Abstract
Objectives
Breast cancer is highly heterogeneous, and certain histopathologic subtypes—particularly invasive lobular carcinoma—exhibit low glucose metabolism that limits 18F-fluorodeoxyglucose positron emission tomography/computed tomography (18F-FDG PET/CT) performance. Fibroblast activation protein inhibitor (FAPi) imaging with Gallium-68 (68Ga)-FAPi PET/CT has emerged as a promising alternative for such tumors. The aim of this study was to evaluate the efficacy of 68Ga-FAPi PET/CT in staging, restaging, and recurrence detection of breast cancer, with emphasis on subtypes for which 18F-FDG PET/CT has low sensitivity.
Methods
Twenty-nine women with pathologically confirmed breast cancer (mean age 57.7±11.6 years) were prospectively enrolled. Histopathology (available in 24 patients) showed lobular carcinoma in 16, ductal carcinoma in 6, signet ring cell carcinoma in 1, and mucinous carcinoma in 1. All underwent both 68Ga-FAPi and 18F-FDG PET/CT within one week for staging or restaging. Maximum standardized uptake values (SUVmax) of primary tumors and metastases were recorded for both tracers and compared using paired t-tests.
Results
In 7/29 patients (24.1%), neither 18F-FDG PET/CT nor 68Ga-FAPi PET/CT detected pathological findings. Disease stage increased in 13/29 (44.8%) after 68Ga-FAPi PET/CT, including 10 patients with no pathologic findings on 18F-FDG PET/CT; nine were upstaged to stage 4 and one to stage 3. Additionally, one patient progressed from stage 1 to 2, one from 2 to 3, and another from 3 to 4. Among patients with lobular carcinoma, 8/16 (50%) were upstaged with 68Ga-FAPi PET/CT. Detection of nodal and distant metastases in lobular carcinoma was higher with 68Ga-FAPi than 18F-FDG. 68Ga-FAPi PET/CT also demonstrated higher SUVmax in primary lesions and metastases (p<0.05).
Conclusion
68Ga-FAPi PET/CT provides a significant advantage for staging and restaging breast cancer—especially lobular carcinoma and other low 18F-FDG-avid subtypes—supporting its potential role in clinical decision-making and treatment planning.
Introduction
Breast cancer is the most common malignancy among women and represents a significant public health issue due to its high incidence and mortality rates (1). This heterogeneous disease exhibits considerable variability in biological behavior, histopathological characteristics, and treatment response (2). Accurate staging and restaging are critical components in the diagnosis and management of breast cancer. 18F-fluorodeoxyglucose (18F-FDG) positron emission tomography/computed tomography (PET/CT) is a key modality in breast cancer management, contributing to diagnosis, staging, restaging, treatment assessment, and prognostic evaluation. While 18F-FDG PET/CT demonstrates high diagnostic accuracy, particularly for metastases, its sensitivity is limited for detecting small tumors (<1 cm), peritoneal micrometastases, and lymph node metastases (3). Moreover, its diagnostic efficacy is limited in certain histopathological subtypes, such as lobular carcinoma, which typically demonstrates low metabolic activity (4). These limitations underscores the need for more sensitive and effective imaging modalities in these patients. Studies have demonstrated that PET/CT performed with Gallium-68 (68Ga)-labeled fibroblast activation protein inhibitor (FAPi) a new theranostic candidate, provides superior sensitivity and specificity in detecting primary and metastatic breast cancer lesions compared to 18F-FDG PET/CT, offering improved diagnostic accuracy and staging precision (5,6).
FAPi targets fibroblast activation protein, which is highly expressed in the tumor stroma, providing effective imaging even in cases with low 18F-FDG uptake. 68Ga-FAPi PET/CT has been shown to outperform conventional methods in detecting primary tumors and metastatic lesions with low 18F-FDG affinity. In particular, it is thought to offer enhanced sensitivity for histological subtypes such as lobular carcinoma, for identifying lymph node metastases and distant metastases.
The aim of this study was to evaluate the efficacy of 68Ga-FAPi PET/CT in staging, restaging, and recurrence detection of breast cancer patients, particularly in tumors such as lobular carcinoma, in which the diagnostic value of 18F-FDG is limited.
