CT and MRI of small renal masses.

BJR Received: 31 January 2018 © 2018 The Authors. Published by the British Institute of Radiology Revised: 11 April 2018 https://doi.org/10.1259/bjr.20180131 Accepted: 16 April 2018 Cite this article as: Wang ZJ, Westphalen AC, Zagoria RJ. CT and MRI of small renal masses. Br J Radiol 2018; 91: 20180131. Review Article CT and MRI of small renal masses Zhen J Wang, MD, Antonio C Westphalen, MD and Ronald J Zagoria, MD Department of Radiology and Biomedical Imaging, University of California San Francisco, San Francisco, CA, USA Address correspondence to: Dr Ronald J Zagoria E-mail: ; Abstract Small renal masses are increasingly detected incidentally at imaging. They vary widely in histology and aggressiveness, and include benign renal tumors and renal cell carcinomas that can be either indolent or aggressive. Imaging plays a key role in the characterization of these small renal masses. While a confident diagnosis can be made in many cases, some renal masses are indeterminate at imaging and can present as diagnostic dilemmas for both the radiologists and the referring clinicians. This article will summarize the current evidence of imaging features that correlate with the biology of small solid renal masses, and discuss key approaches in imaging characterization of these masses using CT and MRI. Introduction Renal tumors are incidentally discovered at an increasing frequency due to the widespread use of cross-sectional imaging.1,2 Many of these are 4 cm or less in diameter (clinical stage T1a) and termed small renal masses (SRMs). They vary widely in biological aggressiveness, ranging from benign tumors to high grade renal cell carcinomas (RCCs). As a result, management options for these tumors have evolved over time and range from surgical resection to active surveillance. Imaging is crucial to characterize renal masses and guide management. It is used to differentiate benign tumors from RCCs, and predict RCC histological subtype and grade where possible. Imaging is also used to stage RCC, and provide pre-operative planning. CT is currently the most commonly used modality for initial diagnosis and staging. MRI can provide additional characterization in some cases that may aid in management decision. Dedicated renal mass CT and MRI protocols, where images are acquired before and after the administration of intravenous contrast material at prescribed timings, are usually performed to optimize renal mass characterization. However, evidence shows not all incidentally detected renal masses require such a complete assessment. For example, a homogenous hyperdense renal mass that measures greater than 70 Hounsfield units (HU) on a non-enhanced CT represents a benign hyperdense cyst in 99% of cases,3 and likely does not require additional imaging with contrast. Similarly, a homogeneous low attenuation lesion measuring between −10 and 20 HU is presumed to be a benign cyst without the need for additional imaging.4 While ultrasound can be useful in assessing whether a mass is cystic or solid, its use for renal mass characterization is limited by low sensitivity for small lesions, operator dependence, and other technical limitations. Though contrast enhanced ultrasound with microbubble agents is emerging as a useful modality to characterize previously indeterminate renal lesions,5–7 it is not currently widely available. In this review, we will summarize the current evidence of the biology of small solid renal masses, and discuss key approaches in imaging characterization of these masses using CT and MRI. Our discussion will focus on the well-circumscribed renal cortical tumors, and not tumors exhibiting clearly aggressive features. Renal pseudotumors Before establishing a diagnosis of a renal tumor, one should always consider the possibility of tumor mimics, since misdiagnosis could result in inappropriate management such as surgical resection. Renal pseudotumors can usually be correctly diagnosed by noting several imaging features as well as the clinical history. Focal hypertrophy of the renal parenchyma can be properly identified by noting that the “tumor” and normal renal parenchyma demonstrate the same enhancement pattern, and by recognizing adjacent parenchymal scarring. Images obtained during the corticomedullary phase may be particularly helpful in these cases to demonstrate the normal corticomedullary differentiation in Wang et al BJR Figure 1. A 68-year-old female with renal pseudotumor secondary to focal parenchymal hypertrophy. (a) Nephrographic phase image shows a bulge in the right lateral renal contour (long arrow) that mimics a tumor. There is adjacent renal cortical scarring (short arrow). (b) Corticomedullary image confirms the bulge in the renal contour represents normal renal parenchyma (long arrow), which appears more prominent due to adjacent scarring (short arrow). the hypertrophied renal parenchyma (Figure 1). Focal pyelonephritis can be differentiated from a renal tumor by noting the presence of a striated or heterogeneous nephrogram, edema in the renal parenchyma adjacent to the lesion, and perinephric fat stranding. Acute renal infarcts can be distinguished from tumors based on the lack of renal contour deformity, wedge-shaped lesions with sharply demarcated straight margins, lack of contrast enhancement, and presence of a cortical rim nephrogram over the infarct. Infarcts usually occur when patients have known risk factors, such as atrial fibrillation, and symptoms of pain (Figure 2). When the differentiation of focal pyelonephritis or acute infarct from a tumor is uncertain, a short-interval follow-up scan can be obtained as these pseudotumors should evolve fairly rapidly, while a tumor is less likely to change significantly over a short time interval. Renal pseudoaneurysms can be diagnosed by observing an enhancement pattern that follows that of the vasculature. Figure 2. A 63-year-old female with renal infarct. (a) Initial contrast-enhanced CT performed for abdominal pain shows an ill-defined right renal lesion (long arrow) and small amount of perinephric fat stranding (short arrow). Patient also had history of atrial fibrillation. Given the history, renal infarct was suspected. (b) Contrast-enhanced CT performed 3 months later shows evolution of the right renal lesion with decrease in its size, confirming that the lesion is not a tumor, and is more consistent with renal infarct given clinical history. 2 of 12 birpublications.org/bjr Renal tumor subtypes and biology of small renal masses Clear cell RCC is the most common subtype and accounts for 70–80% of all RCCs. It is followed by papillary RCC (~10%), chromophobe RCC (~5%), and other rare subtypes of RCCs.8 Clear cell RCCs are usually considered the most aggressive of the more common subtypes.9 Yet, some studies have shown that tumor stage, grade and clinical performance, but not histology, were independent predictors of overall survival.10 Among renal masses 4 cm or less, approximat (...truncated)