F-PSMA-1007 versus F-FDG PET/CT in the detection of hepatocellular carcinoma.

Introduction
Liver cancer is the sixth most common cancer worldwide and the fourth most prevalent cause of cancer-related mortality globally, and hepatocellular carcinoma (HCC) is the most common type of liver cancer, accounting for about 90% of cases (1, 2). Early diagnosis of new or recurrent HCC in at-risk patients offers the most favorable opportunity for effective treatment and enhances long-term disease-free survival (3). Unlike many other malignant tumors, HCC can be diagnosed by imaging based on non-invasive criteria without confirmatory pathology (4). Therefore, imaging plays a critical role in the detection and diagnosis of HCC (5). To date, imaging in HCC mainly relies on computed tomography (CT) and magnetic resonance imaging (MRI) (6, 7), both of which rely on change in size, contrast enhancement and wash-out characteristics to diagnose lesions suspicious for HCC (8). Anatomic imaging can be limited by atypical imaging characteristics, reduced resolution in small lesions and is complicated by altered parenchymal architecture on a background of significant liver cirrhosis and patient factors including body habitus and previous treatment. Molecular imaging offers the advantage of detecting functional abnormalities that often precede anatomical changes identifiable through morphological imaging in oncological diseases (9). Among molecular imaging techniques, Fluorine-18 fluorodeoxyglucose (18F-FDG) positron emission tomography/computed tomography (PET/CT) has been extensively studied in HCC patients (10). However, 18F-FDG has a limited role in the evaluation of patients with HCC, as this tumor type exhibits 18F-FDG avidity in fewer than half of cases, particularly in well-differentiated HCC (11).
Prostate-specific membrane antigen (PSMA), also referred to as glutamate carboxypeptidase type II, is a transmembrane protein encoded by the gene FOLH1, first identified in prostate cancer cells in 1987 (12, 13). It has been observed that PSMA is not exclusively expressed by prostate cancer cells; rather, it is also present on the surface of various cancer cell types and neovascular endothelial cells associated with different solid tumors (14). Given that PSMA is overexpressed by neovascular endothelial cells in numerous malignancies, including HCC, this provides a rationale for employing PET/CT or PET/MRI with PSMA-radioligands in tumors exhibiting low 18F-FDG uptake, thereby evaluating molecular pathways beyond glucose metabolism. This study aimed to comparatively assess the diagnostic performances of 18F-PSMA-1007 PET/CT and 18F-FDG PET/CT in HCC and to assess factors associated with the sensitivity of 18F-PSMA-1007 PET/CT in detecting HCC and intrahepatic tumor lesions.

Materials and methods

Patients
This is a post-hoc analysis of a prior prospective study conducted at the First Affiliated Hospital of Zhengzhou University from June 2023 to October 2024. Forty-four patients with suspiciously incipient HCC determined by clinical manifestations and conventional imaging techniques (CT, MRI and ultrasound) were included in this study. They underwent both 18F-FDG and 18F-PSMA-1007 PET/CT examinations with an interval of one day before surgical treatment. According to Standardization for diagnosis and treatment of hepatocellular carcinoma (2022 edition), for patients who underwent surgery or biopsy, the definitive diagnosis was confirmed by pathology. In cases where patients received transarterial chemoembolization (TACE), the HCC diagnosis was based on a specific imaging pattern characterized by hyperenhancement in the arterial phase and washout in the venous or delayed phase, as observed on contrast-enhanced CT or MRI in the context of liver cirrhosis (3, 15). The inclusion criteria were as follows: (1) HCC diagnosed by pathology; (2) patients exhibiting typical imaging features of HCC on contrast-enhanced CT or MRI who had not undergone surgery or biopsy; (3) signed informed consent; and (4) willingness to accept follow-up. Exclusion criteria included: (1) patients who had received local or systemic treatment; (2) patients with concurrent other types of tumors; and (3) patients with mental disorders who were unable to cooperate with the examination. This study received approval from the Ethics Committee (2022-KY-0482).

