⁶⁸Ga-FAPI renal cortical uptake: a diagnostic and predictive biomarker for kidney disease in PET/CT imaging

Introduction

Gallium-68-labeled fibroblast activation protein inhibitor (⁶⁸Ga-FAPI) is a promising imaging agent that targets fibroblast activation protein (FAP), which is overexpressed in cancer-associated fibroblasts (CAFs) (1,2). FAPI accumulates in various tumors (3,4) and provides a higher target-to-background ratio with low brain uptake compared to ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) (5,6). In addition to its wide range of cancer imaging applications, ⁶⁸Ga-FAPI has potential in non-neoplastic diseases such as inflammatory conditions (7-9), as FAP overexpression occurs during the remodeling of the extracellular matrix (6).

Previous studies and cases have revealed that patients with kidney disease have elevated levels of FAPI uptake in both kidneys (10-15). The level of FAPI uptake is negatively correlated with estimated glomerular filtration rate (eGFR) and positively correlated with the severity of renal pathological changes (5,6). This may be due to the early-stage accumulation of fibroblasts and matrix remodeling associated with renal fibrosis, characterized by extracellular matrix accumulation, a common pathological condition across various stages of kidney disease (16,17). These findings suggest the potential utility of ⁶⁸Ga-FAPI positron emission tomography (PET) imaging in assessing kidney disease (18).

Previous studies have summarized the imaging characteristics of ⁶⁸Ga-FAPI PET/computed tomography (CT), indicating that ⁶⁸Ga-FAPI PET/CT has potential applications in kidney disease. Meanwhile, it has also been observed that patients with kidney disease may not exhibit increased renal FAPI uptake, whereas healthy individuals may show elevated renal FAPI levels. Accurately interpreting images with increased renal FAPI uptake on ⁶⁸Ga-FAPI PET/CT remains challenging. Furthermore, it is uncertain whether this uptake serves as a warning or predictive indicator for impending kidney disease in patients not yet diagnosed. A better understanding of its diagnostic and predictive value can enhance radiologists’ confidence in interpreting these reports.

The primary methods for monitoring kidney disease currently involve laboratory tests, such as blood creatinine, eGFR, urine analysis, and renal tubular function testing (19). Although these tests can be influenced by factors such as age, gender, and urine concentration (20-22), kidney puncture—considered the gold standard for diagnosis—poses a high risk and is not always feasible. Additionally, the accuracy of biopsy results may be compromised by pathological changes at the puncture site (21,23). Consequently, laboratory tests remain the most widely used diagnostic tools for acute kidney disease (AKD) and chronic kidney disease (CKD) in clinical practice.

Therefore, this study investigated the diagnostic capabilities of increased renal ⁶⁸Ga-FAPI uptake in kidney diseases and its potential to predict impending conditions, with the goal of enhancing radiologists’ image interpretation. We present this article in accordance with the STARD reporting checklist (available at https://qims.amegroups.com/article/view/10.21037/qims-2025-1270/rc).

Methods

Patients

Patients with malignant tumors who underwent whole-body ⁶⁸Ga-FAPI PET/CT between February 2022 and December 2022, with renal function test results obtained within 7 days before or after the scan, and regular renal function follow-ups within one-year post-examination, were enrolled. The exclusion criteria were as follows: (I) patients with renal tumors; (II) patients who had undergone kidney surgery; and (III) patients with urinary obstructive diseases. All patients provided informed consent for the use of their clinical data in this retrospective analysis. The trial was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. This study was approved by the Biomedical Ethics Review Committee of West China Hospital (No. 20221637).

PET/CT imaging

PET/CT was conducted using routine protocols. PET imaging began 50–70 minutes after intravenous administration of ⁶⁸Ga-FAPI-04. The injected activity was 2 MBq/kg (0.0541 mCi/kg). CT acquisition employed a tube voltage of 120 kV and dose-modulated effective tube current of 70–200 mAs on a uMI780 system [United Imaging Healthcare (UIH), Shanghai, China], with a slice thickness of 3 mm. PET scanning followed immediately after CT in three-dimensional (3D) mode (matrix 192×192) at 3 minutes per bed position. PET data were reconstructed iteratively (2 iterations, 20 subsets), using CT-based attenuation correction. PET images were then fused with the low-dose CT to enhance anatomical localization. The PET/CT datasets were co-registered and displayed with dedicated software (Image Fusion, UIH).

