Study design and data collection
This retrospective study included consecutive patients referred to PET/CT for staging of alveolar echinococcosis between the years 2005 and 2019 at the University Hospital of Zurich. Imaging data of MRI and contrast-enhanced CT was included in the analysis if performed within a time frame of three months before or after the PET/CT.
Clinical follow-up was performed in all patients by reviewing electronic patient charts. We collected all relevant clinical data (such as patient demographics, laboratory data, and clinical and treatment information) at the time of initial diagnosis, at discontinuation of benzimidazole therapy, and at the last recorded clinical visit (last follow-up December 2020).
The local ethics committee, namely the Kantonale Ethikkomission Zürich, approved the study protocol (BASEC-Nr. 2018 − 01855) and all patients examined between 2016 and 2019 signed written informed consent to the retrospective use of their clinical data for research. For patients scanned between the years 2005 and 2015, informed consent was waived, due to retrospective inclusion by the local ethics committee, namely the Kantonale Ethikkomission Zürich (study protocol BASEC-Nr. 2018 − 01855). All procedures were performed in accordance with the 1964 Helsinki declaration and its later amendments or comparable ethical standards.
Imaging data acquisition
All imaging examinations followed basic study protocols. For PET/CT, patients fasted for at least four hours, FDG dosage was body-weight adjusted, the uptake time was standardized to 60 minutes in supine position, a non-enhanced CT scan was performed and used for attenuation correction, and data was acquired with arms overhead whenever possible. Body weight, height, and blood glucose level were measured prior to imaging, and blood glucose levels < 12 mmol/l were accepted . Five different types of PET/CT scanners were used throughout the study period, i.e. Discovery STE, Discovery LS, Discovery RX, Discovery MI, and Discovery 690 (all GE Healthcare, Waukesha, WI). To compensate for differences in the sensitivity of the different PET/CT scanner generations, we measured the metabolic activity in normal/non-infected liver tissue and in the mediastinal blood pool for reference.
Contrast-enhanced CT was performed of the abdomen or chest and abdomen after intravenous injection of 80 ml iodinated contrast material (Visipaque® 320, GE Healthcare), timed for imaging at the portal venous phase with a tube voltage of 120 kV and a tube current–time product of 100–320 mAs.
MRI examinations were performed following a standard liver MRI protocol, which included at least T2-weighted sequences with and without fat saturation in axial and/or coronal plane and T1-weighted sequences with fat saturation before and after i.v. contrast administration in arterial (typically 30 s), portal venous (typically 60–90 s) and delayed phases (typically 120–240 s). Extracellular contrast agents were used from different vendors. Imaging was either performed at 1.5 or 3.0 T (Aera, Avanto or Skyra, Siemens Healthineers, Siemens, Erlangen, Germany; GE Signa HDxt or GE MR750w, GE Healthcare, Waukesha, WI; Ingenia or Achieva, Philips Healthcare, Best, the Netherlands) using dedicated phased array channel coils.
Image analysis and definitions
All PET/CT and contrast-enhanced CT data sets were retrospectively reanalysed in consensus by two experienced and doubly board certified nuclear medicine physicians and radiologist on a dedicated workstation (Advantage Workstation, Version 4.6; GE Healthcare Biosciences, Pittsburgh, PA). All MRI data were reanalysed by a radiologist and a doubly board certified nuclear medicine physician and radiologist in consensus. Readers were blinded to all clinical patient outcome data, and collected the following data:
For PET/CT, contrast-enhanced CT, and MRI, readers quantified the number of detectable hepatic and extrahepatic lesions, and the size of the largest lesion. With PET/CT and contrast-enhanced CT, the extent of disease was staged (i.e. PNM stage (P = parasitic mass in the liver; N = involvement of neighboring organs; M = metastasis) as defined by the WHO Informal Working Group on Echinococcosis [2, 13–15], the CT findings were classified according to the EMUC-CT classification [16, 17], and the presence and pattern of lesion calcifications was determined.
MRI findings were classified, according to the recommendations by Kodoma et al.  (i.e. type 1: multiple small cysts without a solid component; type 2: multiple small cysts with a solid component; type 3: a solid component surrounding a large and/or irregular cysts with multiple small cysts; type 4: a solid component without cysts; type 5: a large cyst without a solid component).
For contrast-enhanced CT and MRI, the presence of contrast enhancement was rated, using a four-point score (score 1: no enhancement; score 2: no clear enhancement; score 3; suspicion of enhancement; score 4: clear contrast enhancement).
Furthermore, quantitative imaging parameters were measured in PET/CT, i. e. maximum and peak standardized uptake value (SUVmax and SUVpeak) of FDG, in the largest and/or most FDG-avid manifestation as well as in non-infected liver tissue (the latter for reference). SUVmax is defined as the maximum itensity voxel, SUVpeak as the average activity concentration within a 1 cm3 spherical volume of interest centered on the hottest voxel, both within a defined subspace of the PET image matrix.
Finally, serum samples were tested at the Institute of Parasitology, University of Zurich. EMII/3–10 or its derivative EM-18 were used, which are encoded by part of the EM-10 gene sequence, and the EM-VF Western blot for serological confirmation of alveolar echinococcosis .
Clinical follow-up of all patients was performed by reviewing electronic patient charts. Patient data were recorded at the time of staging and at the last recorded clinical visit (latest retrospective follow-up in December 2020). Patient demographics, laboratory data, and clinical information were assessed for all patients, including data on patient survival, duration of benzimidazole therapy and time to reach no detectable levels of Em-18 antibodies.
Statistical analyses were performed using commercially available software (Stata/SE, Version 16.1, StataCorp, College Station, TX). Variables were expressed as median and IQR (25th, 75th percentiles) or percentages. We used tests for trend of survivor functions to assess the association of tertiles of SUVmax, SUVpeak, and SUVratio, the five categories of the KODOMA score, and the four categories of the EMUC CT classification with the duration of benzimidazole therapy and the duration to reach no detectable levels of Em-18. We used the log-rank test to assess the association of curative versus non-curative surgery with the duration of benzimidazole therapy and the duration to reach no detectable levels of Em-18. Kaplan-Meier estimates were used to describe survival from date of diagnosis to last clinical follow-up or death at 1 and 5 years. A p-value of ≤ 0.05 was considered to indicate statistical significance.