Study design and patients
This is a single-center retrospective cohort study of patients with biochemically proven pHPT who underwent preoperative localization of the suspected parathyroid adenoma using 11C-choline PET/CT in a tertiary referral hospital.
The medical charts of all patients who underwent 11C-choline PET/CT between April 2015 and February 2017 were reviewed. Patients eligible for inclusion were those ≥ 18 years, diagnosed with biochemically confirmed pHPT, and who underwent 11C-choline PET/CT for the localization of the suspected adenoma. In total, 23 patients underwent 11C-choline PET/CT for the localization of an adenoma. All patients had biochemically confirmed pHPT (calcium and PTH values above the upper limit of normal). However, one patient was known to have multiple endocrine neoplasia type I, and one patient was diagnosed with familial hypocalciuric hypercalcemia (FHH); these two patients were excluded from this analysis. Of the analyzed patients that were operated (67%), the 11C-choline PET/CT correctly identified the location of the adenoma in all cases. Of the analyzed patients that were not operated (33%), the 11C-choline PET/CT was positive in all but one patient. This patient was only included in the inter-observer analysis of this study and not in the analysis of the optimal uptake time of the tracer and the radioactivity to be administered (data not shown).
The medical charts were reviewed to determine the injected activity of 11C–choline in MBq, gender, age, length, weight, preoperative PTH, and corrected calcium (for calculation of corrected calcium refer to Additional file 1).
Data obtained from patient records were anonymously stored using study-specific patient codes in a password-protected database. The study was exempt for collection of informed consent after reviewing by the Medical Ethics Committee Groningen (registration number 2016/413).
11C-choline was produced on site as described in . Further details on production, patient preparation, and PET/CT acquisition can be found in Additional file 2. In the first nine patients, PET/CT images were taken directly after injection of 7.0 ± 0.5 MBq/kg [range 6.1–7.4 MBq/kg] 11C-choline for up to 40 to 60 min postinjection (p.i.). After an interim analysis of the first nine patients had been performed to determine the most suitable uptake time of the tracer to reduce overall scan duration, all subsequent patients were scanned dynamically 20 min after the injection of mean 5.9 ± 1.4 MBq/kg [range 4.2–8.5 MBq/kg] for a duration of 10 min.
11C-Choline PET/CT analysis
Of the first nine patients, the images were reconstructed in time frames of 10 min (0–10 to 50–60-min p.i.) to determine the optimal uptake time of the tracer. Also, these images were reconstructed in scan durations of 5 min (20–25 and 25–30 min), 2.5 min (20–22.5 to 27.5–30 min), and 1 min (20–21 to 29–30 min) to assess image quality as a function of scan duration used as a surrogate for variation in administered radioactivity.
For each of these newly created images, accumulated activity of 11C-choline in the adenoma was (semi) quantitatively evaluated by placing an automatic volume of interest (VOI) with a threshold of 50% of the maximum tracer uptake in the lesion, using an in-house software (ACCURATE) . In the surrounding normal background tissues (descending aorta, thyroid, shoulder muscle, and the first thoracic vertebrae (T1)), a spherical or cubical VOI, with a fixed size, was positioned. In the thyroid gland, the cubical VOI was positioned in an area on the contralateral side of the adenoma in healthy appearing thyroid tissue. VOI’s were copied and transferred to scans of all time frames through an automatic linking feature and manual correction if needed. In each of these newly created images, peak, mean, and maximum standardized uptake values corrected for body weight (SUVpeak, SUVmean, and SUVmax) were obtained for adenoma and background organs: descending aorta, thyroid, shoulder muscle, and T1. SUVmax represents the maximum tracer uptake seen across all voxels within the volume of interest. SUVpeak represents the average uptake in a 1 mL spherical volume of interest positioned such to yield the highest value across all possible locations of the lesion (or organ volume of interest). SUVmean is the mean SUV within the volume of interest. SUVmean was used for analysis of the background tissues, because it provides the most accurate and precise SUV in case of regions with an almost uniform uptake, and SUVpeak was used for the adenoma being the preferred SUV metric to quantify tracer uptake in (small) lesions besides SUVmax . Moreover, SUVmax was used for all as this metric is and has been used frequently and is therefore included to allow comparison with other studies.