Image analysis

Two experienced nuclear medicine physicians, blinded to the patients’ renal function, analyzed the ⁶⁸Ga-FAPI PET/CT images using both visual and semiquantitative methods. The maximum and mean standardized uptake values (SUVmax and SUVmean) of each patient were measured at separate locations of the bilateral renal cortex (superior, middle, and inferior layer) (13). The region of interest (ROI) of each site was a spherical area with a diameter of 1 cm. The renal SUVmax and SUVmean for each patient were calculated as the average of six ROIs in bilateral kidneys. Individuals whose SUVmax exceeded the upper threshold of normal renal uptake determined in this study were defined as having increased renal uptake of FAPI. The SUVmax and SUVmean of the blood pool were measured in a spherical area with a diameter of 2 cm in the descending aorta.

Diagnosis and follow-up of kidney disease

After the ⁶⁸Ga-FAPI PET/CT examination, each enrolled patient was followed for at least 12 months by a physician blind to the PET/CT results to assess kidney disease development and gather risk factors. Diagnoses of AKD and CKD were made based on criteria established by the Kidney Disease Improving Global Outcomes (KDIGO) organization (Table 1) (23). Markers of renal damage included albuminuria, urine sediment abnormalities, electrolyte imbalances linked to tubular disorders, and structural changes identified through histology or imaging. Patients without kidney disease were classified as non-kidney disease (NKD). The presence of risk factors for kidney disease prior to the ⁶⁸Ga-FAPI PET/CT examination was assessed through medical record review and verbal interviews. Key risk factors included diabetes, hypertension, nephrotoxic drug use, and severe electrolyte disorders (24,25).

Statistical analysis

Statistical analyses were performed using the software SPSS 28.0 (IBM Corp., Armonk, NY, USA) and MedCalc (MedCalc Software, Ostend, Belgium). Data were presented as mean ± standard deviation or median and interquartile range (IQR). The t-test or non-parametric tests were used to compare variables between groups. Pearson or Spearman correlation analyses assessed relationships between variables. The percentile method recommended by the American Clinical and Laboratory Standards Institute (CLSI) was used to establish the upper limit of SUVmax for normal kidneys (26). The diagnostic performance of ⁶⁸Ga-FAPI PET/CT was assessed using the receiver operating characteristic (ROC) curve and its area under the curve (AUC), along with diagnostic sensitivity, specificity, accuracy, positive predictive value (PPV), and negative predictive value (NPV). The minimum sample size was calculated using PASS software (https://www.ncss.com/software/pass/) based on the incidence rate of kidney disease. Cox proportional hazards model analysis and Kaplan-Meier survival analysis were used for predictive analysis. A P value <0.05 was considered statistically significant.

Results

Demographics and clinical characteristics of participant cohort

From February 2022 to December 2022, 145 patients were included from 196 consecutive patients who underwent whole-body ⁶⁸Ga-FAPI PET/CT scans in our institution. The study flow diagram is presented in Figure 1. Demographics and clinical characteristics of the 145 patients (41 women and 104 men, average age 59.3±11.27 years, range, 23–83 years) are shown in Table 2. The median follow-up time after the ⁶⁸Ga-FAPI PET/CT examination was 17.3 months (range, 12.8–22.6 months, IQR, 14.8–19.2 months).

Figure 1 The patient flow diagram outlines the details of patient selection and exclusions. ⁶⁸Ga-FAPI, gallium-68-labeled fibroblast activation protein inhibitor; PET/CT, positron emission tomography/computed tomography.

Of the 145 patients, 30 were diagnosed with AKD (n=20, 13.8%) or CKD (n=10, 6.9%) before or during the follow-up period. Among these 30 patients, 18 were classified as present kidney disease patients, diagnosed either before the PET/CT examination or within 7 days afterward (Figure 2A). The remaining 12 were identified as impending kidney disease patients, exhibiting normal renal function results within 7 days post-PET/CT but later diagnosed with kidney disease during follow-up examinations (Figure 2B). The remaining 115 patients did not develop kidney disease throughout the follow-up period (Figure 2C). Additionally, 93 patients (64.1%) had other risk factors for kidney disease beyond cancer before the PET/CT examinations.