Materials and Methods
Patients
This single-center prospective clinical study enrolled 29 female patients (mean age 57.7±11.6 years) with pathologically confirmed breast cancer. All patients underwent both 18F-FDG and 68Ga-FAPi PET/CT imaging within one week for staging or restaging. The inclusion criteria were as follows: I) histologically confirmed invasive breast carcinoma (primary staging or restaging), II) a prior 18F-FDG PET/CT performed within the clinical work-up, III) 18F-FDG PET/CT that showed: a) no abnormal 18F-FDG uptake (18F-FDG-negative), b) low-grade, equivocal, or clinically non-explanatory 18F-FDG uptake, insufficient to account for clinical suspicion, rising tumor markers, or abnormalities on other imaging modalities, IV) a clinical expectation that 68Ga-FAPi PET/CT might clarify disease extent or reveal additional metastatic sites not visualized on 18F-FDG PET/CT, V) age ≥18 years and ECOG performance status <3. Exclusion criteria were: I) 18F-FDG PET/CT demonstrating clear and disseminated 18F-FDG positive metastatic (stage IV) disease, for which 68Ga-FAPi PET/CT was not expected to provide further diagnostic benefit and would only add unnecessary radiation exposure, II) 18F-FDG-avid primary breast cancers in which staging was already conclusive based on 18F-FDG PET/CT findings, III) history of a second primary malignancy.
Informed consent was obtained from all patients. This prospective study was approved by the Ethics Committee of Yeditepe University (decision no: 1576, date: 02.03.2022).
Preparation and Quality Control of 68Ga-FAPi
68Ga-DOTA-FAPi-46 (used in 19 patients) and 68Ga-DOTA-FAP-2286 (used in 10 patients) were prepared using a modular-based fully automated synthesizer (GRP V4, Scintomics GmbH, Germany). The use of two FAP-targeting ligands was related to radiopharmaceutical availability and logistical changes in the local radiochemistry workflow. The imaging acquisition protocol, scanner, reconstruction parameters, and interpretation criteria were kept consistent throughout the study. The study was not designed as a head-to-head comparison of these two ligands. Briefly, the 68Ga obtained from the 68Ge/68Ga generator (iThemba LABS) was sent to the reaction vial containing DOTA-FAPi-46 or DOTA-FAP-2286. After completion of the labelling process, the reaction solution was purified using an extraction cartridge and then subjected to sterile filtration to prepare the final patient dose. The total synthesis time was 25 minutes. The radiochemical purity and radiolabeling efficiency of 68Ga-DOTA-FAPi-46 and 68Ga-DOTA-FAP-2286 were determined by combining a radioactive detector with reversed-phase high-pressure liquid chromatography. 68Ga-FAPi with a radiochemical purity of ≥95% was administered to patients.
18F-FDG and 68Ga-FAPi PET/CT Imaging
Whole-body 18F-FDG PET/CT and 68Ga-FAPi PET/CT scans were performed using a PET scanner (Discovery PET/CT 710, General Electric Medical Systems, Milwaukee, WI, USA) with high resolution, time-of-flight function, and LYSO crystal and integrated with 64-slice CT scanner. The average activity of 18F-FDG was 425±64 MBq (range: 340-548 MBq) [11.5±1.7 mCi (range: 9.2-14.8 mCi)], 68Ga-FAPi was 243±57 MBq (range: 122-312 MBq) [6.6±1.5 mCi (range: 3.3-8.4 mCi)]. Patients were positioned supine on the PET scanner bed 60 minutes after intravenous injection of radiopharmaceuticals. CT and PET images were acquired from the vertex region to mid-thigh. 68Ga-FAPi PET/CT scans were performed within 7 days after 18F-FDG PET/CT.
Evaluation of 18F-FDG and 68Ga-FAPi PET/CT Images
The presence of primary breast lesions and metastatic lesions, as well as changes in disease stage, on 18F-FDG and 68Ga-FAPi PET/CT scans were noted. Uptake in the tumor was measured by maximum standardized uptake value (SUVmax) using circular regions of interest drawn around the lesions with focal uptake in transaxial slices and automatically adapted to a 3D voxel area.
The final disease status of PET-positive and PET-negative findings was determined using a predefined composite reference standard. Histopathological confirmation was used whenever clinically available. For lesions without histopathological confirmation, a 68Ga-FAPi PET/CT-positive finding was accepted as true-positive only when all of the following criteria were fulfilled: anatomical/radiological correlation on diagnostic contrast-enhanced CT and/or magnetic resonance imaging (MRI); compatible findings on follow-up or correlative imaging performed at least 6 months after the index PET/CT, including 68Ga-FAPi PET/CT, 18F-FDG PET/CT, CT, and/or MRI; and confirmation by multidisciplinary tumor board assessment. Follow-up imaging findings were interpreted together with the clinical course and treatment status. PET findings were not used as the sole reference standard. All images were reviewed by consensus by three senior nuclear medicine physicians for confirmation.
