Comparison of the diagnostic accuracy between ¹⁸F-FAPI-04 PET/CT and ¹⁸F-FDG PET/CT in the clinical stage IA of lung adenocarcinoma

Introduction

Lung cancer is the leading cause of cancer-related deaths with high rates of morbidity and mortality (1). Most patients are diagnosed at an advanced stage, missing optimal treatment opportunity and increasing the risk of death, particularly in low- and middle-income countries (2). Early detection of lung cancer is crucial for high-risk patients, particularly for identifying stage IA lung cancer. Timely intervention and treatment can enhance a patient’s prognosis. Fluorine18-labeled-fluorodeoxyglucose (¹⁸F-FDG) positron emission tomography/computed tomography (PET/CT) is widely used for the staging and restaging of lung cancer, as recommended by the National Comprehensive Cancer Network (NCCN) guidelines (3-5). However, false-negative results may occur when evaluating nodules measuring below 1.0 cm (T1a), adenocarcinoma in situ (AIS), or minimally invasive adenocarcinoma (MIA) (6,7). The latter mainly depends on CT morphological diagnosis, but its specificity is not satisfactory, especially for patients with chronic lung diseases, which poses a particular challenge (6).

Researchers have introduced positron-labeled fibroblast activation protein inhibitors (FAPIs) for cancer imaging, targeting fibroblast activating protein (FAP) overexpressed in cancer-associated fibroblasts (CAFs) (8-10). Several studies have confirmed that positron-labeled FAPI PET/CT is superior to ¹⁸F-FDG PET/CT in evaluating advanced lung cancers, such as lymph nodes, pleural, liver, and bone metastases. However, its accuracy remains uncertain for early-stage lung adenocarcinomas (LUAD), such as stage IA LUADs (11-14).

Therefore, we aimed to compare the diagnostic value of ¹⁸F-FDG PET/CT and ¹⁸F-FAPI-04 PET/CT imaging in stage IA LUADs while assessing FAP expression using immunohistochemistry in resected specimens to evaluate the correlation between the uptake levels of ¹⁸F-FAPI-04 radiopharmaceuticals and pathological characteristics. We present this article in accordance with the STARD reporting checklist (available at https://jtd.amegroups.com/article/view/10.21037/jtd-24-1658/rc).

Methods

Study design and population

The study was approved by the Clinical Research Ethics Committee of Maoming People’s Hospital (No. PJ2023MI-K003-01) and the Declaration of Helsinki (as revised in 2013). All patients provided written informed consent before participating in the study. It was registered at Chictr.org.cn (registration number: ChiCTR2400080737). The surgical pathology of focal lesions was taken as the gold standard, and the receiver operating characteristic curves of the two samples were compared to estimate the sample size. The PASS (version: 11.0.7) software was employed to calculate the sample size, and the subsequent parameters were adopted: power (1 − beta) =0.8, alpha level (significance level) =0.05, and the alternative hypothesis: two-sided test. A total of 56 patients were required.

Inclusion criteria

The inclusion criteria were: (I) age >18 years; (II) Eastern Cooperative Oncology Group (ECOG) score of ≤1; (III) lung nodules ≤3 cm detected on CT scan and had undergone tumor markers examination; (IV) informed consent; and (V) no history of chemotherapy, radiotherapy, or surgical resection before the PET/CT scan.

Exclusion criteria

The exclusion criteria were: (I) lung nodules >3 cm; (II) participants who were pregnant or suspected to be pregnant; (III) interval between ¹⁸F-FDG PET/CT and ¹⁸F-FAPI-04 inspection >1 week; (IV) more than two primary malignant tumors; and (V) lack of a final histological diagnosis.

Preparation of ¹⁸F-FDG and ¹⁸F-FAPI-04

¹⁸F-FDG and ¹⁸F-FAPI-04 were obtained from Dongcheng AMS Pharmaceutical Co., Ltd. (Dongguan, Guangdong, China), utilizing a 20 MeV cyclotron (CYPRIS HM-20, Sumitomo, Kyoto, Japan). Synthesis of the radiotracer ¹⁸F-FAPI-04 strictly adhered to the protocol published by Wei et al. (15), and the radiochemical purity exceeded 95%.