Figure 2 Representative images of patients with present kidney disease, imminent kidney disease and non-kidney disease. (A) Patient A, a 74-year-old female with thyroid cancer and a 2+ year history of chronic kidney disease, had a renal cortex SUVmax of 3.91 and an eGFR of 36.95 mL/min/1.73 m² before the PET/CT scan. (B) Patient B, a 78-year-old male with gastric cancer, had a renal cortex SUVmax of 2.20 despite no prior kidney dysfunction. His eGFR was 74.2 mL/min/1.73 m² post-scan but decreased to 56.34 mL/min/1.73 m² by day 30, remaining below 60 mL/min/1.73 m² for over 3 months. In the MIP (A1,B1) and PET images (A2,B2) of patients A and B, increased radiotracer uptake and visible renal cortex contours (black arrows) are evident. (C) Patient C, a 69-year-old male with esophageal cancer, showed no clinical signs of kidney disease prior to PET/CT, with a renal cortex SUVmax of 1.28 and an eGFR of 94.36 mL/min/1.73 m². No increased radiotracer uptake or visible renal cortex contours were seen in the MIP (C1) and PET images (C2) (black arrows). No kidney function abnormalities were observed during follow-up. CT, computed tomography; eGFR, estimated glomerular filtration rate; MIP, maximum intensity projection; PET, positron emission tomography; SUVmax, maximum standardized uptake value.

The correlation between eGFR and SUVs

The patients included in the study were categorized into three groups based on eGFR according to the staging of CKD: eGFR <60 mL/min/1.73 m², 60≤ eGFR <90 mL/min/1.73 m², and eGFR ≥90 mL/min/1.73 m². The group with lower eGFR had a significantly higher median SUVmax compared to the other groups (P<0.001, Table 3). Additionally, patients with kidney disease exhibited a higher median SUVmax compared to those without kidney disease [2.34 (IQR, 2.14–2.73) versus 1.71 (IQR, 1.56–1.82), P<0.001]. However, there was no significant difference in the median SUVmax between patients with and without risk factors for kidney disease [1.81 (IQR, 1.63–2.18) versus 1.68 (IQR, 1.57–1.82), P=0.075].

Spearman analysis and scatter plot revealed a significant inverse correlation between renal SUVmax and eGFR, as well as between renal SUVmean and eGFR, with correlation coefficients of −0.454 and −0.416 (P<0.001, Figure 3A,3B). The median SUVmax and SUVmean of the blood pool for included patients were 1.26 (IQR, 1.09–1.46) and 0.86 (IQR, 0.74–1.01), which were also negatively correlated with their eGFR, with correlation coefficients of −0.306 and −0.311, respectively (P<0.001, Figure 3C,3D).

Figure 3 The scatter plots and linear regression curve illustrate the relationship between estimated glomerular filtration rate (mL/min/1.73 m²) and ⁶⁸Ga-FAPI concentration in the renal cortex (A,B) and blood pool (C,D). ⁶⁸Ga-FAPI, gallium-68-labeled fibroblast activation protein inhibitor; eGFR, estimated glomerular filtration rate; SUV, standard uptake value.

The diagnostic ability of ⁶⁸Ga-FAPI PET/CT for kidney disease

In 115 patients who did not develop kidney disease during the follow-up period, the median SUVmax of their kidneys was 1.67 (range, 1.12–4.02; IQR, 1.54–1.81). Based on the percentile method recommended by CLSI, these 115 patients were randomly assigned to an experimental group (n=81) and a validation group (n=34) in a 7:3 ratio. The experimental group had a median SUVmax of 1.64 (IQR, 1.54–1.79). The right 95th percentile of SUVmax for the experimental group, calculated using the robust method was 2.05 [95% confidence interval (CI): 1.96–2.14], which was established as the upper reference limit of SUVmax for normal kidneys.

Based on the ROC curve analysis (Figure 4A), the optimal SUVmax threshold for diagnosing kidney disease, determined using the Youden index, was found to be 1.97. The corresponding AUC was 0.946 (95% CI: 0.908–0.983). Based on the SUVmax cutoff values calculated using the Youden index and the percentile method, we propose using an SUVmax =2.00 as a more clinically convenient threshold for diagnosing kidney disease.

