Use of 18F-FDG PET/CT to Differentiate Ectopic Adrenocorticotropic Hormone-Secreting Lung Tumors From Tumor-Like Pulmonary Infections in Patients With Ectopic Cushing Syndrome

Background Ectopic adrenocorticotropic hormone (ACTH)-secreting lung tumors represent the most common cause of ectopic Cushing syndrome (ECS). Pulmonary opportunistic infections are associated with ECS. The present study aimed to evaluate the usefulness of 18F-FDG PET/CT for differentiating ectopic ACTH-secreting lung tumors from tumor-like pulmonary infections in patients with ECS. Methods We retrospectively reviewed the imaging data of 24 patients with ECS who were suspected to have ACTH-secreting lung tumors and underwent 18F-FDG PET/CT between 2008 and 2019. Eleven patients with lung tumors and 4 with pulmonary infections also had additional somatostatin receptor imaging (99mTc-HYNIC-TOC SPECT/CT or 68Ga-DOTATATE PET/CT). Results In total, 18 patients had lung tumors and six had pulmonary infections. The primary source of ECS remained occult in the six patients with pulmonary infections. The maximum standardized uptake value (SUVmax) for pulmonary infections was significantly higher than that for tumors (P = 0.008). Receiver operating characteristic analysis revealed that a cut-off SUVmax of 4.95 helped in differentiating ACTH-secreting lung tumors from infections with 75% sensitivity and 94.4% specificity. For the 11 patients with ACTH-lung tumors, somatostatin receptor imaging (SRI) was positive in 6; while for the 4 with pulmonary infections, SRI was positive in 2. The sensitivity and specificity of somatostatin receptor imaging (SRI) for detecting ACTH-secreting lung tumor was 54.5% and 50%. Conclusions Our findings suggest that pulmonary infections exhibit significantly higher FDG uptake than ACTH-secreting lung tumors in 18F-FDG PET/CT. An SUVmax cut-off value of 4.95 may be useful for differentiating the two conditions. Our results also suggested that SRI may not be an effective tool for differentiating the two conditions given the relatively low specificity.


INTRODUCTION
10%-15% of Cushing syndrome is caused by ectopic adrenocorticotropic hormone (ACTH)-secreting tumors. In such cases, resection of the tumors can have curative effects. The most common tumors associated with ECS are pulmonary carcinoids and small cell lung carcinoma (SCLC), followed by thymic carcinoids, pancreatic neuroendocrine tumors, medullary thyroid carcinoma, and pheochromocytoma (1). 18 Ffluorodeoxyglucose (FDG) positron emission tomography (PET)/computed tomography (CT; 18 F-FDG PET/CT) has been shown to be an effective modality for localizing ectopic ACTH-secreting tumors causing ECS. Pulmonary carcinoids generally demonstrate low to moderate metabolic activity because of their low proliferation rate and slow growth. Meanwhile, ACTH-producing SCLC can show positive findings on 18 F-FDG PET/CT, although the reported number of ECS-causing SCLCs detected by 18 F-FDG PET/CT is quite small. This is probably because the patients are rapidly diagnosed by conventional cross-sectional imaging and do not undergo further 18 F-FDG PET/CT (2).
ECS due to ectopic ACTH-secreting tumors is associated with markedly elevated ACTH levels. This results in high circulating glucocorticoid levels, which could affect cell-mediated immunity (3) and impair immune function by inhibiting the phagocytic function of alveolar macrophages and reducing neutrophil recruitment to the infected areas. This results in an increased incidence of opportunistic bacterial and fungal infections (4,5). The four most common infections associated with ECS are cryptococcosis, aspergillosis, nocardiosis, and pneumocystosis (6), with the lung being the most frequently involved site. Pulmonary infections can exhibit varied radiographic findings and may appear as nodules or masses simulating lung tumors (7). Thus, it could be difficult for conventional anatomic imaging to differentiate tumor-like pulmonary infections from lung tumors. FDG is a nonspecific tracer that accumulates in areas of infection. Pulmonary cryptococcosis, aspergillosis, nocardiosis, and pneumocystosis have been reported to show high metabolic activity and mimic lung malignancies on 1 8 F-FDG PET/CT (8)(9)(10)(11). In the clinical setting of immunosuppression resulting from ECS, surgery for the removal of pulmonary infectious lesions can deteriorate the patient's condition. Therefore, discrimination of infections and tumors is crucial for avoiding unnecessary surgical intervention. The primary goal of this retrospective study was to evaluate the usefulness of 18 F-FDG PET/CT for differentiating ectopic ACTH-secreting lung tumors from tumor-like pulmonary infections in patients with ECS. In addition, as somatostatin receptor-based imaging (SRI) has been increasingly used for the detection of occult ECS tumors, we also reviewed the SRI imaging findings in 15 patients and investigated whether the SRI findings were able to differentiate the two conditions.