Patient preparation and ¹⁸F-FDG PET/CT protocol

As recommended by the EANM guidelines for tumor imaging in version 2.0, all patients fasted for ≤4 h before ¹⁸F-FDG injection for PET/CT scanning (16). Blood glucose levels ≤11 mmol/L (approximately 200 mg/dL) were required before ¹⁸F-FDG injection. All patients received manual administration of 4.44 MBq/kg (0.12 mCi/kg) of ¹⁸F-FDG, and patients were instructed to lie on the bed. PET/CT acquisition commenced 60±5 min post-¹⁸F-FDG injection using a Discovery MI scanner (GE Healthcare, Milwaukee, WI, USA). A low-dose CT scan (120 kV, 80 mA, 3.75-mm thickness) with dose modulation was employed for attention correction and anatomical localization prior to PET imaging. Subsequently, PET data were acquired. PET acquisition ranged from the skull to the upper thigh, requiring five to eight bed positions per patient and two min per bed position. Patients maintained shallow breathing during the PET acquisition. High-resolution breath-hold CT (120 kV, 150 mA, 1.25-mm thickness) scans were acquired for all patients. PET image reconstruction was performed using the Bayesian penalized likelihood algorithm (Q. Clear, GE Healthcare) with a penalization factor (b=750). Attention correction was performed using CT transmission data.

Patient preparation and ¹⁸F-FAPI-04 PET/CT protocol

The same patients underwent ¹⁸F-FDG and ¹⁸F-FAPI-04 PET/CT within 1 week. The patients were advised to refrain from engaging in vigorous physical activity before ¹⁸F-FAPI-04 PET/CT, and no additional specific preparations were required. ¹⁸F-FAPI-04 PET/CT was performed 1 h after administration of 4.44 MBq/kg (0.12 mCi/kg) of ¹⁸F-FAPI-04 following the same operation procedure and imaging analysis as ¹⁸F-FDG PET/CT.

Imaging analysis

All acquired data were transferred to an Advantage Workstation (AW version 4.7, GE Healthcare), where attention-corrected PET, CT, and fused PET/CT images (transaxial, coronal, and sagittal) were reviewed and analyzed. Two experienced nuclear medicine physicians (>5 years) independently reviewed all images and were blinded to the medical histories of the participants (X.L. and N.Z.). Any discrepancies were resolved through discussion with a senior expert (>15 years old, H.X.L.). To eliminate intra-observer variability in the quantitative analysis, PET/CT images were quantitatively evaluated by a single nuclear physician (H.X.L.).

The long diameters of the lung nodules were measured using high-resolution computed tomography (HRCT) images. Any focal accumulation of ¹⁸F-FDG or ¹⁸F-FAPI-04 exceeding that of the surrounding tissue was interpreted as a positive lesion, indicating suspected malignancy (12). A semi-automatic 3D delineation was utilized to encompass the entire tumor, exhibiting avidity for ¹⁸F-FDG or ¹⁸F-FAPI-04. A 3D isocontour volume of interest (VOI), established at 41% of the maximum standardized uptake value (SUVmax) threshold, was applied in accordance with the EANM guidelines (16). SUVmax, mean standardized uptake value (SUVmean), and peak standardised uptake value (SUVpeak) within a 1-cm³ spherical volume were automatically generated. Metabolic tumor volume (MTV) and total lesion glycolysis (TLG), as assessed by ¹⁸F-FDG, were documented alongside the corresponding values determined using ¹⁸F-FAPI-04 [FAPI avid tumor volume (FTV) and total lesion FAP expression (TLF), respectively].

The uptake of ¹⁸F-FDG or ¹⁸F-FAPI-04 in the normal lung parenchyma was assessed by using a 2-cm-diameter region of interest (ROI) in the contralateral normal lung parenchyma on the axial slice corresponding to the maximal diameter of the lung nodules. The SUVmean of the normal lung parenchyma was also recorded. The tumor-to-background ratio (TBR) was computed by dividing the SUVmax of the tumor by the SUVmean of the contralateral normal lung parenchyma. The formula is as follows:

TBR=SUVmaxoftumorSUVmeanofcontralateralnormallungparenchyma[1]

Histology and immunohistochemistry

A board-certified and experienced (>10 years) pathologist (Q.W.H.) specializing in oncology who was blinded to the patient’s clinical status performed all macro- and microscopic examinations of all surgical specimens. The degree of differentiation in invasive non-mucinous adenocarcinomas was also categorized as moderately or poorly differentiated, based on the 2021 World Health Organization classification of lung tumors (17). The degree of differentiation, tumor size (long-axis diameter), pathological tumor-node-metastasis (TNM) stage (I, II, or III), and tumor stroma ratio (TSR) were documented. The assessment of the TSR was based on previous reports (18,19). Routine microscopic examination of 5-µm hematoxylin-eosin (H&E) stained sections from the primary tumor was analyzed. Utilizing a 5× microscope objective (50× total magnification), the most invasive part of the primary tumor was selected for further evaluation. The region typically comprises two components, the tumor cell component and the stromal component, quantified as percentages.

The percentage of tumor cells within a single field of view was evaluated at 100× magnification within the area, and the remaining percentage was the TSR. For example, the area of tumor cells accounts for 30% of the field of view, 70% of the stromal component, and 70% of the TSR. Calculate the TSR with a value interval of 10% that is, 20%, 30%, and so on. Two to four different fields of view were selected for the evaluation, and the final value was determined based on the highest TSR value. To evaluate the TSR within the tumor, tumor cell infiltration must be present around the stroma within the selected field of view. Areas with more stroma and no tumor cells surrounding the stroma could not be selected for evaluation.