Figure 4 ROC curve of SUVmax on ⁶⁸Ga-FAPI PET/CT for diagnosing kidney disease (A) and Kaplan-Meier curve for kidney disease-free survival rate (B). ⁶⁸Ga-FAPI, gallium-68-labeled fibroblast activation protein inhibitor; PET/CT, positron emission tomography/computed tomography; ROC, receiver operating characteristic.

The testing group (n=64) to evaluate the diagnostic ability of ⁶⁸Ga-FAPI PET/CT for kidney disease included patients with present kidney disease (n=18), as well as validation group of NKD patients and immediate-future kidney disease patients (n=46). In the testing group, there were no statistically significant differences in the diagnostic sensitivity, specificity, accuracy, PPV, and NPV for present kidney disease when comparing threshold SUVmax =2.05 to SUVmax =2.0. The values were as follows: sensitivity 94.4% (17/18) versus 94.4% (17/18) (P>0.99), specificity 63.0% (29/46) versus 58.7% (27/46) (P=0.960), accuracy 71.9% (46/64) versus 68.8% (44/64) (P=0.699), PPV 50.0% (17/34) versus 47.2% (17/36) (P=0.816), and NPV 96.7% (29/30) versus 96.4% (27/28) (P=0.960).

The predictive potential of ⁶⁸Ga-FAPI PET/CT for impending kidney disease

Using SUVmax =2.0 as the threshold, 43 patients exhibited increased renal ⁶⁸Ga-FAPI uptake. Among these, 17 had existing kidney disease at the time of the PET/CT scan, 12 NKD patients progressed to immediate-future kidney disease, and the remaining 14 had persistent NKD.

Although each patient was followed for at least 12 months, the median duration between the detection of elevated renal ⁶⁸Ga-FAPI uptake and the diagnosis of immediate-future kidney disease was 29 days (range, 14–109 days; IQR, 23.25–67.00 days). In our cohort, NKD patients with increased renal ⁶⁸Ga-FAPI uptake had a significantly higher incidence of immediate-future kidney disease compared to those without increased uptake, at 46.2% versus 0.0% (P<0.001, Figure 4B). Notably, during the entire follow-up period, none of the NKD patients with normal renal uptake of ⁶⁸Ga-FAPI were diagnosed with kidney disease.

Cox proportional hazards model analysis revealed that age and renal SUVmax were independent risk factors for immediate-future kidney disease (P<0.001), whereas gender and other high-risk factors prior to PET/CT examination were not (P<0.001).

Discussion

Consistent with previous research, this study showed an inverse correlation between eGFR and renal uptake of ⁶⁸Ga-FAPI. The study also demonstrated high efficacy of ⁶⁸Ga-FAPI PET/CT in diagnosing kidney disease with an AUC of 0.946, yielding a high diagnostic sensitivity and NPV for kidney disease. In addition, the study found that increased renal uptake of FAPI is a risk factor for immediate-future kidney disease, which has a certain predictive and warning effect on upcoming kidney disease.

Increased uptake of ⁶⁸Ga-FAPI in pathologically confirmed renal fibrosis patients with or without clinical renal function impairment had already been revealed in some case reports (10,11) and a study conducted by Zhou et al. (12). Another retrospective study found a negative correlation between eGFR and ⁶⁸Ga-FAPI uptake in the kidneys (13), which was further confirmed by Wang et al. (14) in a larger cohort and found that increased ¹⁸F-AlF-NOTA-FAPI-04 uptake can also occur in patients with AKD and CKD (14). Additionally, an animal experiment conducted by Mao et al. found a positive correlation between the degree of renal fibrosis and the degree of renal uptake of FAPI (15).

Previous studies had examined the imaging features of kidney disease using FAPI PET/CT. Building on these findings, our study validated the diagnostic ability of ⁶⁸Ga-FAPI PET/CT for kidney disease, demonstrating a sensitivity of 94.4% and a specificity of 63.0%. Although this study assessed the diagnostic capabilities and predictive potential of FAPI PET/CT for kidney disease, we do not recommend its use as a primary method for diagnosing kidney disease, as it is not the most convenient, cost-effective, or safe option. The findings should serve as a reference for nuclear medicine physicians when reporting on patients who underwent ⁶⁸Ga-FAPI PET/CT for other indications.