For lesion-site-based detection analysis, the number of patients positive at each metastatic site was determined using a composite reference standard derived from all available patient data. This included histopathological confirmation when available, contrast-enhanced CT, MRI when clinically performed, other conventional imaging findings, follow-up imaging, tumor marker evolution, clinical course, and multidisciplinary consensus. Therefore, detection rates were not calculated solely from 18F-FDG PET/CT and 68Ga-FAPi PET/CT findings, but were correlated with the parameters mentioned above.
Statistical Analysis
Statistical analyses were performed using SPSS software (version 25.0, IBM Inc.). Descriptive analyses were conducted to assess the characteristics of the patients and their tumors. The mean and standard deviation were calculated for normally distributed measurements, and the median and range for non-normally distributed measurements. The SUVmax of the primary tumor areas, axillary lymph nodes and metastases on the 18F-FDG and 68Ga-FAPi PET/CT images were recorded and statistically compared using the paired t-test. Chi-square test was used to evaluate the staging accuracy of 18F-FDG PET/CT and 68Ga-FAPi PET/CT. A p value of less than 0.05 was considered statistically significant.
Results
Pathology reports from previous surgeries/biopsies were available for all patients; however, a documented histopathological subtype was available in 24 patients: lobular carcinoma in 16, ductal carcinoma in 6, signet-ring cell carcinoma in 1, and mucinous carcinoma in 1. In 5 patients, the pathology reports did not include a documented histopathological subtype. Thirteen patients (44.8%) were enrolled for staging purposes, and 16 (55.2%) were enrolled for restaging purposes.
68Ga-FAPi PET/CT demonstrated positive positive findings in 22 patients, whereas 18F-FDG PET/CT revealed positive findings in only 12 patients. In seven patients (24.1%), neither 18F-FDG PET/CT nor 68Ga-FAPi PET/CT detected any pathological findings. Among these patients, two were undergoing initial staging, while five were undergoing restaging. No pathological findings were detected in these patients by other imaging modalities or during follow-up.
On 68Ga-FAPi PET/CT, the most frequently detected metastatic site was bone (41.4%, n=12), followed by axillary lymph nodes (34.5%, n=10), primary tumor lesions (34.5%, n=10), distant lymph nodes (24.1%, n=7), visceral metastases (20.7%, n=6), and peritoneal metastases (17.2%, n=5) (Table 1).
The detection of lymph nodes and distant metastases was higher with 68Ga-FAPi PET/CT than with 18F-FDG PET/CT (Table 2). Furthermore, 68Ga-FAPi PET/CT showed a higher SUVmax in primary tumor foci and metastases (p<0.05) (Table 3). The mean SUVmax of primary lesions was significantly higher on 68Ga-FAPi PET/CT than on 18F-FDG PET/CT, 15.08±4.30 vs. 2.30±1.98, respectively, p<0.001. Axillary lymph node metastases also demonstrated significantly higher uptake on 68Ga-FAPi PET/CT, with SUVmax of 23.54±16.46 compared with 5.30±11.22 on 18F-FDG PET/CT, p=0.032. For distant lymph node, visceral, peritoneal, and bone metastases, no 18F-FDG SUVmax comparison was possible because these sites were not detected on 18F-FDG PET/CT in our cohort. The mean SUVmax on 68Ga-FAPi PET/CT were 14.07±6.50 for distant lymph node metastases, 10.66±4.50 for visceral metastases, 9.94±2.15 for peritoneal metastases, and 16.32±9.31 for bone metastases.
In 5 out of 13 patients (38.4%) referred for staging, the disease stage was higher on 68Ga-FAPi PET/CT compared to 18F-FDG PET/CT. Following 68Ga-FAPi PET/CT, one patient was upstaged from stage 1 to stage 2, one from stage 2 to stage 3, and another from stage 3 to stage 4 according to the American Joint Committee on Cancer 8th edition. 68Ga-FAPi PET/CT also revealed stage 4 disease in two patients where no findings were observed on 18F-FDG PET/CT. All patients who were upstaged to stage 4 (a total of 3 patients) were found to have bone metastases, as detected by 68Ga-FAPi PET/CT, highlighting its superior sensitivity in identifying metastatic lesions compared to 18F-FDG PET/CT.
Of the 21 patients referred to our clinic for restaging due to elevated tumor markers or suspected recurrence on conventional imaging during follow-up, 18F-FDG PET identified recurrence in 8 patients, while 68Ga-FAPi PET/CT detected recurrence in 16 patients (Figure 1). In 5 patients, no lesion was identified by either modality.