According to the TSR evaluation results, using 50% as the cut-off value, cases with a TSR value higher than 50% were defined as the stromal-rich group, and cases with a TSR value less than or equal to 50% were defined as the stromal-poor group.

Immunohistochemical (IHC) staining was performed to confirm FAP expression in tumor tissues. Surgical specifications were fixed and embedded before sectioning and immunostaining with the FAP-alpha antibody (1:100; ab207178, Abcam, Cambridge, UK). All surgical specimens were stained and analyzed to measure the intensity and percentage of FAP-positive cells. The staining intensity was graded into four classes, and image acquisition was performed using the Mantra multispectral imaging platform.

FAP expression was assessed in cross-sectional areas encompassing tumors and adjacent non-malignant tissues. A semi-quantitative IHC score was determined by the aforementioned pathologist, who evaluated the percentage of FAP-positive cells and their staining intensity under a light microscope at ×10/20 magnification.

The FAP intensity score was graded as zero (none), one (weak), two (intermediate), or three (strong), whereas the semi-quantitative percentages of FAP-positive cells were scored as follows: zero (0%), one (1–25%), two (26–50%), three (51–75%), and four (76–100%). The final FAP-IHC score, which ranged from zero to 12, was calculated by multiplying the intensity and percentage values. Based on this scoring system, tumors were categorized as follows: negative or marked as ‘0’ (IHC score =0), mild or marked as ‘1’ (IHC score between 1 and 4), moderate or marked as ‘2’ (IHC score between 5 and 8), intense or marked as ‘3’ (IHC score between 9 and 12) (20,21).

Statistical analysis

All statistical analyses were carried out using R (version 4.3.2) and RStudio (version 2023.12.1 + 402). The normality of all numerical variables was tested by Probability-Probability (PP) plots, and the result distribution was normal or approximately so. All numerical variables were examined through parameter tests. A paired Student’s t-test was employed to compare paired data. Student’s t-test was conducted on both groups. The Spearman correlation was utilized for correlation analysis. Continuous variables are presented as mean ± standard deviation (SD), and categorical variables are shown as frequency and percentage. A two-tailed probability value <0.05 was considered statistically significant.

Results

Population

The study included 20 patients, two of whom had two lung nodules each, one had three, and the remaining had one nodule each, for a total of 24 lung nodules. All lung nodules were resected and confirmed to be LUADs by histopathology and immunohistochemistry. However, specimens were not available for six patients (seven lung nodules) who underwent surgery at other hospitals. FAP-IHC staining was performed for the remaining 17 nodules. The patient characteristics are shown in Table 1. A flowchart of the research design is presented in Figure 1.

Figure 1 Flowchart of research and design. ¹⁸F, fluorine 18; FDG, fluorodeoxyglucose; PET/CT, positron emission tomography/computed tomography; FAPI, fibroblast activation protein inhibitor.

Comparison of ¹⁸F-FDG and ¹⁸F-FAPI-04 uptake in all nodules

The SUVmean of ¹⁸F-FAPI-04 was significantly lower than that of ¹⁸F-FDG in the contralateral normal lung parenchyma (0.46±0.16 vs. 0.50±0.19; P=0.046). On ¹⁸F-FDG imaging, the SUVmax, TBR, MTV, and TLG of all tumors were 1.51±0.95, 1.98±1.75, 2.08±2.02, and 2.88±2.83, respectively. On ¹⁸F-FAPI-04 imaging, the SUVmax, TBR, FTV, and TLF of all tumors were 3.1±1.55, 4.47±3.75, 2.7±2.14, and 6.37±6.43, respectively. There were significant differences in SUVmax and TBR between ¹⁸F-FDG and ¹⁸F-FAPI-04 uptake in all tumors (P<0.001, P=0.04 respectively) (Table 2). ¹⁸F-FAPI-04 PET/CT was easier to observe and diagnose than was ¹⁸F-FDG PET/CT for stage IA LUAD, especially AIS and MIA (P=0.001) (Figure 2). Moreover, the TLF of ¹⁸F-FAPI-04 in the IAC group was significantly higher than those in the AIS and MIA groups (Table 3). The TBR of ¹⁸F-FAPI-04 was significantly higher than that of ¹⁸F-FDG in moderately and well-differentiated lesions (8.76±5.88 vs. 4.19±4.03, P=0.001; 5.85±3.84 vs. 2.77±2.87, P=0.001). The TBR of ¹⁸F-FAPI-04 showed a statistically significant increase compared with ¹⁸F-FDG in stage IA1 and IA2+3 (4.47±3.75 vs. 1.98±1.75, P=0.04; 8.58±5.34 vs. 4.14±3.91, P<0.001) (Table 2).