Radiopharmaceutical
18F-PSMA-1007 precursor, cassettes, and reagents for the synthesis of 18F-PSMA-1007 were procured from ABX Advanced Biochemical Compounds (Radeberg, Germany). The synthesis of 18F-PSMA-1007 was conducted using the HM-20 cyclotron and CFN-100 synthesis module from Sumitomo Corporation (Japan). The synthesis of 18F-FDG was carried out according to the methodology outlined by Gallagher et al. (16). The radiotracer was achieved with a radiochemical purification yield exceeding 98%, as determined by radio-thin-layer chromatography and high-performance liquid chromatography analysis. All products were prepared utilizing advanced technology under aseptic and pyrogen-free conditions.

PET/CT imaging
Whole-body 18F-FDG and 18F-PSMA-1007 PET/CT scans were obtained on the same PET/CT scanner [Biograph TruePoint64 (52) ring, Siemens, Germany]. Whole-body static 18F-FDG PET/CT scans were obtained as a routine procedure. Whole-body 18F-PSMA-1007 PET/CT images were acquired 60 min after tracer injection, as referenced by Kuten et al. (17). The doses of 18F-FDG and 18F-PSMA-1007 were determined according to patient weight: < 60 kg (200 MBq), 61∼90 kg (250 MBq), > 90 kg (300 MBq). Low-dose CT (120 keV and 100 mAs per section, pitch of 0.8 mm) scans were obtained for attenuation correction and image fusion. PET images were acquired in the 3D mode. After attenuation correction of PET images with CT data, and whole-body PET and CT images were generated through iterative reconstruction.

Image analysis
18F-FDG and 18F-PSMA-1007 PET/CT images were assessed by two experienced, certified nuclear medicine physicians blinded to other imaging and pathology results. All scans were anonymized and independently reviewed by each physician to mitigate subjective bias. Concordance in cases with differing results was achieved through consensus. The mean standardized uptake values (SUVmean) of primary lesions (T) was obtained by outlining a volume of interest that included the lesion in all three planes in 18F-FDG and 18F-PSMA-1007 PET/CT images. Moreover, regions of interest with a diameter of 1 cm were drawn from lesion-free liver tissue (L), the abdominal aorta (A), and the right medial gluteal muscle (M) for SUVmean measurements. Using these three background SUVmean values, tumor-to-normal liver parenchyma (T/L), tumor-to-abdominal aorta (T/A), and tumor-to-gluteal muscle (T/M) ratios were calculated separately. A lesion was considered to be positive on the basis of the visual judgment of elevated uptake in the tumor tissue by 2 experienced nuclear medicine physicians independently, supported by the calculation of the T/L of 18F-FDG and 18F-PSMA-1007, respectively.

Statistical analysis
SPSS version 26.0 statistical software (IBM Corp., Armonk, NY) was employed for data analysis. Categorical variables were presented as frequency and percentage (%), and continuous variables conforming to normal distribution were expressed as mean ± standard deviation (SD), whereas continuous variables not conforming to normal distribution were represented as median [interquartile range (IQR)]. The McNemar’s test, Chi-square test, and Fisher’s exact test were employed to compare categorical variables. The Paired t-test was utilized to compare dependent variables that followed a normal distribution. Two-tailed P < 0.05 was considered statistically significant.

Results

Patient characteristics
Forty-four patients were included in the current study, comprising 26 individuals who underwent hepatic surgery, 10 administered TACE, and eight who received biopsy only. With the exception of two patients diagnosed with benign hepatic nodules and one patient with lung cancer and hepatocellular carcinoma coexisting, the remaining 41 patients were diagnosed with HCC. Thus, 41 HCC patients with 62 intrahepatic lesions were ultimately included in the statistical analysis. The study flowchart is presented in Figure 1. According to microvascular invasion (MVI) number and distribution, 8, 12 and 5 patients were categorized into the M0 (no MVI), M1 (≤ 5 MVI in adjacent liver tissue ≤ 1 cm away from the HCC), and M2 (> 5 MVI or MVI in adjacent liver tissue > 1 cm away from the HCC) groups, respectively. According to histologic grade of HCC (18), 2, 12 and 11 patients were categorized into histologic grade I, II and III, respectively. The general characteristics of the 41 HCC patients are summarized in Table 1.

