Computed Tomography, Magnetic Resonance Imaging and 11C-metomidate Positron Emission Tomography for Evaluation of Adrenal Incidentalomas

Background: Given the higher sensitivity of modern computed tomography (CT) scanners, adrenal incidentalomas are being discovered increasingly often. This implies a growing quantitative diagnostic and clinical problem. CT and/or magnetic resonance imaging (MRI) and usually thorough hormonal testing are routinely used to determine the origin of these lesions. Recently, positron emission tomography (PET) using the tracer (11)C-metomidate (MTO) has been established as an alternative diagnostic method with high sensitivity for identifying adrenocortical lesions. The aim of this study was to evaluate the clinical use and value of MTO-PET compared to CT and MRI in the characterisation and work-up of adrenal incidentalomas.

Methods: Initially, we retrospectively evaluated 20 adrenal incidentalomas in patients who had undergone CT, MRI and MTO-PET and from whom we had either histopathological diagnosis or clinical follow-up data. After this analysis we conducted a prospective study in order to compare the imaging modalities. In the latter study, 24 incidentalomas were imaged by CT, MRI and MTO-PET and the results were correlated to those from histopathology (n=8) and clinical diagnosis after follow-up (n=16).

Results: In the retrospective analysis, MRI and especially MTO-PET, correlated well to histopathology and clinical diagnosis after follow-up, whereas specificity with CT was low. This was possibly due to the presence of several haematomas/fibrosis which were misdiagnosed as adrenocortical adenomas. In the prospective cohort, sensitivity and specificity with CT were 0.71 and 1.0, respectively, and further characterisation by MRI increased these values to 0.86 and 1.0, whereas maximum sensitivity and specificity were reached when MTO-PET was added.

Conclusion: The diagnosis of an adrenocortical adenoma may be established by CT in most patients and by MRI in an additional number. For the few remaining patients needing further characterisation, MTO-PET is advantageous as an additional imaging modality.