Figure 2 A 58-year-old female (Patient 12) presented with a moderately differentiated invasive adenocarcinoma (1.8 cm in diameter, stage IA2) located in the apical segment of the right upper lobe. (A-D) ¹⁸F-FAPI-04 PET/CT images (MIP, PET, CT, and fusion) revealed significantly increased metabolic activity within the lesion (red arrows) (SUVmax =6.0, P<0.05). The TBR was calculated to be 12.0, and the TLF was measured at 14.4 cm³. (E-H) In contrast, the corresponding ¹⁸F-FDG PET/CT images (MIP, PET, CT, and fusion) demonstrated slightly elevated metabolism within the lesion (red arrows; SUVmax =1.6; TBR =2.7; TLF =4.0 cm³). Additionally, radioactive concentrations are observed in the right rib fracture [black arrows (A) and (E)]. ¹⁸F, fluorine 18; FAPI, fibroblast activation protein inhibitor; PET/CT, positron emission tomography/computed tomography; MIP, maximum intensity projection; SUVmax, the maximum standardized uptake value; TBR, the tumor-to-background ratio; TLF, total lesion FAP expression; FDG, fluorodeoxyglucose.

Correlation between ¹⁸F-FAPI-04 uptake and histological FAP expression in stage IA LUAD lesions

The FAP-IHC score was 5.59±4.24, and the percentage of FAP-positive cells was 53.70%±32.49%. Moderately differentiated stage IA LUAD exhibited significantly increased FAP-IHC score (8.44±3.71, P=0.001) and a higher percentage of FAP-positive cells (76.33±18.16, P<0.001).

The FAP-IHC score of IAC lesions was significantly higher than that of AIS and MIA lesions (7.09±4.48 vs. 2.83±1.84, P=0.02). The FAP-IHC score of moderately differentiated lesions was significantly higher than that of well-differentiated lesions (8.44±3.71 vs. 2.38±1.77, P=0.001) (Figure 3). The SUVmax of ¹⁸F-FAPI-04 in the FAP-IHC 1’, 2’, and 3’ groups was 2.68±1.43, 3.44±0.87, and 5.12±1.07, respectively. Notably, the SUVmax of the FAP-IHC 3’ group was significantly higher than that of the IHC 1’ group, demonstrating a statistically significant difference (P=0.005) (Figure 4A).

Figure 3 ¹⁸F-FAPI-04 PET/CT uptake and FAP-IHC score in well and moderately differentiated types of stage IA LUAD. (A-E) A 76-year-old female (Patient 08) presented with a well-differentiated MIA (diameter 1.5 cm, stage IA2) in the posterior segment of the left superior lobe. (A-C) PET, CT and fusion images revealed slightly increased metabolism of the lesion (red arrows, SUVmax =2.5, TBR =5.0, TLF =1.3 cm³). (D,E) H&E staining demonstrated the stromal-rich group (TSR >50%), while FAP-IHC exhibited mild expression (IHC score =1’, ×20). (F-J) A 51-year-old male (Patient 11) presented with a moderately differentiated IAC (diameter 1.7 cm, stage IA2) in the apical segment of the right upper lobe. (F-H) PET, CT and fusion imaging revealed increased metabolism in the lesion (red arrows, SUVmax =2.9, TBR =7.3, TLF =2.4 cm³). (I,J) H&E staining indicated the stromal-poor group (TSR ≤50%), and FAP-IHC displayed moderate tumor stroma expression (IHC score =2’, ×20). ¹⁸F, fluorine 18; FAPI, fibroblast activation protein inhibitor; PET, positron emission tomography; CT, computed tomography; FAP, fibroblast activating protein; IHC, immunohistochemical; LUAD, lung adenocarcinoma; MIA, minimally invasive adenocarcinoma; SUVmax, the maximum standardized uptake value; TBR, the tumor-to-background ratio; TLF, total lesion FAP expression; TSR, tumor-stroma ratio; IAC, invasive adenocarcinoma; H&E, hematoxylin and eosin.