All patients were included in the inter-observer agreement analysis. The acquired image data at 20-min p.i. was post-processed with varying scan duration of 1, 2.5, 5, and 10 min, respectively.
From all patients, two scans with a scan duration of 1 min and 2.5 min, both scans with a duration of 5 min, and the scan with a duration of 10 min were randomly selected.
Four observers visually interpreted the scans in four rounds, with increasing scan duration per round. Observers were asked to identify and localize any abnormally increased 11C-choline uptake and to localize it in the right upper, left upper, right lower, left lower, or ectopic zones. Next, in case of a focal increased uptake, per location, they had to score how certain they were that the uptake was indeed increased (certainty of increased uptake (CIU)) and how certain they were that the uptake could be attributed to an adenoma (certainty of adenoma (CA)). They completed a standard scoring form per patient, per round including a 5-point scale to score their CIU and CA, with “1” being totally unsure and “5” being totally sure. Further details of the inter-observer analysis and assessment can be found in Additional file 3.
Patient characteristics are described using means and standard deviations (SD) or medians and ranges for continuous variables (depending on normal distribution). All statistical analyses were performed with IBM SPSS Statistics for Windows, Version 23.0 (Armonk, NY: IBM Corp.). P values of < 0.05 were considered statistically significant. For multiple testing, the Bonferroni correction was applied. Graphics were generated using GraphPad Prism, Version 7.02 (La Jolla, CA, USA).
Optimal uptake time of tracer
Of the first nine patients, scanned for up to 40 to 60 min, contrast ratios (adenoma to muscle, T1, aorta, and thyroid tissue) were calculated to determine the optimal uptake time of the tracer. Contrast ratios were calculated by dividing the SUVpeak for the adenoma by the SUVmean for the background. The ratios in every time frame were compared with the ratios in the upcoming time frame using the Wilcoxon signed-rank test as normality of the data could not be shown.
The SUVmean for thyroid and SUVpeak for adenoma (of the first nine patients) in every uptake time were compared with the same SUVs in the upcoming uptake time using the Wilcoxon signed-rank test as normality of the data could not be shown. Using the Bonferroni correction, p values < 0.01 were considered statistically significant.
Radioactivity to be administered
In the analysis of the radioactivity to be administered, only the first eight patients (scanned for up to 40 to 60 min) were included, since no post-processed images could be made in scan durations of 1, 2.5, and 5 min for one patient, because the original raw data was no longer available. The post-processed images with a scan duration of 10 min was, however, still available.
To assess the effect of injected radioactivity on SUVmax, SUVmean, and SUVpeak for adenoma and background tissues, the SUVs in the shorter scan durations (1, 2.5, and 5 min) were compared to the 10-min scan duration (at the determined optimal uptake time) to determine whether statistical differences were observed, using the Wilcoxon signed-rank test as normality of the data could not be shown. For these analyses, the average SUVs of patients per scan duration were calculated, since the 1-, 2.5-, and 5-min scan durations had multiple measurements of SUV (10, 4, and 2, respectively) and the 10-min scan duration only had 1 measurement. In this way, we could assess if using different scan durations would result in different SUV data, on average, i.e., if it would result in a systematic bias. Using the Bonferroni correction, p values < 0.017 were considered statistically significant.
Inter-observer agreement was calculated per possible location (right upper, left upper, right lower, left lower, ectopic) by comparing the results of the location of the adenoma using the Fleiss’ kappa. Interpretations of the kappa values were as follows: < 0.00 poor, 0.00–0.20 slight, 0.21–0.40 fair, 0.41–0.60 moderate, 0.61–0.80 good, and > 0.81 almost perfect agreement .