Patients
We retrospectively reviewed 18 F-FDG PET/CT scans obtained for localizing the source of ectopic ACTH secretion in all patients with ECS in our department between 2008 and 2019. The inclusion criteria were as follows: (a) confirmed diagnosis of ACTH-dependant Cushing syndrome; (b) negative finding on pituitary MRI; (c) lung nodules suspected on chest CT images. Exclusion criteria were as follows: (a) confirmed tumoral source other than in the lung (i.e. thymus, gastrointestinal tract and pancreatic neuroendocrine tumor), (b) unavailable histopathological result of suspected lung lesion. Eventually, 24 patients with suspicious ACTH-secreting lung tumors were included in the present study. The diagnosis of ECS was confirmed by clinical presentations combined with laboratory tests including low-dose dexamethasone suppression test (LDDST), high-dose dexamethasone suppression test (HDDST), CRH test, inferior petrosal sinus sampling (IPSS). The head MRI results of all patients suggested that the pituitary gland was normal. Pulmonary CT indicates pulmonary nodules, but the nature is unclear. Fifteen patients also underwent additional 99m Tc-HYNIC SPECT/CT (n=6) or 6 8 Ga-DOTATATE PET/CT (n=9). The reference standard was histopathological diagnosis obtained by either lung surgery or biopsy. There were 11 female and 13 male patients aged 9 to 72 years (mean age, 37.8 ± 17.1 years). This retrospective study of existing patient data and images was approved by the institutional review board of Peking Union Medical College Hospital. The requirement for informed consent was waived. 18

F-FDG PET/CT Study
Following 8 h of fasting and confirming the blood glucose level to be less than 120 mg/dL, 18 F-FDG (5.5 MBq/kg) was intravenously injected. An hour later, PET/CT images were acquired from the mid-thigh to the skull base (2 min/bed position) using a combined PET/CT Biograph (Siemens Co.). All scans were obtained in a three-dimensional model. Tc-HYNIC-TOC Scintigraphy 99m Tc-HYNIC-TOC was synthesized and labeled as previously described (12). After intravenous administration of the tracer, whole-body planar images were acquired using a double-head gamma camera at 1 and 4 hours after injection. Some patients also underwent pulmonary SPECT/CT imaging when there is an increased uptake in the chest.

68
Ga-DOTA-TATE PET/CT Study The 68 Ga-DOTATATE was produced following our previously published procedure (13). The study was carried out on a PET/ CT scanner (Siemens Co.). Patients received an intravenous injection of 68 Ga-DOTATATE (111-148 MBq). A low-dose whole-body CT scan was obtained at 40-60 min post-injection for anatomical localization and attenuation correction. PET scanning followed at 1.5 min/bed position with a 23-slice overlap. Images were reconstructed using an ordered subsets expectation-maximization algorithm and corrected for CT-based attenuation, dead time, random events, and scatter.

