Intense ¹⁸F-Fluoro-deoxyglucose Uptake in Benign Adrenal Nodule

Sir,

Adrenal mass is a clinical entity which can occur in 1–5%[1] of adult patients. A critical question to answer is whether it is benign or malignant. Small tumor size and anatomical imaging characteristics based on their lipid content such as unenhanced computed tomography (CT) attenuation <10 Hounsfield units (HU) and decrease in intensity of signal on out-of-phase magnetic resonance imaging are highly predictive of the benign nature of an adrenal mass. However, a fair number of patients in clinical practice can have adrenal tumors with conflicting clinical and imaging features such as small tumor size and high CT attenuation. ¹⁸F-fluoro-deoxyglucose-positron emission tomography (¹⁸F-FDG-PET) has been shown to be excellent in differentiating malignant from benign adrenal tumors, with a reported sensitivity and specificity ranging from 92–100% to 80–100%, respectively.[23]

Here, we report a 60-year-old female, follow-up case of Stage IIIa adenocarcinoma of the esophagus who presented to us for PET/CT study. The patient had already received chemotherapy as the primary treatment and did not have any complaint at present. ¹⁸F-FDG PET/CT has performed approximately 60 min, following intravenous injection of 370 MBq of ¹⁸F-FDG. PET/CT scan from skull to mid-thigh was acquired in a whole-body full-ring PET/CT scanner (GE Discovery STE16 camera). Incidentally, the left adrenal gland was found to be showing markedly increased FDG uptake (standardized uptake value maximum [SUVmax] - 7.3, CT attenuation - 40 HU) which raised suspicion for possible malignancy. No abnormal FDG uptake was noted elsewhere in the body. Hence, this increased adrenal gland uptake was followed up with a contrast-enhanced computed tomography (CECT) scan and histopathological examination. CECT scan (done elsewhere) revealed a heterogeneously enhancing soft tissue nodule in the left adrenal gland. Histopathological examination showed predominantly benign adrenal cortical cells with no atypia and malignant cells. The patient had undergone repeat PET/CT study 1 year later which showed no significant change in the left adrenal gland tracer uptake (SUVmax - 7.0) as compared to the previous study [Figures 1 and 2].

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Figure 1 Maximum intensity projection image of WB positron emission tomography/computed tomography scan showing increased fluorodeoxyglucose uptake (standardized uptake value maximum - 7.0) in the left suprarenal region with no other abnormal focus of fluorodeoxyglucose uptake elsewhere

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Figure 2 Transaxial computed tomography and fused positron emission tomography/computed tomography images showing intense fluorodeoxyglucose uptake in the left adrenal gland

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Most of the studies performed for the evaluation of role of FDG-PET in the diagnosis of nature of adrenal lesions have shown FDG-PET to be useful for differentiating between benign and malignant lesions based on the SUV values.[24] An SUV value of more than 3.1[4] or that of liver[2] is considered as cutoff value for the differentiation between benign and malignant lesions.

Although most of the benign lesions show low SUV values, some studies state that one in five lesions showing SUV > 3.1 can be benign.[5] Dong et al.[6] reported that apart from malignant masses, increased FDG uptake can be seen in benign adrenal lesions such as adrenal hyperplasia, functioning adrenal masses, histoplasmosis, tuberculosis, blastomycosis, myelolipoma, ganglioneuroma, and adrenal hemorrhage. In conclusion, any isolated adrenal gland uptake should always be followed up with histopathological examination. Nonfunctioning benign adrenal lesions should be considered a possible false-positive FDG-PET finding for malignancy while evaluating adrenal incidentalomas, even when intense FDG uptake is revealed.