The Role of Delayed 18F-FDG PET Imaging in the Follow-up of Patients with Alveolar Echinococcosis

F-18-FDG PET has already proved its usefulness in the follow-up of patients with alveolar echinococcosis (AE) and has been proposed as a surrogate marker for therapeutic decisions on structured treatment interruption by benzimidazoles. However, standard PET acquisition (1 h after F-18-FDG injection) lacks sensitivity, and the parasite may stay viable even if F-18-FDG perilesional uptake has disappeared. The aim of our study was to evaluate the usefulness of delayed F-18-FDG PET in the management of AE patients. Methods: During a 6-y period, 120 PET scans using F-18-FDG were obtained for 70 AE patients treated by benzimidazoles, without selection. All patients underwent whole-body imaging on a PET/CT device 1 h after F-18-FDG injection (4 MBq/kg), as well as an acquisition focused on the liver 3 h after the injection. We also analyzed the results of serologic tests. Results: Of the 57 scans considered negative at the standard acquisition, 13 (22.8%) became clearly positive at the delayed acquisition, and 6 (10.5%) became indeterminate at the delayed acquisition. Furthermore, 20 of 22 scans interpreted as indeterminate at the standard acquisition were considered positive because of clear perilesional F-18-FDG uptake at the delayed acquisition. Thus, delayed acquisition changed the interpretation in 32.5% of cases. Moreover, of 44 patients treated by benzimidazoles and followed for more than 2 y by regular F-18-FDG PET scans and specific AE serology, 11 (25%) presented pathologic F-18-FDG uptake at the delayed acquisition but not at the standard one. In these patients, the treatment was continued despite negative results on standard F-18-FDG PET and negative serologic findings. On the other hand, in 7 patients with negative delayed F-18-FDG PET and negative serology, the treatment was safely interrupted with no evidence of disease recurrence during 8-37 mo (mean, 23 mo). Conclusion: Our study clearly demonstrated that delayed F-18-FDG PET greatly facilitated the differentiation between active and inactive liver lesions in AE patients. Also, our results strongly suggested that the combination of delayed F-18-FDG PET and specific serology would prevent most of the recurrences observed after premature interruption of the treatment based only on standard F-18-FDG PET.