Image Interpretation and Statistical Analysis
The images were reviewed by two experienced nuclear medicine physicians, who visually inspected the images and performed semi-quantitative measurements based on the maximum standard uptake value (SUV max ), which is determined by selecting the point of maximum FDG uptake within the lesion. For 99m Tc-HYNIC SPECT/CT and 68 Ga-DOTATATE PET/CT, the images were interpreted as positive if the tracer uptake in the lesion was higher than surrounding background.
All data are expressed as mean ± standard deviation. Differences between groups were analyzed using the Student t test, nonparametric analysis, and c2 test. The cut-off SUV max for differentiating pulmonary infections from ACTH-secreting tumors was obtained via receiver operating characteristic (ROC) analysis with calculation of areas under the curve (AUCs) and sensitivity and specificity values. The correlation between ACTH level and SUVmax value was evaluated using the Pearson correlation coefficient. A P-value of <0.05 was considered statistically significant. All statistical analyses were performed using SPSS Statistics (version 21.0, IBM SPSS Inc., IBM, Chicago, IL, USA).

Patients
Among the 24 patients, 18 patients with 18 lesions were diagnosed with ectopic ACTH-secreting tumours (typical carcinoids, n =12; atypical carcinoids, n = 5; SCLC, n =1) while six patients with eight lesions were diagnosed with pulmonary infections (cryptococcosis, n = 3; aspergillosis, n = 4; pulmonary abscess, n = 1). Therefore, a total of 26 lesions were analyzed in this study. The patient characteristics are shown in Table 1. The clinical presentations are summarized in Table 2. After surgical resection of the lesions, all patients in the tumor group were relieved of all symptoms, with serum cortisol and ACTH levels returning to normal. On the other hand, the source of ectopic ACTH secretion remained occult in patients with pulmonary infections. One patient of the infection group died of cryptococcal meningitis after surgery resection of pulmonary nodule. Thirteen of the tumor group (4 typical carcinoids, 9 typical carcinoids) had follow up information. Two patients with atypical carcinoids developed recurrence, while no patient died during the follow-up period, 18

