Imaging of adrenal masses

Abstract

Adrenal pathology may be discussed based on hormonal functionality of the adrenals, appearances on imaging modality, or pathological determination. There are three main categories of adrenal function. Hyperfunctional states include Conn's or Cushing's syndrome. Lesions with normal function may be detected incidentally. Hypofunctional states may occur from idiopathic Addison's disease or some bilateral adrenal pathology. The most common modalities for characterization of adrenal pathology are non-enhanced CT, often followed by contrast CT or chemical shift MRI. The common appearance on non-enhanced CT is a well-defined homogeneous lesion with low-density due to the microscopic fat present and adrenal adenomas. When density criteria are not met, many of these may be characterized as adenomas by washed out of contrast or signal decrease using in phase and out-of-phase MRI sequences. Other non-invasive modalities may incidentally discover adrenal lesions, but are not typically used in the work-up. NP-59 is an uncommonly used nuclear medicine technique which is very specific for adenoma when correlated with pathology on other imaging studies. In the rare cases where non-invasive imaging is non-specific, fine needle aspiration or core biopsies may be necessary. However, biopsies have associated risks including infection and hemorrhage. The imaging appearance of an adrenal lesion is often specific such that further imaging is not necessary. These lesions include adrenal adenoma, pheochromocytoma, myelolipoma, adrenal cyst, and some large adrenocortical carcinomas. However, the findings in lesions such as metastasis, smaller primary adrenal carcinomas, lymphoma, granulomatous disease, and many adenomas are not as specific. In the proper clinical situation, follow-up imaging may be necessary, or biopsy may be warranted.

Introduction

Computed tomography (CT), MRI, and ultrasound enable physicians to peer into the human body and non-invasively diagnose a wide variety of disease processes. As these technologies improve, reliance on medical imaging techniques increases. Imaging of the adrenals especially benefits from the improved resolution and tissue characterization that new techniques allow. The adrenals are very small organs, and adrenal pathology may often be measured in millimeters. As more and more adrenal lesions are diagnosed, a better understanding of the specific characteristics of many adrenal abnormalities is gained. In many cases, the imaging diagnosis is definitive, requiring no further evaluation. In less-specific lesions, it is important to understand the clinical settings in which an adrenal lesion demands further evaluation and when expectant follow-up is warranted. This review will discuss the typical characteristics of many adrenal lesions and describe the algorithms and dilemmas that occur when such lesions are detected.

An approach to adrenal imaging begins with knowledge of the clinical history surrounding the diagnosis. A large irregular adrenal lesion discovered as an incidental finding has different implications than a small adrenal lesion in a patient with paroxysmal hypertension. Many decision pathways use function as a primary branch point. Adrenal lesions are described in the setting of adrenal hyperfunction, hypofunction, or normal function. Indications for evaluation of adrenal hyperfunction include hormonal abnormalities such as Cushing's (due to hypersecretion of cortisol), Conn's syndrome (due to hypersecretion of aldosterone), virilization, or feminization. Lesions discovered incidentally or in the presence of known malignancy may warrant further evaluation. Hypoadrenalism is often idiopathic, but the patient may benefit from the exclusion of treatable etiologies. The most commonly encountered situation is an adrenal nodule in a patient with normal adrenal function. In the past, the evaluation of adrenal lesions required invasive procedures such as venous sampling or biopsy. Now, however, most lesions are characterized by CT or MRI. The rest require nuclear medicine studies or biopsy.