Introduction
Liver cancer is the 6th most frequent malignancy and the 4th most common cause of cancer-related deaths worldwide. Hepatocellular carcinoma (HCC), which develops due to major risk factors such as hepatitis B virus, hepatitis C virus, aflatoxin-containing foods, and non-alcoholic steatohepatitis, accounts for 75%-85% of primary liver cancers (1). Conventional dynamic contrast-enhanced imaging methods, including computed tomography (CT) and magnetic resonance imaging (MRI), are routinely used in the diagnosis of HCC, with 62%-82% sensitivity and over 90% specificity. Nodules larger than 1 cm show high contrast enhancement in the arterial phase of CT and MRI and wash out in venous and late phases (2). Alpha-fetoprotein (AFP), a serum biomarker, is one of the most commonly used markers in HCC screening and diagnosis. However, its sensitivity and specificity are unsatisfactory, especially in early-stage HCCs (3). The histopathological examination of the tumor tissue is the gold standard in the definitive diagnosis of HCC, but it may cause tissue damage and seeding along the biopsy tract (4).
18Flourine-fluorodeoxyglucose (18F-FDG) positron emission tomography(PET)/CT is associated with the aggressiveness of HCC; moderately and well-differentiated HCCs exhibit a low 18F-FDG uptake, while poorly differentiated HCCs show high uptake (5,6). 18F-FDG PET/CT shows a high sensitivity in detecting lymph nodes and extrahepatic metastases, which are poor prognostic factors for HCC, but show a low sensitivity in detecting primary HCC lesions (7,8).
Prostate-specific membrane antigen (PSMA) is a type 2 transmembrane protein and is overexpressed in prostate cancer (PCa). 68Ga-PSMA PET/CT is widely used in staging, evaluating the treatment response, and assessing relapse in PCa (9). However, in many solid tumors, including HCCs, high PSMA uptake indicates neoangiogenesis (10,11).
In this prospective study, we compared the metabolic parameters obtained from 18F-FDG PET/CT and 68Ga-PSMA-11 PET/CT and investigated the association between PET parameters and serum AFP in patients with HCC.
Material and Methods
Patient Characteristics
Fourteen patients [13 males; 1 female; mean age: 63.8±6.0 years (58-76)] were included in this study. Twelve patients had a Child-Pugh (CP) score “A” cirrhosis, and two patients had a CP-B cirrhosis. Twelve patients had a newly diagnosed HCC, one patient had a history of transarterial chemoembolization (TACE), and one had radiofrequency ablation + TACE for HCC. While six patients had a histopathological confirmation, eight patients were diagnosed with HCC based on radiological findings and serum AFP levels. Patients were recruited after dynamic MRI of the upper abdomen, and 18F-FDG and 68Ga-PSMA-11 PET/CT imaging studies were conducted in the same week. AFP and routine laboratory tests and 18F-FDG PET/CT were performed for all patients on the same day. Patients who previously received chemotherapy or had a history of hepatic tumor surgery were excluded from the study. This study was conducted in concordance with the local good clinical practice guidelines and current laws. The Local Ethics Committee of İstanbul Training and Research Hospital approved this study under the decision number: 2018/1297. Written informed consent was obtained from all patients.
PET/CT Scan and Evaluation
Whole-body PET/CT imaging was performed 60 min after intravenous injection of 18F-FDG (3.5-5.5 MBq/kg) and 68Ga-PSMA-11 (2-2.5 MBq/kg) in a PET/CT scanner [mCT 20 ultra HD LSO PET/CT (Siemens molecular imaging, Hoffmann Estates, Illinois, USA)] on different days. CT imaging was performed in the craniocaudal direction with a 5 mm slice thickness and rotation time of 0.5 sec [80-140 kV, 20-266 mAs, 0.8 pitch, and 512x512 matrix (personalized settings determined by the automatic exposure control system; automatically defined by the software used by the manufacturer, depending on the patient)]. Then, PET imaging was performed in the same range through the craniocaudal direction for 2 min for each PET bed; ultra HD images were acquired using the time of flight + true X algorithm at iteration two and subset 16 values for reconstruction.