DISCUSSION
Ectopic ACTH-producing tumors account for 15%-20% of cases of ACTH-dependent Cushing syndrome. Lung carcinoids and SCLC represent the most common tumors associated with ECS, and the resection of the responsible tumors can have curative effects (14). There is no consensus regarding the usefulness of 18 F-FDG PET/CT for localizing the source of ectopic ACTH secretion, even though it is the most commonly used molecular imaging method in clinical practice. A nodule or mass-like lesion in the lung that demonstrates abnormal activity on 18 F-FDG PET/CT tends to be interpreted as an ACTH-secreting tumor and might be surgically resected. However, in clinical practice, the resected pulmonary 'tumor' occasionally turns out to be an infectious lesion most often caused by fungus. In such cases, surgery is unnecessary and may deteriorate the patient's condition. The present study included 18 patients with ectopic ACTH-secreting lung tumors and six patients with pulmonary infections. To the best of our knowledge, this is the first study to describe and compare the features of ACTH-secreting lung tumors and pulmonary infectious pseudotumors using 18 F-FDG PET/CT. This discrimination is important because the two conditions require different treatment plans.
We found that a cut-off SUV max of 4.95 maximized the sensitivity and specificity for the differentiation of pulmonary infections from ACTH-secreting tumors. Specifically, the findings indicated that a pulmonary nodule or mass-like lesion with a SUV max of ≥4.95 was more likely to be an infectious lesion. Our study included only one SCLC, and it was the only lesion with a SUV max of >4.95 in the tumor group (SUV max , 7.7). SCLCs generally exhibit high FDG uptake on PET/CT because of their aggressiveness and high metabolic activity (15). The SCLC was underrepresented in our series, probably because most SCLCs are rapidly diagnosed by conventional cross-sectional imaging and do not require 18 F-FDG PET/CT or other nuclear imaging modalities for localization (2).
The present study showed significantly higher FDG accumulation in infectious lesions than in pulmonary carcinoids. The reason for the low FDG uptake of pulmonary carcinoids is that  most of the lesions (17/18) are well-differentiated neuroendocrine neoplasms (16). As mentioned above, patients with poorly differentiated neuroendocrine neoplasms such as SCLC are rarely examined by FDG PET/CT, which is why we have fewer patients in this group. Among the eight infectious lesions, only two showed low FDG uptake with a SUV max of <4.95. One of the lesions (Patient 1, SUV max , 1.2) was due to cryptococcosis, and it was the smallest lesion among the infectious lesions (0.7 cm in diameter). The other infectious lesion with low FDG uptake was an aspergilloma (Patient 4, SUV max , 1.0). Pulmonary aspergillosis can be divided into four subtypes on the basis of clinical and radiological findings: aspergilloma, allergic bronchopulmonary aspergillosis, chronic necrotizing aspergillosis, and invasive pulmonary aspergillosis (IPA) (17). The first three subtypes are also considered to be noninvasive pulmonary aspergillosis (NIPA) (17). Kim et al. evaluated the FDG PET/CT scans of 24 patients with pulmonary aspergillosis (8 IPA and 16 NIPA) and concluded that an isometabolic pattern on FDG PET/CT most likely represented NIPA (18). NIPA is a chronic infection with low virulence and a mild inflammatory reaction, which might attribute to the low metabolic activity on 18 F-FDG PET/CT. Somatostatin receptor-based imaging techniques including octreoscan-SPECT/CT and 68Ga-DOTATATE PET/CT have also been shown to be helpful in localizing the source of ECS (19). 68 Ga-SSTR PET/CT demonstrated high sensitivity in detecting ECS tumors. Two recent systematic reviews reported a sensitivity of 76.1% and 81.8% for 68 Ga-SSTR PET/CT (2,20). However, in this study, the sensitivity (54.5%) of SRI ( 68 Ga-DOTATATE PET/CT and 99m Tc-HYNIC-SPECT/CT) was lower than previously reported. One reason may be publication bias as negative results tend to be not reported. Another reason might be FIGURE 2 | A receiver operating characteristic curve for measuring the accuracy of the SUV max as a parameter for distinguishing pulmonary ACTHsecreting tumors from pulmonary infection. The area under the curve is 0.833. A cut-off SUV max of 4.95 or greater is predictive of pulmonary infection with 75% sensitivity and 94.4% specificity.  Our results also showed that the specificity of SRI (50%) is quite low as two cases of the infection group showed positive finding on 99m Tc-HYNIC-TOC SPECT/CT, which were misdiagnosed as the culprit tumors of ECS and were then surgically resected. Inflammatory disease demonstrating increased tracer uptake on SRI is not uncommon in the literature because somatostatin receptor is known to be expressed on activated macrophages in inflammatory process (22,23). These findings suggested that SRI may not effectively discriminate tumor-like infectious lesions from ACTH-secreting lung tumors.
The main limitations of this study were the small sample size, which does not allow for powerful statistical analysis, and retrospective design. In addition, the number of patients who had follow-up data is small, thus we did not perform statistical analysis on survival. The prognostic value of 18 F-FDG PET/CT needs to be investigated in future study. Finally, as 6 of the 15 patients who had SRI had 99m Tc-HYNIC-TOC SPECT/CT, we could not perform semi-quantitative analysis. Future studies are required to investigate whether there is a difference in intensity of 68 Ga-DOTATATE uptake between lung infectious lesions and tumors.
In conclusion, although pulmonary infectious lesions associated with ECS and ACTH-secreting lung tumors might exhibit similar morphological features, the former may show higher FDG activity on 18 F-FDG PET/CT.   An SUVmax of 4.95 may help differentiate the two conditions. While our results suggested that SRI may not be an effective tool for differentiating the two conditions given the relatively low specificity.

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/supplementary material. Further inquiries can be directed to the corresponding authors.

ETHICS STATEMENT
Written informed consent was not obtained from the individual(s) for the publication of any potentially identifiable images or data included in this article.

AUTHOR CONTRIBUTIONS
GH, YJ, FL, and XC contributed to the design and implementation of the research, to the analysis of the results, and to the writing of the manuscript. All authors contributed to the article and approved the submitted version.