2-deoxy-2-[¹⁸F]fluoro-D-glucose (¹⁸F-FDG), known as “the century molecule,” is currently the most widely used oncological tracer. ¹⁸F-FDG positron emission tomography/computed tomography (PET/CT) is a valuable molecular imaging method widely used in the clinical diagnosis, staging and efficacy monitoring of various diseases. However, due to the pathological characteristics of glucose metabolism, ¹⁸F-FDG is highly distributed in normal organs including the brain, heart, and liver (1–3). Many physiological variations and pitfalls of whole-body ¹⁸F-FDG PET/CT imaging have been reported (4–6). In the pelvic region, activity retention within the urinary tract and ¹⁸F-FDG uptake in the normal intestine over very short segments are common sources of false-positives (7). At the same, ¹⁸F-FDG PET/CT lacks specificity in differentiating inflammation (8–10).

The menstrual cycle complicates the imaging of the female reproductive system. Gynecologic malignancies, including cervical, endometrial, vulvar, and ovarian carcinoma show ¹⁸F-FDG activity (11–13). ¹⁸F-FDG uptake in follicular ovarian cysts or hemorrhagic luteal cysts has previously been reported (13–15)]. In recent years, there have been reports of focal ¹⁸F-FDG uptake in the ovaries and uterus associated with the menstrual cycle in premenopausal women (14, 16–18), which may result in false positives on ¹⁸F-FDG PET imaging or malignancy being overlooked for physical uptake. This may be a challenge to nuclear medicine physicians. In order to identify if ovarian ¹⁸F-FDG uptake is physiological or pathological, the traditional method requires ¹⁸F-FDG PET/CT in different menstrual periods, which is not conducive to early clinical diagnosis. This has led to the development of more tracers.

Quinoline-based ligands targeting cancer-associated fibroblasts are promising radiopharmaceuticals in multiple tumors. Fibroblast activation protein (FAP) is a type II transmembrane glycoprotein expressed in dimer form on the surface of the tumor-associated cell matrix (CAFs) (19, 20). Evidence indicates that FAP is highly expressed in a variety of tumors, especially colorectal, ovarian, pancreatic, and hepatocellular carcinomas characterized by a strong desmoplastic reaction (21). CAFs with high FAP expression are associated with an adverse prognosis by promoting invasion, angiogenesis, micro-environmental immune suppression, and metastasis (22). ⁶⁸Ga-DOTA-FAPI-04 showed the most favorable PET imaging properties, including low nanomolar affinity to FAP, near-complete internalization of FAP-bound radioactivity, and rapid blood clearance (23). This is a promising diagnostic and therapeutic target because of its low uptake in normal tissues and high target/non-target ratio.

To our knowledge, there have been no systematic investigations of ovarian FAPI metabolism. Herein, we present the results of a retrospective study of FAPI ovarian uptake patterns, which could help identify physiological and pathological changes for disease staging and formulate optimal treatment strategies.

This is a retrospective analysis of a sub-cohort of patients from a previously acquired prospective database. Data were screened from the study previously registered at the clinical trial center and approved by the Clinical Research Ethics Committee of our Hospital. The study was conducted in accordance with the 1964 declaration of Helsinki and its subsequent amendments or similar ethical standards. In all cases, PET/CT scans were performed according to clinical needs or other protocols approved by our institutional review committee.

A total of 78 female patients underwent ¹⁸F-FDG and ⁶⁸Ga-DOTA-FAPI-04 PET/CT examination within 3 days. Ovarian uptake was observed on ¹⁸F-FDG PET/CT images in 29 patients, 15 of whom were confirmed by pathology (n = 11) or follow-up (n = 4). This retrospective analysis included 14 patients with a mean age of 38.6 (range, 19–52 years). They were mainly in secretory phase (11/14 for FDG, 12/14 for FAPI) and proliferative phase (3/14 for FDG, 2/14 for FAPI). The demographic and clinical characteristics of the subjects are summarized in Table 1.

The FDG and FAPI PET/CT images showed that there were significant differences in the ovarian uptake between the two groups, most being unilateral. There was no significant difference in SUV between the left and right ovary (t = 0.272 for FDG, and t = 0.592 for FAPI). For the FDG group, the ovary SUVmax (4.89 ± 1.84) was statistically significantly higher than that of liver (1.57 ± 0.44) and pelvic muscle (0.86 ± 0.19) (P < 0.05). The average Le/Li ratio and TBR were 3.38 ± 1.81, and 5.81 ± 1.98, respectively. Physiological FDG uptake is associated with the menstrual cycle and occurs mainly in late hyperplasia and early secretion (Table 2). There was no significant difference between the two phases (t = 1.26, p = 0.23). Focal endometrial FDG uptake was observed in 4 patients (median SUVmax4.5, Figure 1). For the FAPI group, the average SUV of ovary, liver and muscle were 1.53 ± 0.37, 0.45 ± 0.10, and 1.64 ± 0.29, respectively. The difference between ovary and liver was statistically significant (t = 11.108, p < 0.05), while the difference between ovary and muscle was not (t = −1.33, p = 0.21). The average Le/Li ratio and TBR were 3.57 ± 1.26, and 0.94 ± 0.19, respectively. The uterus of the 14 subjects showed intense FAPI activity (average SUVmax 12.7, Figure 2). In addition, abnormal FAPI uptake in the broad ligament of the uterus was observed in a patient, which was manifested as a stripe of increased FAPI-avid (SUVmax 3.9). The TBR of the two groups was statistically different (t = 9.42, p < 0.05), while Le/Li ratio was not (Table 3). This is because normal liver has a high FDG uptake and a low FAPI uptake (one of the advantages of FAPI in displaying lesions, Figure 3).