False-negative findings were observed in two patients: one had no uptake in the primary tumor, and the other had mucinous carcinoma with lung metastasis; these lesions were undetected by both 18F-FDG PET/CT and 68Ga-FAPi PET/CT.
Discussion
Although 18F-FDG PET/CT is a highly valuable method for the diagnosis and staging of breast cancer, it may yield negative findings in several histopathological subtypes of breast tumors, particularly in invasive lobular carcinoma and certain subtypes of ductal carcinoma (7). In such cases, both the diagnosis of the primary tumor and the staging of the disease can be compromised. 68Ga-FAPi PET/CT stands out due to its low background uptake and high uptake in these tumor types (8). Recent studies have found that 68Ga-FAPi PET/CT demonstrates generally high tumor uptake in breast cancer and shows a higher metastasis detection rate compared with 18F-FDG PET/CT, particularly for bone and peritoneal metastases (9,10).
In our study, the lesion detection capabilities of both modalities across different regions are presented in Table 2. The reported detection rates were calculated using this composite reference standard. Accordingly, distant lymph node metastases were present in 7 patients and were detected by 68Ga-FAPi PET/CT in all 7 patients. Visceral metastases were present in 7 patients and were detected by 68Ga-FAPi PET/CT in 6 patients. Peritoneal metastases were present in 5 patients and were detected by 68Ga-FAPi PET/CT in all 5 patients. These findings suggest that 68Ga-FAPi PET/CT may provide additional diagnostic value in lesion sites that are often difficult to detect with 18F-FDG PET/CT in low-18F-FDG-avidity breast cancer subtypes. According to these results, 68Ga-FAPi was found to be highly effective, with detection rates of 100% for distant lymph nodes, 85.7% for visceral organ metastases, and 100% for peritoneal metastases. In contrast, 18F-FDG PET/CT was unable to detect these lesions. In the literature, the detection rates of 18F-FDG PET/CT for lobular breast carcinoma have been reported to vary between 50% and 90% (11). However, in our study, these rates were significantly lower. The key point emphasized in this study is that this group of tumors, for which 18F-FDG has limited diagnostic value and often fails to detect, can be visualized with the novel molecular agent 68Ga-FAPi.
While primary tumors and axillary lymph nodes can often be detected with the help of the CT component of 18F-FDG PET/CT, distant lymph node, visceral organ, and peritoneal metastases are frequently missed in this patient group. In our study, the detection rates for axillary lymphadenopathy did not differ significantly between the two imaging modalities for staging. However, the number of lesions detected with 68Ga-FAPi PET/CT was higher compared to 18F-FDG PET/CT. Regarding the ability to detect the primary tumor, no significant difference was observed between the two modalities. The absence of a significant difference between the two imaging modalities in detecting the primary tumor may be attributed to the limited sample size. On the other hand, the SUVmax of the primary tumors were found to be higher with 68Ga-FAPi PET/CT. SUVmax for all localizations were significantly higher with 68Ga-FAPi compared to 18F-FDG (Table 3). Elboga et al. (12) performed a retrospective analysis involving 48 patients with invasive breast cancer, demonstrating that 68Ga-FAPi PET/CT identified more primary breast lesions and additional metastatic lesions with greater uptake values compared to 18F-FDG PET/CT. The effectiveness of 18F-FDG PET/CT was notably reduced in the invasive lobular carcinoma subtype, emphasizing the necessity for a more efficient imaging modality, especially for this specific patient group in their study. Similarly, Ballal et al. (13) observed increased radiotracer uptake using 68Ga-DOTA.SA.FAPi in lymph nodes corresponding to CT findings in the axillary region, where no uptake was detected with 18F-FDG PET/CT. The findings of this prospective study highlight the significant impact of 68Ga-FAPi PET/CT imaging on accurate staging of breast cancer, particularly in cases with lobular carcinoma.
According to our results, disease upstaging was observed in 44.8% of patients, emphasizing the diagnostic superiority of 68Ga-FAPi PET/CT compared to 18F-FDG PET/CT. This demonstrates the enhanced sensitivity of 68Ga-FAPi PET/CT in detecting previously undetected metastases or disease extent. Notably, 50% of patients with lobular carcinoma were upstaged after 68Ga-FAPi PET/CT imaging, emphasizing the potential of this modality to address the limitations of 18F-FDG in this histological subtype, where lower glycolytic activity often leads to false negatives. The capability of FAPi to identify fibrotic or stromal components of tumors, which are prominent in lobular carcinoma, may account for this improvement. In the study conducted by Kratochwil et al. (14) it was reported that 68Ga-FAPi PET/CT demonstrates high affinity for tumor stromal components, enabling high sensitivity even in tumors with low 18F-FDG avidity. Our study findings are consistent with the literature and can be attributed to the ability of 68Ga-FAPi PET/CT to effectively visualize the microscopic stromal and fibrotic components of tumors.