18F-FDG and 68Ga-PSMA-11 PET/CT images were both evaluated by two nuclear medicine physicians with at least 10 years of experience in PET/CT, and decisions were made with consensus. Both PET/CT studies were scored visually. 1: Low uptake (equal or less than liver), 2: Moderate uptake (slightly higher than liver), 3: High uptake (markedly higher than liver). SUVmax of primary lesions were acquired by drawing a volume of interest to include the lesion in all three planes in 18F-FDG and 68Ga-PSMA-11 PET/CT. Moreover, regions of interest of 1 cm diameter were drawn from lesion-free liver tissue (L), abdominal aorta (A), and right medial gluteal muscle (M) for background maximum standard uptake value (SUVmax). Using these three background SUVmax, tumor to normal liver parenchyma (T/L), tumor to abdominal aorta (T/A), and tumor to gluteal muscle (T/G) parameters were calculated separately.
Statistical Analysis
SPSS version 21.0 software (IBM Corporation, Armonk, New York, USA) was used for statistical analyses of the variables. The normality of one-variable data was tested with the Shapiro-Francia test, while variance homogeneity was evaluated using Levene’s test. Mann-Whitney U test was used to compare independent and non-normally distributed variables, while the Wilcoxon signed-rank test was used to compare dependent and normally distributed variables. Pearson and Spearman’s rho tests were used to analyze the correlation of variables. The variables had a 95% confidence interval, and a p value less than 0.05 was considered significant.
Results
Six patients exhibited a bilobar involvement, while eight patients had a lobar involvement. On MRI, nine patients showed a mosaic enhancement pattern, and five patients showed a homogeneous enhancement pattern. The median size of primary tumors on MRI is 80.5 mm (20 mm-140 mm). The smallest lesion detected on 68Ga-PSMA PET had a diameter of 8 mm, and the lesion had been described on MRI.
On the visual evaluation, five patients (36%) showed a low 18F-FDG uptake in the primary lesion, three patients (21%) showed a moderate 18F-FDG uptake, and six patients (43%) showed a high 18F-FDG uptake. In contrast, one patient (7%) showed low 68Ga-PSMA uptake, two patients (14%) showed moderate 68Ga-PSMA uptake, and 11 patients (79%) showed high 68Ga-PSMA uptake. Four patients with low 18F-FDG uptake showed high 68Ga-PSMA uptake (Figure 1), while one patient exhibited low uptake with both 18F-FDG and 68Ga-PSMA (Figure 2). Two patients with moderate 18F-FDG uptake showed higher 68Ga-PSMA uptake (Figure 3). In contrast, one patient with moderate 68Ga-PSMA uptake showed higher 18F-FDG uptake (Table 1).
The total number of liver lesions on 68Ga-PSMA PET/CT, MRI, and 18F-FDG PET/CT are 61, 57, and 30, respectively. The number of liver lesions on 68Ga-PSMA PET/CT and MRI were significantly higher than 18F-FDG PET/CT (p=0.042 and 0.026, respectively). There was no statistically significant difference between the number of liver lesions on MRI and 68Ga-PSMA PET/CT (p=0.593) (Table 2).
68Ga-PSMA PET/CT revealed a pathologically increased radiotracer uptake in the abdominal lymph nodes of four patients. Of these four patients, one patient had no 18F-FDG uptake. Two patients had 18F-FDG and 68Ga-PSMA-positive mediastinal lymph nodes, while the remaining patient, who was later histopathologically diagnosed with anthracosis, had only 18F-FDG uptake. One patient had focally increased radiotracer accumulation in the prostate gland on both 18F-FDG and 68Ga-PSMA PET/CT, consistent with a synchronous tumor in the prostate.
The median SUVmax of primary lesions in 18F-FDG and 68Ga-PSMA PET/CT were 6.45 (range: 3.7-21.3) and 16.7 (range: 9.3-48.9), respectively. When median T/L, T/A, and T/G ratios were compared, T/L ratio had no statistically significant difference between 18F-FDG and 68Ga-PSMA (p=0.331), whereas T/A and T/G were significantly higher in 68Ga-PSMA than 18F-FDG (p=0.002 and 0.002, respectively).
