands in salivary gland tissue is mainly caused by nonspecific uptake [18,33]. Further research will be necessary to investigate the exact mechanism of radioligand accumulation in salivary glands. (b) At present, there is no uniform classification standard for 68 Ga-PSMA-11 uptake in lacrimal glands and major salivary glands. e clinical experience of 68 Ga-PSMA-11 PET/CT image interpretation is still limited. e uptake patterns of 68 Ga-PSMA-11 in salivary and lacrimal glands may need to be analyzed in future large sample studies and to develop accurate and reliable classification criteria. 3) was slightly lower than that in the right parotid gland (UR 5.6). In addition, the thyroid uptake of 99m TcO − 4 was significantly increased; this patient had a history of hyperthyroidism and took propylthiouracil for half a year.

68
Ga-PSMA-11 PET/CT imaging shows promising results in terms of lacrimal gland and major salivary gland uptake function. In contrast to SGS, 68 Ga-PSMA-11 PET/CT can visualize small glands, such as the lacrimal gland and sublingual gland, and is less affected by surgery. 68 Ga-PSMA-11 PET/CT may be used as a supplementary examination tool for SGS in the evaluation of salivary gland function. is study is a preliminary exploratory study, and the results of this study still need to be verified in subsequent large-scale studies.

Data Availability
e data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare that they have no conflicts of interest.