Physiological uterine uptake of 68Ga-FAPI-04 may limit its application in uterine pathology

Purpose 68 Ga-labeled broblast activation protein inhibitor ( 68 Ga-FAPI) has been useful in the imaging of desmoplastic reaction in different tumors, as well as brosis in inammatory diseases. As we have found that most female patients showed avid uterine uptake of 68 Ga-FAPI, we sought to further investigate the physiological uptake of 68 Ga-FAPI by the uterus and analyze its characteristics in women of childbearing age, in perimenopause, and postmenopause. rst detail analysis of the uterine uptake on 68 Ga-FAPI-04 PET/MRI.


Introduction
Fibroblast activation protein (FAP) is regarded as an excellent target for diagnosis and therapy in oncology [1,2]. 68 Ga-labeled FAPI-04 (a speci c inhibitor of FAP) is used as a PET tracer for tumor diagnosis by visualizing desmoplastic reaction [3,4]. It also has the potential for non-malignant diseases associated with tissue remodeling, such as myocardial infarction, sarcoidosis, chronic in ammation, brosis of lung and liver, rheumatoid arthritis, and possibly also atherosclerosis [5][6][7][8][9][10]. The clinical applications of 68 Ga-FAPI-04 PET in different diseases are being de ned.
Some studies have reported the physiological changes in the female reproductive system using 18 F-FDG PET [11][12][13]. In our clinical studies to evaluate the diagnostic value of 68 Ga-FAPI-04, it occurred to us that elevated 68 Ga-FAPI-04 uptake in the uterus were noted in most female patients. We reviewed the literature and case reports on clinical studies of 68 Ga-FAPI-04 and found that uterine uptake of the tracer is very common [14][15][16]. Uptake patterns varied from patient to patient. There are no further discussions of the physiological uptake of 68 Ga-FAPI-04 by the uterus. Primary and metastatic malignant uterine tumors are common [17]. The uptake of 68 Ga-FAPI-04 by the uterus may affect the evaluation of uterine lesions, and the factors related to uterine uptake are still lacking.
On the basis of the evaluation of all female patients in our institute who underwent 68 Ga-FAPI-04 PET/MRI, the aim of this study was to analyze the physiological uptake and characteristics of 68 Ga-FAPI in the uterus, and to further evaluate its value in the diagnosis of uterine diseases. The owchart and clinical information of the included patients are shown in Fig. 1 and Table 1, respectively. Patients without uterine malignancy were divided into three groups: 13 (mean age, 34.92 ± 4.55 years) of reproductive age, six (mean age, 49.17 ± 1.17 years) in the perimenopausal stage, and 22

Material And Methods
(mean age, 59.36 ± 6.51 years) who were postmenopausal. For females of reproductive age, 12 patients were in the luteal phase and one in the ovulatory phase at the time of examination. One of the perimenopausal patients was experiencing menses. From the participants without uterine malignancy (41), 25 (60.9%) had a history of uterine invasive operation due to benign causes, including cesarean section (14,34.1%), pregnancy termination (7, 17.1%), intrauterine device implantation (4, 9.8%), and myomectomy (1, 2.4%).
Patterns of uterine uptake of 68 Ga-FAPI-04 The 68 Ga-FAPI-04 and 18 F-FDG SUVmax of uterus and uterine volume among different groups are summarized in Table 2. Lower 68 Ga-FAPI-04 uptake was noted in postmenopausal women compared with reproductive age and perimenopausal females. Higher accumulation of FAPI in the surgical group was found than that in patients with no surgical history (P = 0.047). Of the patients with leiomyomas, two were perimenopausal and four were postmenopausal. In the perimenopausal and postmenopausal periods, the patients with leiomyomas showed higher 68 Ga-FAPI-04 uptake. The uterine fundus, body, and cervix had different uptake values (SUVmax 8.27 ± 3.71, 9.44 ± 4.59 and 6.32 ± 4.26, respectively, P < 0.001, measured on coronal PET images).
Imaging features of reproductive age, perimenopausal, and postmenopausal patients In Fig. 2, three patients in reproductive, perimenopausal, and menopausal periods showed varying degrees of FAPI uptake in the uterus. MRI and CT images showed the expected uterine volume decrease with increasing age. 68 Ga-FAPI-04 uptake was higher in patients of reproductive and perimenopausal stages than in postmenopausal patients ( Fig. 2a-l). The images showed that the smallest uterine size and oldest age correlated with the lowest FAPI uterine uptake values. On 18 F-FDG PET/CT scans, the uterus had low FDG uptake, with no signi cant differences among individuals ( Fig. 2m-x). In addition, mild uptake was noted in the ovaries in this set of patients (yellow arrows).

