Imaging of incidentally discovered adrenal masses.

Abstract

IntroductionIn the last decade the increased use of cross-sectional imaging,especially computed tomography (CT) and magnetic resonanceimaging (MRI), has led to an increase in the number ofincidentally discovered adrenal masses. Even in patients with aknown malignancy, the majority of these will be benign,nonhyperfunctioning adenomas (Oliver et al., 1984). Rapidlyevolving imaging techniques used to evaluate these masses hasmeant that intervention is now indicated in only a minority ofpatients. The approach to their investigation must include aconsideration of several factors including: diagnostic costs,discomfort to the patient, risks from biopsy, consequences offalse-positive results, and disease prevalence.This article reviews the imaging appearances of the mostcommon causes of an incidentally discovered adrenal mass, andsuggests a strategy for the investigation of such masses.PrevalenceAdrenal masses of varying pathology are found in 0·35-4·4% ofpatients imaged with CT for reasons other than suspectedadrenal pathology (Glazer et al., 1982; Prinz et al., 1982;Abeccassis et al., 1985; Belldegrun et al., 1986; Herrera et al.,1991), and are usually referred to as adrenal ‘incidentalomas’.Large autopsy series of more than 1000 patients report aslightly higher incidence of 1·4-5·7% (Russi & Blumenthal,1945; Devenyi, 1967; Kokko et al., 1967; Granger & Genest,1970; Russel et al., 1972; Commons & Callaway, 1984). It islikely that with the increased use of abdominal CT and MRI,improving spatial resolution, and the use of thinner slices, therewill be a further increase in the prevalence of such incidentallydiscovered adrenal masses.Differential diagnosisThe vast majority of truly incidental adrenal masses (i.e. thosediscovered in patients with no other pathology) are adrenaladenomas. The incidence of other adrenal masses such asmyelolipomas, adrenal cysts, phaeochromocytomas, carci-noma and metastases varies considerably in the literaturedepending on the size of the series (Kloos et al., 1995). Evenin patients known to have malignant disease, adrenaladenomas are often more common than metastases (Oliver etal., 1984).The first step in investigating an adrenal mass detectedincidentally is to establish whether it is biochemically active.If shown to be biochemically active the mass is dealt withaccordingly. If the mass is not hyperfunctioning a substantialdifferential diagnosis remains. Some of these masses, such asmyelolipomas and cysts, have specific imaging features whichmay allow characterization of the mass on imaging alone. Inpatients with an adrenal mass which cannot be characterizedon imaging alone, the problem is then to differentiate benignfrom malignant lesions. In patients with a known malignancythe differential diagnosis is essentially between adrenalmetastases and an adrenal adenoma, whilst in patients withouta known malignancy adrenal carcinoma must also beconsidered.Biochemical investigationThe major hyper-secretory syndromes associated with adrenalmasses are Cushing’s syndrome, Conn’s syndrome andphaeochromocytomas. Cushing’s syndrome may be clinicallyobvious, and can be confirmed by grossly elevated urinary freecortisol or, with greater sensitivity, by a failure of serumcortisol at 0900h to suppress to<50nmol/l in the standard low-dose dexamethasone suppression test (Perry & Grossman,1997). However, it may be noted that some, possibly themajority, of ‘nonfunctioning’ adenomas may also show afailure of suppression, although presumably in these cases theoverall secretory rate remains below or within the normal range(Tsagarakis et al., 1998). Patients with Conn’s syndrome willdemonstrate hypokalaemic alkalosis, although this may bemasked by a low sodium diet: further investigation will reveal asuppressed plasma renin and elevated aldosterone:renin ratio(Vallotton, 1996b). Such tumours are usually small and havethe signal characteristics of adenomas. Finally, secretoryphaeochromocytomas can be identified by elevated urinarycatecholamine excretion using either GCMS or HPLC withelectrochemical detection: the sensitivity of this techniquecurrently approaches 100% (Ross et al., 1993; Bouloux &Fakeeh, 1995).