Figure 4 The uptake of ¹⁸F-FAPI-04 is significantly higher with higher FAP-IHC scores (3’ vs. 1’) and in stromal-poor tumors compared to stromal-rich tumors, and it demonstrated diagnostic efficacy in differentiating IAC from AIS and MIA. (A) Bar chart demonstrated a significant difference in¹⁸F-FAPI-04 SUVmax between FAP-IHC score 1’ and 3’ groups. (B) The ROC curve represented the diagnostic efficacy of ¹⁸F-FAPI-04 SUVmax for differentiating IAC from AIS and MIA. (C) Bar chart showed the difference in ¹⁸F-FAPI-04 SUVmax between the stromal-poor group (TSR ≤50%) and the stromal-rich group (TSR >50%). (D) The ROC curve represented the diagnostic efficacy of ¹⁸F-FAPI-04 TBR in differentiating IAC from AIS and MIA. Statistical significance was indicated as *, P<0.05; **, P<0.01; ns, not statistically significant. FAPI, fibroblast activation protein inhibitor; SUVmax, the maximum standardized uptake value; FAP, fibroblast activating protein; IHC, immunohistochemical; IHC 1’, immunohistochemical; mild or marked as ‘1’; IHC 2’, immunohistochemical, moderate or marked as ‘2’; IHC 3’, immunohistochemical, intense or marked as ‘3’; AUC, area under the curve; AIS, adenocarcinoma in situ; MIA, minimally invasive adenocarcinoma; IAC, invasive adenocarcinoma; TBR, the tumor-to-background ratio; TSR, tumor-stroma ratio; ¹⁸F, fluorine 18; ROC, receiver operating characteristic.

The optimal threshold of the ¹⁸F-FAPI-04 SUVmax for differentiating IAC lesions from AIS and MIA lesions was determined to be 1.3 (Figure 4B). The SUVmax of ¹⁸F-FAPI-04 of the stromal-rich group (TSR >50%) and the stromal-poor group (TSR ≤50%) was 2.61±1.66 and 4.24±0.88, respectively, with statistically significant differences (P<0.05) (Figure 4C). There was no significant difference in the FAP-IHC score between the stromal-rich group (TSR >50%) and the stromal-poor group (TSR ≤50%) (P=0.17). The ¹⁸F-FAPI-04 TBR cutoff value for distinguishing IAC lesions from AIS and MIA lesions was established as 5.6 (Figure 4D).

FAP was significantly expressed in the stromal cells of LUAD and was found to be expressed in LUAD cells (4/17). PET/CT findings at different FAP expression levels are shown in Figure 5.

Figure 5 PET/CT findings at different FAP expression levels are shown. (A,B) A 50-year-old male (Patient 02) presents with moderately differentiated IAC located in the posterior segment of the left superior lobe (1.5 cm in diameter, stage IA2). (A) The fusion image from an ¹⁸F-FAPI-04 PET/CT scan reveals increased metabolic activity within the lesion (arrow; SUVmax =2.3, TBR =4.6, and TLF =2.8 cm³). (B) FAP-IHC staining showed mild expression in the tumor stroma (FAP-IHC score =1’, ×20). (C,D) A 52-year-old female (Patient 09) with well-differentiated adenocarcinoma AIS located in the posterior basal segment of the right lower lobe (1.1 cm in diameter, stage IA2). (C) The fusion image from an ¹⁸F-FAPI-04 PET/CT scan shows increased metabolic activity within the lesion (arrow: SUVmax =4.5, TBR =9.0, TLF =1.3 cm³). (D) FAP-IHC staining reveals moderate expression in the tumor stroma (FAP-IHC score =2’, ×20). (E,F) A 66-year-old male (Patient 14) with moderately differentiated IAC located in the dorsal segment of the left lower lobe (1.2 cm in diameter, stage IA2). (E) The fusion image from an ¹⁸F-FAPI-04 PET/CT scan shows increased metabolic activity within the lesion (arrow: SUVmax =4.3, TBR =21.5, TLF =3.5 cm³). (F) FAP-IHC staining revealed intense expression of FAP in the tumor stroma (FAP-IHC score =3’, ×20). (G,H) A 73-year-old female (Patient 03) with a moderately differentiated IAC located in the posterior segment of the right upper lobe (1.0 cm in diameter, stage IA1). (G) The fusion image from an ¹⁸F-FAPI-04 PET/CT scan shows increased metabolic activity within the lesion (arrow: SUVmax =3.5, TBR =11.7, TLF =2.2 cm³). (H) FAP-IHC staining reveals moderate expression in the tumor stroma (FAP-IHC score =2’, ×20). (I,J) A 63-year-old female (Patient 05) with moderately differentiated IAC located in the posterior basal segment of the left lower lobe (1.2 cm in diameter, stage IA2). (I) The fusion image from an ¹⁸F-FAPI-04 PET/CT scan shows increased metabolic activity within the lesion (arrow: SUVmax =6.1, TBR =8.7, TLF =18.6 cm³). (J) FAP-IHC staining revealed intense expression of FAP in the tumor stroma (FAP-IHC score =3’, ×20). (K,L) A 63-year-old male (Patient 13) with moderately differentiated IAC located in the posterior basal segment of the left lower lobe (2.5 cm in diameter, stage IA3). (K) The fusion image from an ¹⁸F-FAPI-04 PET/CT scan shows increased metabolic activity within the lesion (arrow: SUVmax =4.1, TBR =20.5, TLF =4.6 cm³). (L) FAP-IHC staining revealed intense expression of FAP in the tumor stroma (FAP-IHC score =3’, ×20). IHC 1’, immunohistochemical; mild or marked as ‘1’; IHC 2’, immunohistochemical, moderate or marked as ‘2’; IHC 3’, immunohistochemical, intense or marked as ‘3’. PET/CT, positron emission tomography/computed tomography; FAP, fibroblast activation protein; IAC, invasive adenocarcinoma; ¹⁸F, fluorine 18; FAPI, fibroblast activation protein inhibitor; SUVmax, the maximum standardized uptake value; TBR, the tumor-to-background ratio; TLF, total lesion FAP expression; IHC, immunohistochemical; AIS, adenocarcinoma in situ.