Of our six patients with histopathology results, one was reported as having poorly differentiated HCC, two, as well-differentiated HCC, and three, as HCC without specifying differentiation levels. 18F-FDG (T/B: 1.48) and PSMA (T/B: 1.44) uptake in the patient with poorly differentiated HCC were similar, and a moderate radiopharmaceutical uptake was observed in both studies. The primary tumors in the two patients with well-differentiated HCC showed a low level of radiopharmaceutical uptake (T/B: 1.11 and 1.12) in 18F-FDG PET/CT and intense radiopharmaceutical uptake (T/B: 2.74 and 3.96) in PSMA PET/CT. Two of the three patients whose differentiation level was not specified showed intense 18F-FDG (T/B: 3.86 to 2.76) and PSMA (T/B:10.1 to 3.98) uptake, while one patient showed low 18F-FDG (T/B: 0.86) and intense PSMA (T/B: 2.1) uptake.
When the relationship between the laboratory results and PET parameters was examined, serum AFP levels showed a statistically significant positive correlation with 18F-FDG T/A ratio only (r=0.641 p=0.007). However, there was no correlation between 68Ga-PSMA parameters and serum AFP (p>0.05).
Discussion
In this prospective study, we investigated the contribution of 68Ga-PSMA PET/CT to the evaluation of HCCs. The most important findings in this study were primary tumors showing higher 68Ga-PSMA uptake on visual assessment and the ability of 68Ga-PSMA to detect more primary and metastatic lesions compared with 18F-FDG. Conventional imaging methods, such as MRI, are routinely used as the first choice in the diagnosis of HCC due to the typical enhancement pattern with hyperenhancement in the arterial phase and wash out in the portal and late venous phases (12). Although MRI is generally sufficient for diagnosis, it cannot provide information about the biological behavior of HCC. PET radiopharmaceuticals, especially 18F-FDG, are helpful in this context. 18F-FDG PET/CT appears as an important non-invasive diagnostic tool, especially in terms of detecting metastatic lesions in HCC. It is known that 18F-FDG PET/CT findings constitute a stronger prognostic factor than the nodule’s size and number, as described in Milan criteria. Because 18F-FDG avidity may predict the risk of relapse in patients who are planned to undergo liver transplantation, resection, or ablation, it may have a direct effect on the transplantation and ablation outcome (13,14). However, 18F-FDG PET/CT has a low sensitivity in HCC due to overexpression of multidrug resistance protein and increased glucose-6-phosphatase activity in HCC cells, and its use in routine clinical practice is limited (15,16). Therefore, different radiopharmaceuticals have been investigated for the evaluation of primary and extrahepatic metastases of HCCs. Agents with high sensitivity, including 18F-fluorocholine and 11C-acetate, have a relatively poorer availability and, therefore, a limited use (17,18).
Non-prostate solid tumors may exhibit a wide endothelial PSMA expression, associated with neoangiogenesis and vascular growth factor regulation (19,20,21). Recent studies have shown that HCC shows a higher 68Ga-PSMA uptake compared with 18F-FDG (11,22). In our study, 68Ga-PSMA PET/CT revealed more lesions than 18F-FDG PET/CT, and the lesions showed a higher PSMA uptake compared with 18F-FDG.
A recent study 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. Fibrolamellar HCC, normal hepatic tissue, and non-neoplastic cirrhotic tissue are reported to not overexpress PSMA. 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) (23).
In a study that evaluates seven patients and 37 lesions, Kesler et al. (11) demonstrated 68Ga-PSMA uptake to be much higher than the background hepatic activity in 36/37 lesions. Twenty-eight lesions with no 18F-FDG uptake showed high 68Ga-PSMA uptake, while eight lesions showed both 18F-FDG and 68Ga-PSMA uptake. In their study involving 19 patients, Kuyumcu et al. (22) reported that 68Ga-PSMA uptake was higher than 18F-FDG uptake in nine patients. Four patients had a higher 18F-FDG uptake compared with 68Ga-PSMA, while two patients showed no uptake (22). In our study, 13 patients had an increased 68Ga-PSMA uptake, while nine patients had an increased 18F-FDG uptake. Four patients with no 18F-FDG uptake had a high 68Ga-PSMA uptake, however, one patient showed neither 18F-FDG nor 68Ga-PSMA uptake. One patient with moderate 68Ga-PSMA uptake exhibited a higher 18F-FDG uptake. Because of these results, the staging and treatment strategy can be changed through using 68Ga-PSMA PET/CT instead of 18F-FDG PET/CT for metabolic imaging in patients with HCC.