Comparison of 18F-PSMA-1007 with 18F-FDG in patient-based analysis
Among the 41 HCC patients, 20 were positively identified by both 18F-PSMA-1007 and 18F-FDG PET/CT, 15 were positively identified by 18F-PSMA-1007 alone, and 1 was not positively identified by either imaging tracer. The sensitivity of 18F-PSMA-1007 PET/CT in detecting HCC patients was superior compared with 18F-FDG PET/CT (85.4% vs. 61.0%, P = 0.041, Table 2). 18F-PSMA-1007 PET/CT demonstrated greater sensitivity than 18F-FDG PET/CT in the detection of well- or moderately differentiated HCC patients (P = 0.003). 18F-PSMA-1007 PET/CT detected 13 of the 14 well- or moderately differentiated HCC patients, whereas 18F-FDG PET/CT detected only 2 in these cases (Figure 2). The sensitivity of 18F-FDG PET/CT was associated with histologic grade (P < 0.001), while that of 18F-PSMA-1007 PET/CT was not correlated with those clinical and pathological features, such as cirrhosis, AFP levels, tumor number, MVI, or histologic grade (all P > 0.05). These findings suggested that 18F-PSMA-1007 PET/CT was more sensitive than 18F-FDG PET/CT in the detection of well- or moderately differentiated HCCs.

Comparison of 18F-PSMA-1007 with 18F-FDG in lesion-based analysis
18F-PSMA-1007 PET/CT showed a better sensitivity in detecting intrahepatic lesions compared with 18F-FDG PET/CT (82.3% vs. 50.0%, P = 0.001, Table 3). 18F-PSMA-1007 PET/CT was more sensitive than 18F-FDG PET/CT in detecting small intrahepatic lesions (≤ 2 cm in diameter) (P = 0.049) and well- or moderately differentiated intrahepatic lesions (P < 0.001), but there were no significant sensitivity differences between the 2 tracers in the detection of HCCs > 2 cm in diameter (both P > 0.05) and poorly- differentiated or undifferentiated HCCs (P > 0.05). The sensitivities of 18F-PSMA-1007 PET/CT were significantly related to the size of intrahepatic lesions (both P < 0.05). These findings indicated that 18F-PSMA-1007 PET/CT was more sensitive than 18F-FDG PET/CT in the detection of small and well- or moderately differentiated HCCs, and 18F-PSMA-1007 PET/CT was more sensitive in detection of big intrahepatic lesions.

Uptake intensities of 18F-PSMA-1007 an18F-FDG in HCC(patient-based analysis)
Among the 41 HCC patients, the uptake of 18F-FDG and 18F-PSMA-1007 in intrahepatic lesions, normal liver parenchyma, the abdominal aorta, and the right medial gluteal muscle was assessed, respectively (Table 4). The SUVmean of intrahepatic lesions (T) for 18F-FDG and 18F-PSMA-1007 PET/CT were 3.56 (2.21-7.98) and 21.42 (12.76-44.71), respectively. The SUVmean of normal liver parenchyma (L) for 18F-FDG and 18F-PSMA-1007 PET/CT were 2.33 (2.02-3.14) and 7.83 ± 2.32. A statistically significant difference was observed among T, L value between 18F-FDG and 18F-PSMA-1007 (all P < 0.001). The values for T, L with 18F-PSMA-1007 PET/CT were significantly higher than those for 18F-FDG (Figure 3). The SUVmean of the abdominal aorta (A) for 18F-FDG and 18F-PSMA-1007 PET/CT were 1.65 (1.32-1.9) and 1.12 (0.83-1.54), respectively. The SUVmean for the right medial gluteal muscle (M) in 18F-FDG and 18F-PSMA-1007 PET/CT were 0.89 (0.83-1.10) and 0.52 (0.41-0.59), respectively. Notably, both A and M values for 18F-PSMA-1007 PET/CT were significantly lower than those for 18F-FDG in each patient (all P < 0.001). When comparing the T/A, T/M, and T/L ratios, T/A, T/M, and T/L exhibited statistically significant differences between 18F-FDG and 18F-PSMA-1007 (all P < 0.001). The T/A, T/G, and T/L ratios for 18F-PSMA-1007 were significantly higher than those for 18F-FDG.