Relationship between ¹⁸F-FAPI-04 uptake and tumor clinicopathological feature

The FAP-IHC score positively correlated with the SUVmax of ¹⁸F-FAPI-04 (r=0.64, P=0.005). The associations among FAPI SUVmax, FAP expression, and clinicopathological tumor features are shown in Figure 6. The differences in ¹⁸F-FAPI-04 PET/CT parameters between the different groups are presented in Table 3. Variations in FAP-IHC scores across different clinicopathological groups are shown in Table 4.

Figure 6 Correlation heat map illustrates the relationship among ¹⁸F-FAPI-04 PET/CT imaging parameters, FAP-IHC, and clinicopathological features of stage IA LUAD. FAPI, fibroblast activation protein inhibitor; SUVmax, the maximum standardized uptake value; TBR, the tumor-to-background ratio; FTV, FAPI avid tumor volume; TLF, total lesion FAP expression; CEA, carcinoembryonic antigen; IHC, immunohistochemical; TSR, tumor-stroma ratio; FAP, fibroblast activation protein; ¹⁸F, fluorine 18; PET/CT, positron emission tomography/computed tomography; LUAD, lung adenocarcinoma.

Discussion

The present study revealed a statistically significant increase in the SUVmax and TBR of ¹⁸F-FAPI-04 compared with ¹⁸F-FDG for stage IA LUAD (P<0.05). This disparity was observed in both pathological subtypes (IAC vs. AIS and MIA) and degrees of differentiation (moderately vs. well-differentiated). Notably, ¹⁸F-FAPI-04 PET/CT is more effective than ¹⁸F-FDG PET/CT for diagnosing stage IA LUAD, particularly in cases of AIS and MIA (P=0.001). This may be because stage IA LUAD is a relatively slow-growing cancer of the lungs that is not active in glucose utilization, but the tumor stroma components proliferate rapidly (22). As the tumor grows, it gradually consists of dense tumor cells and tumor stroma, and both FAPI and FDG uptake gradually increase, this is consistent with the results of Zhou et al. (23). Understanding the behavior of stage IA LUAD helps in early detection of lesions and treatment planning.

The SUVmax of ¹⁸F-FAPI-04 in the stromal-poor group (TSR ≤50%) exhibited a significantly higher value compared with the stromal-rich group (TSR >50%), indicating a statistically significant disparity (P<0.05). The SUVmax of ¹⁸F-FAPI-04 in the FAP-IHC 3’ group exhibited a significantly higher value compared with the FAP-IHC 1’ group, a statistically significant disparity (P=0.005). A strong positive correlation was noted between ¹⁸F-FAPI-04 uptake and the FAP-IHC score (Spearman’s correlation, r=0.64). The findings of this study demonstrate a strong correlation between ¹⁸F-FAPI-04 PET/CT uptake and the expression level of FAP, thereby validating the non-invasive visualization capability of this imaging modality for assessing FAP expression.

The degree of FAP expression within the tumor depends on the activation status of fibroblasts, specifically the percentage of stromal content or the quantity of FAP molecules per fibroblast. This is a crucial limiting factor for the detection of tumor lesions. Given that tumor growth beyond 1–2 mm necessitates the development of supportive stroma, FAPI PET/CT can effectively visualise small lesions ranging from 3–5 mm (8).

Our findings indicate that FAP is expressed in stromal and LUAD cells, consistent with previous studies on FAP expression in various tumor cells, including breast cancer cells, pancreatic cancer cells, and oral squamous cell carcinoma cells (24). Shi et al. and Ding et al. also found that FAP was expressed in pancreatic adenocarcinoma cells (20,25). Further research is required to explore the interactions between tumor cells and CAFs in the tumor microenvironment.

Notably, for the assessment of FAP expression in surgical specimens, we incorporated three parameters, namely the intensity score of FAP, the percentage of FAP-staining positive cells, and the FAP-IHC score. However, the parameter with the strongest correlation was the FAP-IHC score (Spearman correlation, r=0.64), similar to the results reported in the literature (20,26).

