Background: Characterisation of large adrenal lesion is challenging. There is no single robust imaging marker in defining benignity, especially for lesions which are greater than 4cm and are believed to carry increased risk of malignancy.
A radiolabelled cholesterol analog tracer, I-131-Norcholesterol (NP-59), localises to adrenal cortical lesions. It has an established role in Conns syndrome. It is not expected to concentrate in adrenocortical cancer. We aim to assess whether a combination of FDG PET-CT and NP-59 scan could be useful to define benignity of large adrenal lesions.
Methodology: We retrospectively reviewed cases referred to the adrenal service at University College London Hospital over the last six years. Patients with large adrenal mass were included when both FDG PET-CT and NP-59 studies had been undertaken and surgery was subsequently performed. Adrenal uptake of FDG was classified as concerning if the SUV was higher than liver activity. NP-59 study was classed as concerning if there is no concentration of the tracer in the lesion (i.e. higher uptake in the contralateral gland). Histology of the resected adrenal lesion was taken as the gold standard to define benignity.
Results: A total of ten patients were included. All adrenalectomy samples were confirmed to be benign (nine adenomas and one benign cyst). Eight patients had sub-clinical Cushings syndrome pre-operatively. Average size of the lesion was 45 mm. Three lesions had unenhanced CT density of >10 HU and high FDG uptake. Two lesions had low CT density but high FDG uptake. All proven adenomas had high NP59 concentration on the concerning adrenal gland. The benign cyst has fluid CT density and low uptake on both FDG and NP59.
Conclusion: Our results confirm previous observation that NP-59 could be useful to confirm benignity of large adrenal lesions. A larger patient group including adrenocortical cancer would be needed to confirm this.