The uterus with broids or invasive operation
In Fig. 3a-d, when the uterus was involved by broids (red arrows), the uptake of 68 Ga-FAPI-04 increased, especially in the broid margin and myometrium. As shown in Fig. 3e-h, an intrauterine device (IUD, yellow arrows) was associated with intense uptake, implying that patients who had undergone invasive procedures may increase 68 Ga-FAPI-04 uptake around the site.
The uterus with primary or secondary malignancy In our clinical study, there were two patients with malignancy. The patient ( Fig. 4) with metastases to the uterus showed a lesion with relatively low 68 Ga-FAPI-04 uptake, which was opposite to the avid accumulation of 18 F-FDG. The case (Fig. 5) of endometrial adenocarcinoma had intense uptake of 68 Ga-FAPI-04.

Discussion
As a new and promising tumor imaging agent, 68 Ga-FAPI-04 has been gradually applied in diagnosing and staging of tumors [19]. However, avid 68 Ga-FAPI-04 uptake in the uterus of adult women may limit the detection of uterine lesions. Therefore, this study aimed to investigate the accumulation of 68 Ga-FAPI-04 in the uterus. We found that FAPI uptake is related to age and uterine volume. Prior surgery and the presence of broids may increase 68 Ga-FAPI-04 accumulation. FAPI uptake is lower in the normal postmenopausal uterus. Different uptake features were found in two uterine malignancies, one primary and one secondary. To the best of our knowledge, this is the rst detail analysis of the uterine uptake on 68 Ga-FAPI-04 PET/MRI.
In our study, most female patients showed varying degrees of FAPI uptake in the uterus. The adult uterus is a thick walled, hollow, muscular organ [20]. FAP provides required support for muscle ber maturation and speci cation [21][22][23]. Higher accumulations were noted in the uteri of patients of reproductive age and in the perimenopausal period compared with postmenopausal patients. The high uterine FAP activity may result from the tissue remodeling and angiogenesis during the menstrual cycle [23][24][25]. Low uptake was noted in the ovaries, possibly due to the hormone uctuation mainly acting on follicles rather than mesenchyme [26, 27], which also supports the idea that 68 Ga-FAPI-04 PET/MRI may be useful in the diagnosis of ovarian diseases. The two malignant cases showed completely different features. In the rst case, one of metastatic disease, the activity in the lesion was decreased compared with normal uterine tissue. For the other patient with primary endometrial cancer, intensely increased and diffuse uptake involved the entire uterus, which may be confused with physiological uptake. Although the 68 Ga-FAPI-04 PET/MRI displayed the tumor, it does not appear suitable for distinguishing uterine tumors. It appears that high SUVs do not improve the accuracy in assessing whether a tumor has extended or metastasized to the uterus.
Combined with 18 F-FDG and MR images, it may improve the diagnostic e cacy of the malignancy in the uterus.
There are several limitations in this study. First, the impact of the menstrual cycle on 68 Ga-FAPI-04 uptake is worth exploring, as most patients of reproductive age in this group were in the luteal phase. Therefore, more patients in different menstrual phases should be investigated. Second, sex hormone should be examined to provides evidence to support the relationship between sex hormone and FAPI uptake in uterus. Last, the number of patients with uterine malignancy was small, which limited the statistical analysis to distinguish between malignant and benign uptake.
In this preliminary study, our ndings suggest that 68 Ga-FAPI-04 PET may not be a suitable tool for the diagnosis of uterine disease because of its high physiological uptake by the uterus. Increased uterine uptake may be associated with age, uterine volume, surgical history, and the presence of broids.

Con ict of interest
The authors declare that they have no con ict of interest.

Ethical approval
All procedures involving human participants were carried out in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards.   Figure 1 The ow diagram of enrolled patients   (h-j, red arrows), which implied it to be a malignancy.

Figure 5
A postmenopausal female was examined for elevated tumor markers for 1 month. 68Ga-FAPI-04 and 18F-FDG PET both showed avid uptake in the uterus (a and k, SUVmax 13.4 and 21.7, respectively). The uterus was also noted to have slightly inhomogeneous T2 signal and restricted diffusion (b, h, and i) with a size of 4.8 cm × 6.6 cm × 7.0 cm. Hematoxylin-eosin (H&E) staining manifested it as endometrial adenocarcinoma (j). The red arrows point to the uterus.

Supplementary Files
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