The FAP-IHC score for stage IA LUAD varied significantly based on the pathological subtype (IAC vs. AIS and MIA; P=0.02) and the degree of differentiation (moderately vs. well-differentiated; P=0.001). There was no significant difference in the FAP-IHC score between the stromal-rich group (TSR >50%) and the stromal-poor group (TSR ≤50%) (P=0.17). The number of patients in the stroma-poor group (9/17) was comparable to that in the stroma-rich group (8/17). However, the FAP IHC score of the stroma-rich group was lower than expected, possibly owing to the limited sample size included in this study. Sandberg et al. observed that FAP expression was higher in the stroma-rich group (N=9) than in the stroma-poor group (N=16) of colorectal cancer (CRC). Still, the difference was not statistically significant (P >0.05). Sandberg et al. attributed this to the fact that the TSR is evaluated in the most invasive part of the tumor, not the entire tumor (27). Zhao et al. also found a comparable number of cases in the stroma-poor (72/135) and stroma-rich (63/135) groups among patients with advanced non-small cell lung cancer. Moreover, there was a higher prevalence of high FAP expression in the stroma-poor group, although this difference was not statistically significant (P=0.47) (28).

Limitations

The limitations of this study include its small sample size and lack of IHC information for seven of the 24 lesions. This study specifically focuses on stage IA LUAD and does not include poorly differentiated tumors, thus lacking a systematic analysis of the three pathological differentiation types: well, moderately, or poorly differentiated. Future studies should conduct prospective trials involving larger patient populations.

Conclusions

¹⁸F-FAPI-04 PET/CT demonstrates higher SUVmax and TBR than ¹⁸F-FDG PET/CT in the detection of stage IA LUAD. It was re-assured that the ¹⁸F-FAPI-04 uptake of stage IA LUAD was positively correlated with the expression of FAP in vitro.

Acknowledgments

We would like to thank Editage (www.editage.com) for English language editing.

Footnote

Reporting Checklist: The authors have completed the STARD reporting checklist. Available at https://jtd.amegroups.com/article/view/10.21037/jtd-24-1658/rc

Data Sharing Statement: Available at https://jtd.amegroups.com/article/view/10.21037/jtd-24-1658/dss

Peer Review File: Available at https://jtd.amegroups.com/article/view/10.21037/jtd-24-1658/prf

Funding: This work was supported by the funding from the Special Fund Project of Science and Technology in Maoming Guangdong China (No. 2022S001) and the Guangdong Medical Research Fund (No. B2022060).

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://jtd.amegroups.com/article/view/10.21037/jtd-24-1658/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 study was approved by the Clinical Research Ethics Committee of Maoming People’s Hospital (No. PJ2023MI-K003-01) and the Declaration of Helsinki (as revised in 2013). All patients provided written informed consent before participating in the study.

