The institutional review board approved this retrospective study. The need for informed consent was waived due to the retrospective nature of this analysis and data anonymity. The clinical records were reviewed to identify patients for analysis.

The inclusion criterion was the patients who underwent [123I]-MIBG planar and SPECT/CT scan due to suspect of PHEO or PGL from April 2018 to March 2023. The exclusion criteria were as follows: (1) patients without pathologically diagnosed PHEO/PGL based on the assessment of surgical specimens, (2) patients with surgical specimens that were not graded using the PASS and GAPP, and (3) patients with head and neck PGLs because either PASS or GAPP scoring system was not applied.

From April 2018 to March 2023, [123I]-MIBG planar and SPECT/CT scan were performed on 58 consecutive patients with suspected PHEO or PGL. The flowchart of the study patient selection steps is shown in Fig. 1. Among them, 35 without PHEO/PGL were excluded from the analysis; 1) Six patients did not present with intra- or extra-adrenal tumors, and no abnormal [123I]-MIBG uptake was observed in all of them. 2) Twenty-nine had adrenal tumors other than PHEO/PGL (18 cortical adenomas; 4 metastatic adrenal tumors [one hepatocellular carcinoma, one melanoma, one renal cell carcinoma, and one thyroid cancer]; 2 myelolipomas, one adrenal carcinoma, one hemangioma, one liposarcoma, one malignant lymphoma, and one ganglioneuroma), and no [123I]-MIBG uptake was observed in all of them. Two patients with [123I]-MIBG avid adrenal tumors who did not undergo pathological examination were excluded. One patient with head and neck PGL was excluded because neither the PASS nor GAPP scoring system was applied in the assessment.

Flowchart of the study patient selection steps

Finally, 20 (13 men, 7 women; mean [± SD] age: 54 ± 17 [range: 23–76] years) patients were eligible for the analyses. The patient’s medication status was also evaluated because some drugs, such as antihypertensive calcium-channel blockers, can reduce [123I]-MIBG uptake [15,16,17]. Plasma catecholamine concentrations were measured before surgery and within 3 months after [123I]-MIBG imaging (mean ± standard deviation [SD]: 22 ± 20 days; range: − 62 to + 66 days).

We previously examined whether the SUVmax of either myocardial or adrenal [123I]-MIBG can characterize myocardial function in patients with PHEO [18]. However, analyses of the correlation between [123I]-MIBG adrenal uptake and pathological grading in PHEOs or PGLs were not performed in the previous study. Thus, the purpose of this current study is different from the previous study [18], and the current study enrolled 18 former examined patients with PHEO who underwent 18 [123I]-MIBG planar and SPET/CT scans between April 2018 and August 2021.

The planar images of the anterior and posterior views and chest to abdomen (n = 18) or abdomen to pelvis (n = 2) SPECT/CT scan images were collected at 1 day after the intravenous injection of [123I]-MIBG at a dose of 111 MBq (3 mCi) (PDR Pharma, Japan) using a dual-head gamma camera with medium-energy parallel-hole collimators and a multidetector (16-row) spiral CT scan (Siemens Intevo SPECT/CT system; Siemens Medical Solutions USA Inc.). Planar images were acquired with a 256 × 1024 matrix, and the photopeak energy window was 159 (± 15%) keV for [123I]. SPECT/CT scan images were acquired with a matrix size of 128 × 128. A total image with 30 frames was acquired with an acquisition time of 30 s/frame and an angular step of 6°. After SPECT acquisition, CT scan images were acquired using a tube voltage of 130 kV and a dose-modulation algorithm with a quality reference mAs setting of 15 (CAREDose 4D; Siemens Medical Solutions USA Inc.). SPECT data were reconstructed with attenuation and scatter correction using a three-dimensional iterative algorithm (Ordered Subset Conjugate-Gradient Minimizer; Siemens Medical Solutions USA Inc.).

The [123I]-MIBG planar, SPECT, and SPECT/CT scan images were displayed on a workstation (Syngo.via; Siemens Healthcare GmbH, Erlangen, Germany; Advantage Windows Workstation 4.5; GE Healthcare, Milwaukee, WI) and were reviewed by two radiologists who were knowledgeable about the study’s purpose but who were blinded to the patients’ clinical information. The readers independently assessed the tumor [123I]-MIBG uptake on the planar images using the following score scales: uptake score of 0, no uptake; uptake score of 1, faint uptake that is less than that in the liver; uptake score of 2, uptake equal to that in the liver; and uptake score of 3, uptake greater than that in the liver [19]. These independent uptake scores were used to examine interobserver variability. Disagreements were resolved via a consensus decision between the two readers. The consensus uptake scores were used for the quantitative analyses. To determine the presence or absence of [123I]-MIBG uptake in the PHEO/PGL, scores of 0–1 and 2–3 were nonvisible and visible, respectively.

