The Added Value of ⁶⁸Ga-FAPI PET/CT in Patients with Head and Neck Cancer of Unknown Primary with ¹⁸F-FDG-Negative Findings.

Abstract

¹⁸F-FDG PET/CT plays an important role in locating the primary tumor for patients with head and neck cancer of unknown primary (HNCUP). Nevertheless, in some cases it can be challenging to locate the primary malignancy on ¹⁸F-FDG PET/CT scans. Because ⁶⁸Ga-radiolabeled fibroblast activation protein inhibitor (FAPI) PET/CT has promising results in detecting different tumor entities, our study aimed to evaluate the performance of ⁶⁸Ga-FAPI PET/CT for detecting the primary tumor in HNCUP patients with negative ¹⁸F-FDG findings. <b>Methods:</b> Eighteen patients (16 men and 2 women; median age, 55 y; age range, 24-72 y) with negative ¹⁸F-FDG findings were enrolled in this study. All patients underwent ¹⁸F-FDG and ⁶⁸Ga-FAPI PET/CT within 1 wk. Biopsy and histopathologic examinations were performed in the sites with positive ⁶⁸Ga-FAPI PET/CT findings. <b>Results:</b>⁶⁸Ga-FAPI PET/CT detected the primary tumor in 7 of 18 patients (38.89%). Among these 7 patients, primary tumor sites included the nasopharynx (<i>n</i> = 1), palatine tonsil (<i>n</i> = 2), submandibular gland (<i>n</i> = 2), and hypopharynx (<i>n</i> = 2). The primary tumors showed moderate to intensive uptake of ⁶⁸Ga-FAPI (mean SUV_max, 8.79; range, 2.60-16.50) and excellent tumor-to-contralateral normal-tissue ratio (mean SUV_max ratio, 4.50; range, 2.17-8.21). In lesion-based analysis, 65 lymph nodes and 17 bone metastatic lesions were identified. The mean SUV_max of lymph node metastases was 9.05 ± 5.29 for ¹⁸F-FDG and 9.08 ± 4.69 for ⁶⁸Ga-FAPI (<i>P</i> = 0.975); the mean SUV_max of bone metastases was 8.11 ± 3.00 for ¹⁸F-FDG and 6.96 ± 5.87 for ⁶⁸Ga-FAPI (<i>P</i> = 0.478). The mean tumor-to-background ratios of lymph node and bone metastases were 10.65 ± 6.59 versus 12.80 ± 8.11 (<i>P</i> = 0.100) and 9.08 ± 3.35 versus 9.14 ± 8.40 (<i>P</i> = 0.976), respectively. <b>Conclusion:</b> We present the first evidence, to our knowledge, of a diagnostic role of ⁶⁸Ga-FAPI PET/CT in HNCUP. Our study demonstrated that ⁶⁸Ga-FAPI PET/CT has the potential to improve the detection rate of primary tumor in HNCUP patients with negative ¹⁸F-FDG findings. Moreover, ⁶⁸Ga-FAPI had a performance in assessing metastases similar to that of ¹⁸F-FDG.