Additionally, our study advanced previous research by identifying increased renal ⁶⁸Ga-FAPI uptake as a risk factor for imminent renal dysfunction in patients with normal kidney function, demonstrating its predictive potential for impending kidney disease. This patient group may exhibit no pathological or functional changes in the kidneys, or they may have already experienced pathological changes while appearing to be in a compensatory phase of renal function (27,28). However, further research is needed to confirm this predictive finding and to determine whether closer monitoring or preemptive intervention for this group is warranted.

The patients in this retrospective study were all diagnosed with malignant tumors. AKD and CKD are complex conditions that affect millions and can both contribute to and result from cancer (29). Cancer patients face an elevated risk of developing kidney disease due to their treatments and the tumor itself (29,30). The coexistence of cancer and kidney disease adversely impacts prognosis and complicates management (31). Therefore, monitoring renal function in cancer patients is essential for improving outcomes (32).

The lack of specific renal pathological information for the included patients restricted our analysis of the histopathological reasons for increased FAPI uptake in kidney disease. Previous pathological studies have consistently demonstrated that patients with elevated renal FAPI uptake show varying degrees of FAP overexpression in the affected areas (10-12,14). Additionally, the level of FAPI uptake correlates positively with the degree of renal fibrosis, including lesion area, interstitial inflammatory infiltrate, and the severity of interstitial fibrosis and tubular atrophy. Studies by Zhou et al. and Wang et al. have also identified high FAP expression in the renal interstitium and glomeruli, particularly in tubular epithelial cells (12,14). For patients with increased renal FAPI uptake and an unclear diagnosis of kidney disease, it is recommended to conduct relevant routine examinations to facilitate early diagnosis and management or to exclude kidney disease.

The study has several other limitations. Firstly, ⁶⁸Ga-FAPI PET imaging is not yet widely recommended for clinical implications. The cases in this study were drawn exclusively from previous clinical studies involving cancer patients at our institution, lacking healthy or nephrology-based control groups. This may introduce selection bias and limit the generalizability of the findings to broader populations with CKD or AKD. Future research could conduct prospective studies in non-cancer patient populations to further validate these findings. Secondly, the diagnosis of kidney disease in the patients included in this study relied exclusively on laboratory assessments of renal function, with no kidney biopsies performed. This may have resulted in an underestimation of ⁶⁸Ga-FAPI PET/CT’s ability to evaluate compensated or subclinical kidney disease. Future studies should consider incorporating biopsy correlation. Thirdly, the cutoff value SUVmax =2.00 is derived internally using CLSI percentiles and the Youden index, with no external dataset validating its applicability. Finally, although the sample size in this retrospective study exceeds that of previous studies, it remains relatively small. The reference values and findings require further validation through larger-scale studies or more rigorously designed prospective investigations.

Conclusions

The degree of ⁶⁸Ga-FAPI uptake in the renal cortex is negatively correlated with eGFR. ⁶⁸Ga-FAPI PET/CT exhibits promising efficacy in the diagnosis of kidney disease and prediction of imminent development of kidney disease.

Acknowledgments

The authors would like to thank the radiopharmaceuticals team of the Nuclear Medicine Department of West China Hospital, Sichuan University, for providing ⁶⁸Ga-FAPI-04, and the technicians and nurses for collaborating to complete the imaging and post-processing.

Footnote

Reporting Checklist: The authors have completed the STARD reporting checklist. Available at https://qims.amegroups.com/article/view/10.21037/qims-2025-1270/rc

Data Sharing Statement: Available at https://qims.amegroups.com/article/view/10.21037/qims-2025-1270/dss

Funding: The study was funded by the National Key R&D Program of China (No. 2023YFC2414201-1).

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://qims.amegroups.com/article/view/10.21037/qims-2025-1270/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. The trial was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. The study was approved by the Biomedical Ethics Review Committee of West China Hospital (No. 20221637) and informed consent was taken from all individual participants.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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