Discussion
Preclinical studies have suggested that PSMA may regulate tumor cell invasion and neo-angiogenesis by modulating integrin signal transduction in endothelial cells (19). Over the past five years, several studies have evaluated the diagnostic performance of 68Ga-PSMA-11 for detecting HCC lesions in newly diagnosed patients as well as in individuals previously treated with various modalities of local or systemic therapy (20–25). The use of 18F-labeled PSMA-targeting radiopharmaceuticals offers several advantages, including large-scale production, reduced costs, and high-quality imaging due to lower positron energy and an extended half-life (26). Preclinical studies indicated that lesions expressing PSMA exhibited a higher affinity and internalization rate for 18F-PSMA-1007 compared to 68Ga-PSMA-11, resulting in increased radioactivity uptake (27, 28). In this prospective study, we investigated the contribution of 18F-PSMA-1007 PET/CT to the diagnostic value in newly diagnosed HCC. The principal findings of this study indicated that18F-PSMA-1007 demonstrated higher sensitivity in detecting HCC and provided better tumor-to-background contrast in blood and muscle tissues compared to18F-FDG. The sensitivity of 18F-PSMA-1007 is correlated mainly with tumor size.
In accordance with prior research indicating sensitivities ranging from 40% to 68%, the sensitivity of 18F-FDG PET/CT for the detection of HCC was determined to be 61.0% in the present study (29). Comparatively, 18F-PSMA-1007 PET/CT demonstrated superior sensitivity (85.4%) for detecting HCC patients. Of note, 18F-PSMA-1007 PET/CT exhibited a relatively higher sensitivity in identifying well- or moderately differentiated HCC patients (13 of 14, histologic grade I or II) compared to 18F-FDG PET/CT (2 of 14). Furthermore, 18F-PSMA-1007 PET/CT was more sensitive than 18F-FDG PET/CT in detecting well- or moderately differentiated intrahepatic lesions (88.9% vs. 14.8%). 18F-FDG metabolism may exhibit significant variability in HCC contingent upon its differentiation. The poor sensitivity of 18F-FDG PET/CT in detecting low-grade HCC is probably related to enhanced glucose-6-phosphatase activity causing the dephosphorylation of 18F-FDG-6-PO4, which is therefore not trapped in HCC cells, resulting in false-negative results (29, 30). It is well known that neo-angiogenesis serves as a crucial factor in tumor growth, PSMA is overexpressed in the neovascular endothelial cells of HCC (31). Literature indicates that nearly 95% of HCCs express PSMA in tumor neovascularization (32–34); moderate to high levels of PSMA positivity are evident in approximately 80% of HCC cases (35), whereas completely PSMA-negative HCCs constitute a minority (4.1%). Therefore, 18F-PSMA-1007 PET/CT appears to be a promising new approach for the detection of intrahepatic HCC lesions with higher sensitivity compared with 18F-FDG PET/CT.
The sensitivity of 18F-FDG PET/CT was associated with histologic grade and 18F-FDG PET/CT exhibited a relatively lower sensitivity in identifying well- or moderately differentiated HCCs than poorly- differentiated or undifferentiated HCCs (14.3% vs. 100%).
18F-PSMA-1007 PET/CT demonstrates a superior ability to detect well- or moderately differentiated HCCs compared to 18F-FDG and its sensitivity in detecting HCC was independent of tumor differentiation. This provides a new perspective, and these findings may be elucidated by the differing mechanisms of tracer uptake. PSMA is overexpressed in the neovascular endothelial cells of tumors rather than within HCC cells (31). A PSMA-targeting tracer can circumvent the highly heterogeneous avidity exhibited by other tracers targeting the tumor directly. Therefore, the positive incidence of 18F-PSMA-1007 PET/CT detection for HCC may not have a direct relationship with the histological grade of HCC cells. Chen et al. has reported that peritumoral/vascular expression of PSMA is greatly associated with grade 3 HCC (5/6, 83.3%) but can also be observed in grade 2 HCC (10/15, 66.7%). This was associated with the clinicopathological characteristics of HCC (36). As such, 18F-PSMA-1007 PET/CT can make up for the deficiencies of 18F- FDG PET/CT in the detection of low-grade HCC.