Of the 21 patients referred to our clinic for restaging due to elevated tumor markers or suspected recurrence on conventional imaging during follow-up, 18F-FDG PET identified recurrence in 8 patients, while 68Ga-FAPi PET/CT detected recurrence in 16 patients (Figure 2). In 5 patients, no lesion was identified with either modality. In our study, 18F-FDG PET/CT failed to detect recurrence in 72.7% of patients who were later identified as having recurrent disease by 68Ga-FAPi PET/CT. Studies in the literature have demonstrated the superiority of 68Ga-FAPi PET/CT over 18F-FDG PET/CT in detecting primary and metastatic lesions. Kömek et al. (15) showed that 68Ga-FAPi-04 PET/CT detected more lesions compared to 18F-FDG PET/CT and exhibited higher tumor-to-background ratios. Similarly, a study highlighted the superiority of 68Ga-FAPi PET/CT over 18F-FDG PET/CT, particularly in invasive lobular carcinoma (5). Additionally, the study conducted by Chen et al. (16) evaluated the potential use of 68Ga-FAPi PET/CT in the detection, staging, and restaging of various cancer types and compared it with 18F-FDG PET/CT. The results demonstrated that 68Ga-FAPi PET/CT offers higher sensitivity and specificity in various cancer types, particularly in breast cancer. While these studies emphasize the role of 68Ga-FAPi PET/CT in the assessment of primary breast masses, axillary lymph nodes, and distant metastases, none have specifically focused on its utility in detecting breast cancer recurrence. Our study differs from prior investigations by not only validating the diagnostic superiority of 68Ga-FAPi PET/CT in primary staging and metastasis detection, but also by revealing its extended capacity for restaging and recurrence identification.
Study Limitations
The main limitation of the present study is the relatively small sample size, particularly within each histopathological subgroup. Therefore, subgroup-based comparisons should be interpreted with caution, and the present findings should be considered preliminary and hypothesis-generating. Since the study specifically focused on breast cancer patients with low or limited 18F-FDG uptake, patient recruitment was inherently restricted. Larger multicenter prospective studies with standardized imaging protocols, pathological confirmation when feasible, and long-term clinical follow-up are required to validate these findings. This study demonstrates the diagnostic superiority of 68Ga-FAPi PET/CT over 18F-FDG PET/CT, especially in invasive lobular carcinoma and challenging histological subtypes. However, the absence of long-term clinical outcome data in the study can be considered a limitation. Another limitation of the present study is the use of two FAP-targeting ligands, 68Ga-DOTA-FAPi-46 and 68Ga-DOTA-FAP-2286. Although both target the fibroblast activation protein, potential differences in pharmacokinetics, biodistribution, background activity, and lesion uptake may influence SUV-based measurements. Because the study was not designed as a head-to-head comparison and no patient underwent imaging with both tracers, the possible impact of ligand-related differences could not be formally assessed. Prospective studies using a single ligand or employing direct intraindividual comparison protocols are warranted. Another limitation is that histopathological confirmation was not available for every 68Ga-FAPi-positive metastatic lesion. Therefore, final lesion classification was based on a composite reference standard including histopathology when available, correlation with diagnostic contrast-enhanced CT and/or MRI, follow-up imaging for at least 6 months, clinical course, and multidisciplinary tumor board consensus. Future multicenter prospective studies with larger populations and subgroup analyses are needed to confirm its efficacy. Additionally, long-term survival and cost-effectiveness data will support its potential role in breast cancer management and inform personalized treatment strategies.
Conclusion
68Ga-FAPi PET/CT was more effective than conventional 18F-FDG PET/CT in detecting and staging breast cancer in our cohort, especially invasive lobular carcinoma. The study findings support the use of 68Ga-FAPi PET/CT as a complementary imaging tool not for all breast cancers, but particularly for low-18F-FDG-avidity histological subtypes, in whom 18F-FDG PET/CT has limited sensitivity for detecting extra-axillary and distant metastases. Larger multicenter studies with pathological confirmation of metastatic sites are warranted to validate these results and further define the clinical utility of FAPi imaging in this subset of patients.
