Clinical implications and diagnostic utility of ⁶⁸ Gallium FAPI PET CT in patients with testicular germ cell tumors: The prospective phase Ib FAPI-TEST study.

5127 Background: The fibroblast activation protein (FAP) is overexpressed in tumor tissue by cancer associated fibroblasts and radiopharmaceuticals can bind to this target. Recently, the 68 Ga-FAPI PET has emerged as a novel imaging modality for optimized differentiation between benign and malignant lesions. However, data on patients with testicular germ cell tumors (TGCT) are rare. This prospective study aims to investigate the expression, uptake and sensitivity of the 68 Ga-FAPI PET in TGCT patients. Methods: In this prospective observational phase Ib study, patients with TGCT up to clinical stage III and unfavorable TGCT histology in the testis such as teratomas were included. A 68 Ga-FAPI PET was performed in all included patients and analyzed with static (SUVmax/mean). Histology from the testis and whenever available from the metastasis were also stained for FAP and manually categorized into negative, medium and high expression. This study was approved by the local ethics committee (no 2024-17844). Results: We included 12 patients with a mean age of 34.5 (SD 8.81). Clinical stage at time of inclusion were CS II (n = 9), CS1 (n = 1) and CS III (n = 1) as well as one extragonadal TGCT. In eight and four cases the 68 Ga-FAPI PET was performed before and after completion of curative chemotherapy, respectively. Histology was available in six cases (5 = LNs, 1 = mediastinal tumor). FAP expression in the tumor of the testis was high in 50%, and in metastasis 60%. Stratification for high and low FAP lesions was performed according to mean SUVmax of 7.81 (SD 7,9) for all radiographic and histopathologic true positive metastasis. Sensitivity for 68 Ga-FAPI PET was 57% with a specificity 100%. Conclusions: This is the first prospective study that evaluates 68 Ga-FAPI PET in TGCT patients with the largest cohort so far. Stratification for SUVmax lesions above 7.81 showed excellent specificity, and the 68Ga-FAPI PET might potentially serve as a useful novel imaging modality for primary staging in clinical stages IIA and IIB, as well as post-chemotherapy with residual mass.