Both pathological and functional ovarian lesions can show abnormal FDG uptake. Physiologically, FDG accumulates in the female reproductive system, with a high SUV, making it difficult to obtain a clear image with high tumor-background contrast and increasing the diagnostic difficulty. In 2002, Chander et al. confirmed the physiological uptake of the endometrium and ovary in a serial PET/CT follow-up of a patient, and suggested that this performance varies with the menstrual cycle (17, 24, 25). Glucose phosphorylation is an important rate-limiting step in the estrogenic stimulation of uterine glycolysis (14). Hughes assessed endometrial enzymes activity in 252 patients with normal menstrual histories and found that, in normal endometrial tissue, glycogen synthetase activity synthesizes glycogen from glucose in increasing amounts until the midcycle (26). Glycogen phosphorylase then-breaks down glucose during the regressive stage of endometrial activity, causing decreased glycogen levels at the end of the cycle. Increased metabolic demands and inflammatory responses before and after ovulation may lead to increased ovarian FDG uptake. However, not all women with active menstruation experience increased FDG uptake for reasons that remain unclear. This physiological FDG uptake mainly occurs during the late follicular to early luteal phase of the menstrual cycle and is usually round or elliptical, mostly unilateral, and with a SUVmax >3 (25). Subsequent research has evaluated the characteristics of physiological FDG uptake in the ovaries, including other imaging methods and the differential diagnostic value of delayed imaging (17, 24, 25).

Cook et al. warn that a high uptake in the periphery of a benign cyst, such as a follicular ovarian cyst, may mimic a necrotic lymph node (15). Therefore, it is crucial to differentiate between physiological and pathological uptake. Measurement of the serum levels of menstrual cycles or ovarian hormones may help diagnose false positives but does not rule out pathological metabolism. The traditional method to distinguish the physiological and pathological uptake of the ovary requires repeated FDG examination in different physiological cycles, which may delay diagnosis.

SUVmax is the most commonly used index for evaluating metabolism on PET/CT, but differences in nuclides, patients, and equipment may cause certain differences. Therefore, using the target/non-target ratio is obviously more comparable. TBR can better perform semi-quantitative analysis of the uptake of different patients and imaging agents. Research has demonstrated that TBR has independent prognostic abilities for many lesions (27).

This retrospective analysis sought to evaluate the benefit and impact of ⁶⁸Ga-FAPI-PET/CT in a small cohort of patients harboring suspicious ovarian lesions. We retrospectively analyzed the PET/CT images of 14 patients with suspected FDG false-positive ovaries. In contrast to ¹⁸F-FDG, ⁶⁸Ga-DOTA-FAPI-04 has no physiological accumulation in the ovaries and is not affected by the physiological cycle, resulting in higher image contrast and better lesion delineation in the adnexal area of the uterus (Figure 3). ⁶⁸Ga-DOTA-FAPI-04 positron emission tomography produces accurate and comprehensive imaging that can help determine the best treatment strategy. It may improves tumor staging, relapse monitoring, and necessary therapeutic interventions. Tumor lesions exceeding 1–2 mm in size require a supporting stroma (28). As the stroma volume can be larger than the tumor volume, stroma-targeted PET imaging may be more sensitive than glycolysis PET imaging for detecting small lesions with sufficient FAP-expressing stroma (22, 28). In 2019, Clemens et al. quantified the tumor-uptake in FAPI-PET/CT of various primary and metastatic tumors and found that ovarian cancer showed a moderate uptake of FAPI (SUVmax 6-12) (29).

Both malignant and functional ovarian lesions can exhibit abnormal FDG uptake. ⁶⁸Ga-DOTA-FAPI-04 has no physiological uptake and is not affected by the physiological cycle. It has a unique advantage in the diagnosis of ovarian diseases and can accurately differentiate physiological and pathological ovarian lesions in the early stage.

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.

Written informed consent was obtained from the individual(s) for the publication of any potentially identifiable images or data included in this article.

QW, SY, WT, LL, and YC: conception and design. QW: methodology, formal analysis, and writing–original draft. All the authors revised the paper, agreed to the submission of the final version of the manuscript, vouch for the accuracy and completeness of the data, analyses and for the fidelity of this report.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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