The sensitivity of 18F-PSMA-1007 PET/CT was not correlated with those clinical and pathological features, such as cirrhosis, AFP levels, tumor number, MVI, or histologic grade.
This result is inconsistent with prior research; Chen et al. reported that HCCs, arising in the setting of cirrhosis (9/10, 90.0%), show a significantly increased peritumoral/vascular PSMA expression compared with non-cirrhotic HCCs (6/12, 50%) (p<0.05) (36). However, it is associated with the size of intrahepatic lesions. 18F-PSMA-1007 PET/CT was capable of detecting more small HCC lesions (15 of 24, ≤ 2 cm in diameter) than 18F-FDG (6 of 24) in the present cohort, which is consistent with previous studies that consider 18F-FDG is not an ideal tracer for small HCCs (37, 38).
The SUVmean of normal liver parencchyma (L) in 18F-PSMA-1007 PET/CT was found to be higher than that observed with 18F-FDG, indicating notable uptake of 18F-PSMA-1007 in normal liver tissue. This observation can be attributed to the distinct biodistribution of 18F-PSMA-1007 compared to other radiopharmaceuticals, characterized by its higher lipophilicity. Therefore, 18F-PSMA-1007 may be absent in the ureters and bladder due to its predominantly hepatic clearance mechanism (39); this property theoretically renders 18F-PSMA-1007 less suitable for HCC imaging compared to other targeting agents. However, in our study, both the SUVmean of intrahepatic lesions (T) and the T/L ratio for 18F-PSMA-1007 PET/CT were significantly higher relative to 18F-FDG. This is inconsistent with previous similar studies. Gündoğan et al. observed no statistical significance in terms of T/L ratios between 18F-FDG and 68Ga-PSMA PET/CT (23).
The SUVmean of the abdominal aorta (A) and the right medial gluteal muscle (M) in 18F-PSMA-1007 PET/CT were considerably lower than that recorded for 18F-FDG across all patients. Additionally, the T/A, and T/M ratios were significantly higher in the context of 18F-PSMA-1007 compared to 18F-FDG. This finding aligns with the study by Gündoğan et al., which reported a significantly higher T/A and T/M ratios for 18F-PSMA-1007 compared to 18F-FDG. The results indicated that the uptake level of 18F-PSMA-1007 was superior to that of 18F-FDG in intrahepatic lesions, while the background uptake of PSMA in blood and muscle were lower. This characteristic might render 18F-PSMA-1007 more suitable for HCC imaging than 18F-FDG.
Several limitations were noted in the present study. The relatively small sample size in this study limits the reliability of the subgroup analysis results (e.g., based on histologic grade or size grouping), suggesting the need for future multicenter, large-sample studies to further validate these findings. Pathological biopsies were not performed on all lesions, which, whereas not always practical or necessary, may introduce latent bias due to the absence of pathological data in 10 HCC patients diagnosed solely on non-invasive criteria. Additionally, the study cohort consisted of a limited number of patients with suspected HCC who consented to undergo both 18F-PSMA-1007 and 18F-FDG PET/CT examinations, resulting in an inevitable selection bias.

Conclusions
Compared to 18F-FDG as a PET/CT radiopharmaceutical, 18F-PSMA-1007 has tremendous potential value in diagnosing18F-FDG PET/CT negative suspected HCC patients. Furthermore, when the uptake of 18F-FDG and 18F-PSMA-1007 was compared in positive lesions, 18F-PSMA-1007 PET/CT exhibited higher values of SUVmax, T/L, T/A, and T/M. Moreover, as first-line therapy for locally advanced and metastatic HCC consists of a combination of immunotherapy and anti-neoangiogenic treatment (1), PET/CT with PSMA-radioligands may serve as a valuable tool to predict the results of therapy and assess the response to ongoing treatment.