The following semi-quantitative analyses according to the interpreted results of the visual assessment were performed using the dedicated software (Syngo.via; Siemens Healthcare GmbH, Erlangen, Germany) by the abovementioned two readers independently. A previous study has shown the phantom experimental method used to calculate cross-calibration factor for converting SPECT count images to SPECT SUV images [18]. The SUV-related parameters of PHEO/PLG were generated as follows: First, the volume of interest (VOI) was drawn manually around the PHEO/PLG in a suitable reference transaxial plane, excluding the adjacent avid non-PHEO/non-PGL structures using the CT scan images as reference. We defined the SUVmax as the maximum tissue concentration in the structure delineated by the VOI divided by the activity injected per gram of body weight. Next, a threshold of 40% SUVmax was set to automatically delineate the VOI that met or exceeded this threshold. This VOI was used to calculate the mean SUV (SUVmean), tumor volume of [123I]-MIBG uptake (TV_MIBG), and total lesion [123I]-MIBG uptake (TL_MIBG). The TL_MIBG was calculated as the SUVmean multiplied by the TV_MIBG.

The averaged values of these SUV-related parameters obtained by the two readers were used for quantitative analyses.

PHEO or PGL was confirmed via pathological examination in all cases. Hematoxylin and eosin staining and Ki-67 (DAKO, M7240) immunohistochemistry (IHC) staining were performed on a single representative block for each tumor. For the Ki-67 labeling index (%), two of the most highly labeled areas (hot fields, magnification of × 200) were counted by two pathologists.

An experienced pathologist who was blinded to the patients’ clinical information assessed PASS based on the published criteria and the histopathologic criteria for the GAPP score including Ki-67 staining (Supplemental Table 1). The PASS score incorporates 12 histologic parameters (large cell nests/diffuse growth, central necrosis, high cellularity, cellular monotony, tumor cell spindling, high mitotic index [> 3/10 high power fields], atypical mitosis, extension into adipose tissue, vascular invasion, capsular invasion, profound nuclear pleomorphism, and nuclear hyperchromasia) [2]. The PASS scores (range: 1–20) were categorized as < 4 and ≥ 4. PHEO or PGL with a PASS score of ≥ 4 was considered as biologically aggressive [2, 4]. Meanwhile, those with a PASS score of < 4 had a non-metastatic potential [2, 4]. The GAPP grading system was presented by 6 parameters which including 4 histological features (histological pattern, cellularity, coagulation necrosis, capsular/vascular invasion), immunohistochemical (Ki67 labelling index) and biochemical features (catecholamine type) [3]. The final GAPP score was established by incorporating data on dominant tumors secreting catecholamine. Tumors with significantly high plasma or urinary epinephrine levels with or without elevated norepinephrine levels were classified as epinephrine-secreting tumors. These tumors were assigned with a score of 0. Tumors with high norepinephrine levels without elevated epinephrine levels, with or without elevated dopamine levels were classified as norepinephrine-secreting tumors. These tumors were assigned with a score of 1. The nonfunctioning type was assigned with a score of 0. Based on the published cutoff points for risk stratification, the GAPP score was categorized as follows: 0–2, well-differentiated; 3–6, moderately differentiated; and 7–10, poorly differentiated [3]. In our institution, routine genetic testing such as assessment of succinate dehydrogenase subunit B (SDHB) mutation for PHEOs/PGLs was not performed.

The interobserver agreement of the uptake scoring was evaluated using κ statistics analysis. κ was interpreted as follows: < 0.20, slight agreement; 0.21–0.40, fair agreement; 0.41–0.60, moderate agreement; 0.61–0.80, substantial agreement; and ≥ 0.81, almost perfect agreement [20]. The Mann–Whitney U test or the Fisher’s exact test was used to assess the difference between two quantitative variables or to compare categorical data. The Spearman’s rank correlation was used to assess the association between two quantitative variables.

Data were expressed as median, interquartile range (IQR), and/or range. A p value of < 0.05 was considered statistically significant, and all p-values were two-sided. Statistical analyses were performed using MedCalc (MedCalc Software Ltd., Mariakerke, Belgium).