[68Ga]Ga-FAPI-04 PET/CT in a patient with endometriosis: a potential game changer?

Abstract

A 43-year-old woman with a known history of endometriosis presented with dysmenorrhea, dyschezia, and hematochezia. She had previously undergone a hysterectomy, during which infiltrative endometriosis in the bowel was identified. To guide further treatment decisions, a magnet resonance imaging (MRI) was performed, confirming bowel involvement and revealing a circumscribed lesion infiltrating the sigmoid colon. Due to persistent symptoms, the patient opted for surgery. For preoperative planning, a fibroblast activation protein inhibitor (FAPI) positron emission tomography/ computed tomography with [68Ga]Ga-FAPI-04 was conducted. PET images, acquired 10 min after administration 148 MBq of [68Ga]Ga-FAPI-04, showed intense tracer uptake (Fig. A; left column: maximum intensity projection; right column: axial slices of PET/CT fusion and PET; big arrow head: endometriosis in the sigmoid colon, SUVmax 9.8) at the same location as the MRI finding (Fig. B, axial and sagittal slices of contrast enhanced T1-weighted fat saturated sequence), thus confirming the MRI result. Additionally, the PET scan revealed intense uptake in the left ovary and the ligamentum teres uteri bilaterally (Fig. A, small arrowheads; SUVmax: 11.3, 7.3 and 5.4, respectively) suggesting the presence of additional endometriotic lesions undetected by MRI. The patient underwent a second surgical procedure, which histopathologically confirmed a deep infiltrating endometriosis extragenitalis.

Endometriosis, characterized by the ectopic growth of endometrial-like tissue, is a common gynecological disease affecting approximately 10% of reproductive-age women [1]. It can cause significant symptoms, including dysmenorrhea, dyspareunia, infertility, and gastrointestinal disturbances [2]. In addition to its physical effects, endometriosis can also affect mental health [3]. Thus, endometriosis significantly impact the quality of life of patients [4]. Diagnosis is primarily symptom-based, which can be challenging due to their variety and the heterogenic phenotype of endometriosis overlapping with other diseases. A definitive diagnosis is mostly confirmed through diagnostic laparoscopy. While transvaginal ultrasound is the first-line imaging modality, MRI is often used as a second-line tool for assessing disease extent and aiding in surgical planning [5].

This case highlights the potential role of 68Ga-FAPI PET/CT in detecting endometriotic lesions. Not only did the scan confirm the known lesion, but also identified additional sites of involvement undetected by MRI. Many studies have demonstrated the biodistribution of the 68Ga-labeled FAPI tracer, often showing intense uptake in the uterus [6,7,8]. There has been a previous case report showing only faint uptake in one endometriotic lesion [9]. To the best of our knowledge, this is the first confirmed case of endometriosis demonstrated on 68Ga-FAPI PET/CT with clear and intensive uptake. Therefore, 68Ga-FAPI PET/CT could represent a promising diagnostic modality for endometriosis, alongside many malignant diseases in which it has proven effective [10,11,12,13]. Our observation is supported by a recent preclinical study that found FAP expression in the immune microenvironment of endometriosis from various locations [14].

This case may provide a rationale for future studies systematically analyzing FAP expression in endometriosis across larger patient cohorts, investigating the value of 68Ga-FAPI PET/CT. A prospective study was recently initiated and is currently underway investigating this issue (NCT06792318).