Kesler et al. (11) reported extrahepatic involvement in two of seven patients, while Kuyumcu et al. (22) reported extrahepatic involvement in one patient. In our study, four patients had extrahepatic involvement on 68Ga-PSMA PET/CT, whereas 18F-FDG PET/CT failed to reveal the involvement in one of these patients. One patient with 68Ga-PSMA-negative mediastinal lymph nodes, which was later evidenced to be anthracosis by histopathological examination, showed false positivity in 18F-FDG PET/CT. This supports the deduction that 68Ga-PSMA PET/CT may provide more accurate staging than 18F-FDG PET/CT.
Kuyumcu et al. (22) found no statistically significant difference between the mean SUVmax of primary tumor in 18F-FDG and 68Ga-PSMA PET/CT and T/L ratios. The researchers only evaluated T/L ratio but did not analyze T/A and T/G ratios (22). Because it has recently been reported that T/A ratios have a prognostic significance in rectal cancer (24), we analyzed T/A and T/G ratios in our study as well. We observed no statistical significance in terms of T/L ratios between 18F-FDG and 68Ga-PSMA PET/CT, while we found significantly higher T/A and T/G ratios and SUVmax in 68Ga-PSMA PET/CT.
Since patients with histopathology results are few in our study, it will be difficult to make a clear evaluation of the relationship between HCC differentiation and PSMA involvement. However, PSMA uptake was significantly higher than 18F-FDG uptake in our patients with well-differentiated HCC. In our patient with less differentiated HCC, 18F-FDG, and PSMA uptakes were found to be similar compared with the background activity. Low 18F-FDG uptake is an expected finding in patients with well-differentiated and moderately differentiated HCC, and our findings on the relationship between HCC differentiation and 18F-FDG involvement are consistent with the literature (25). Since there are no studies in the literature, no correlation could be made between PSMA PET/CT and HCC differentiation.
In this preliminary study, no relationship was found between 68Ga-PSMA tumor uptake and serum AFP level, suggesting that tumor angiogenesis and AFP production are independent parameters in HCC.
Study Limitations
First limitation of the current study is the relatively small number of patients, although our population is similar to other prospective studies in the literature. Second, not all patients had a histopathologically confirmed diagnosis, and some patients were diagnosed according to typical radiological findings.
Conclusion
68Ga-PSMA PET/CT is superior to 18F-FDG PET/CT in the diagnosis and staging of HCC. These preliminary findings show that 68Ga-PSMA PET/CT has a supportive role for MRI in T staging, especially in demonstrating multicentric tumors, and it can be superior to MRI in demonstrating extrahepatic involvement. High PSMA uptake is promising for PSMA-targeted radionuclide treatments in metastatic HCC, which responds poorly to standard chemotherapy regimens. 68Ga-PSMA PET/CT may also be helpful in evaluating treatment response, warranting further prospective studies in this area.
Ethics
Ethics Committee Approval: The Local Ethics Committee of İstanbul Training and Research Hospital approved this study under the decision number: 2018/1297.
Informed Consent: Written informed consent was obtained from all patients.
Peer-review: Externally and internally peer-reviewed.
Authorship Contributions
Surgical and Medical Practices: C.G., Concept: N.E., C.G., Design: C.G., T.F.Ç., Ö.K., Data Collection or Processing: R.U.G., M.S.Ç., C.G., T.A., Analysis or Interpretation: C.G.,Ö.K., N.E., M.S.Ç., Literature Search: R.U.G., T.A., Writing: C.G., T.F.Ç.
Conflict of Interest: No conflict of interest was declared by the authors.
Financial Disclosure: The authors declared that this study has received no financial support.