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References

1. Siegel RL, Giaquinto AN, Jemal A. Cancer statistics, 2024. CA Cancer J Clin 2024;74:12-49. [Crossref] [PubMed]
2. Leiter A, Veluswamy RR, Wisnivesky JP. The global burden of lung cancer: current status and future trends. Nat Rev Clin Oncol 2023;20:624-39. [Crossref] [PubMed]
3. Ettinger DS, Wood DE, Aisner DL, et al. NCCN Guidelines® Insights: Non-Small Cell Lung Cancer, Version 2.2023. J Natl Compr Canc Netw 2023;21:340-50. [Crossref] [PubMed]
4. Wang J, Wen X, Yang G, et al. The predictive value of (18)F-FDG PET/CT in an EGFR-mutated lung adenocarcinoma population. Transl Cancer Res 2022;11:2338-47. [Crossref] [PubMed]
5. Guo Y, Zhu H, Chen C, et al. A nomogram based on (18)F-fluorodeoxyglucose PET/CT and clinical features to predict epidermal growth factor receptor mutation status in patients with lung adenocarcinoma. Quant Imaging Med Surg 2022;12:5239-50. [Crossref] [PubMed]
6. Sugawara H, Watanabe H, Kunimatsu A, et al. Adenocarcinoma in situ and minimally invasive adenocarcinoma in lungs of smokers: image feature differences from those in lungs of non-smokers. BMC Med Imaging 2021;21:172. [Crossref] [PubMed]
7. Owens C, Hindocha S, Lee R, et al. The lung cancers: staging and response, CT, (18)F-FDG PET/CT, MRI, DWI: review and new perspectives. Br J Radiol 2023;96:20220339. [Crossref] [PubMed]
8. Loktev A, Lindner T, Mier W, et al. A Tumor-Imaging Method Targeting Cancer-Associated Fibroblasts. J Nucl Med 2018;59:1423-9. [Crossref] [PubMed]
9. Kratochwil C, Flechsig P, Lindner T, et al. (68)Ga-FAPI PET/CT: Tracer Uptake in 28 Different Kinds of Cancer. J Nucl Med 2019;60:801-5. [Crossref] [PubMed]
10. Zukotynski KA, Gerbaudo VH. Understanding the Value of FAPI versus FDG PET/CT in Primary and Metastatic Lung Cancer. Radiology 2023;308:e231768. [Crossref] [PubMed]
11. Wei Y, Cheng K, Fu Z, et al. [18F]AlF-NOTA-FAPI-04 PET/CT uptake in metastatic lesions on PET/CT imaging might distinguish different pathological types of lung cancer. Eur J Nucl Med Mol Imaging 2022;49:1671-81.
12. Wang L, Tang G, Hu K, et al. Comparison of (68)Ga-FAPI and (18)F-FDG PET/CT in the Evaluation of Advanced Lung Cancer. Radiology 2022;303:191-9. [Crossref] [PubMed]
13. Wei Y, Ma L, Li P, et al. FAPI Compared with FDG PET/CT for Diagnosis of Primary and Metastatic Lung Cancer. Radiology 2023;308:e222785. [Crossref] [PubMed]
14. Yang Q, Huang D, Wu J, et al. Performance of [18F]FDG PET/CT versus FAPI PET/CT for lung cancer assessment: a systematic review and meta-analysis. Eur Radiol 2024;34:1077-85.
15. Wei Y, Zheng J, Ma L, et al. [18F]AlF-NOTA-FAPI-04: FAP-targeting specificity, biodistribution, and PET/CT imaging of various cancers. Eur J Nucl Med Mol Imaging 2022;49:2761-73.
16. Boellaard R, Delgado-Bolton R, Oyen WJ, et al. FDG PET/CT: EANM procedure guidelines for tumour imaging: version 2.0. Eur J Nucl Med Mol Imaging 2015;42:328-54. [Crossref] [PubMed]
17. Nicholson AG, Tsao MS, Beasley MB, et al. The 2021 WHO Classification of Lung Tumors: Impact of Advances Since 2015. J Thorac Oncol 2022;17:362-87. [Crossref] [PubMed]
18. Mesker WE, Junggeburt JM, Szuhai K, et al. The carcinoma-stromal ratio of colon carcinoma is an independent factor for survival compared to lymph node status and tumor stage. Cell Oncol 2007;29:387-98. [Crossref] [PubMed]
19. Wang K, Ma W, Wang J, et al. Tumor-stroma ratio is an independent predictor for survival in esophageal squamous cell carcinoma. J Thorac Oncol 2012;7:1457-61. [Crossref] [PubMed]
20. Ding J, Qiu J, Hao Z, et al. Prognostic value of preoperative [68 Ga]Ga-FAPI-04 PET/CT in patients with resectable pancreatic ductal adenocarcinoma in correlation with immunohistological characteristics. Eur J Nucl Med Mol Imaging 2023;50:1780-91.
21. Henry LR, Lee HO, Lee JS, et al. Clinical implications of fibroblast activation protein in patients with colon cancer. Clin Cancer Res 2007;13:1736-41. [Crossref] [PubMed]
22. Maiga AW, Deppen SA, Mercaldo SF, et al. Assessment of Fluorodeoxyglucose F18-Labeled Positron Emission Tomography for Diagnosis of High-Risk Lung Nodules. JAMA Surg 2018;153:329-34. [Crossref] [PubMed]
23. Zhou X, Wang S, Xu X, et al. Higher accuracy of [68 Ga]Ga-DOTA-FAPI-04 PET/CT comparing with 2-[18F]FDG PET/CT in clinical staging of NSCLC. Eur J Nucl Med Mol Imaging 2022;49:2983-93.
24. Shi J, Hou Z, Yan J, et al. The prognostic significance of fibroblast activation protein-α in human lung adenocarcinoma. Ann Transl Med 2020;8:224. [Crossref] [PubMed]
25. Shi M, Yu DH, Chen Y, et al. Expression of fibroblast activation protein in human pancreatic adenocarcinoma and its clinicopathological significance. World J Gastroenterol 2012;18:840-6. [Crossref] [PubMed]
26. Spektor AM, Gutjahr E, Lang M, et al. Immunohistochemical FAP Expression Reflects (68)Ga-FAPI PET Imaging Properties of Low- and High-Grade Intraductal Papillary Mucinous Neoplasms and Pancreatic Ductal Adenocarcinoma. J Nucl Med 2024;65:52-8. [Crossref] [PubMed]
27. Sandberg TP, Stuart MPME, Oosting J, et al. Increased expression of cancer-associated fibroblast markers at the invasive front and its association with tumor-stroma ratio in colorectal cancer. BMC Cancer 2019;19:284. [Crossref] [PubMed]
28. Zhao Y, Liu Y, Jia Y, et al. Fibroblast activation protein in the tumor microenvironment predicts outcomes of PD-1 blockade therapy in advanced non-small cell lung cancer. J Cancer Res Clin Oncol 2023;149:3469-83. [Crossref] [PubMed]