Pseudotumors in chronic kidney disease: a distinct entity

We read with great interest the article by Spiesecke et al. (1) in which they conclude that contrast-enhanced ultrasound (CEUS) reliably rules out neoplasm in developmental renal pseudotumors. The biggest advantage of CEUS is that it can be coupled along with a routine ultrasound and thus add to the diagnostic confidence, thereby reducing the need of computed tomography (CT)/magnetic resonance imaging (MRI). The demonstration of enhancement similar to the surrounding renal parenchyma and matching the corticomedullary differentiation points towards a pseudotumor. In contrast, the enhancement pattern of renal cell carcinomas (RCCs) is different from the surrounding parenchyma in at least one phase with disruption of normal vessels (1). We want to highlight certain observations regarding the article and this diagnostic conundrum.

Out of the 32 patients in their study, 15 had chronic renal insufficiency while an additional three had shrunken kidneys. Of these 18 cases, 13 had pseudotumors. We suspect that these pseudotumors in background chronic kidney disease (CKD) may not be developmental lesions; rather, they are sequelae to non-uniform parenchymal disease. Pseudotumor in CKD is a common and underrecognized entity. Due to patchy parenchymal loss (due to various underlying causes of renal dysfunction), there is compensatory hypertrophy and hyperplasia of the relatively spared renal parenchyma (2). These appear as ball-type mass lesions in the background of contracted kidneys. Since these are the relatively preserved areas of renal parenchyma, thus they may show hyperenhancement or early enhancement, as was seen in 5 of the 23 pseudotumors in their study (3).

Contrast-enhanced cross-sectional imaging (CE-MRI and/or CE-CT) was done in all their patients despite the presence of renal dysfunction in a significant proportion (glomerular filtration rate [GFR]  < 15 mL/min/1.73 m² in 15 patients). We agree that contrast administration helps tremendously in characterizing pseudotumors as they show enhancement matching that of the cortex and medulla in all phases. However, the administration of iodinated contrast should be avoided in the presence of renal dysfunction because of potential risk of contrast-induced nephropathy (in patients with eGFR < 45 mL/min/1.73 m²) (4). Due to the risk of nephrogenic systemic fibrosis, gadolinium-based MR contrast agents should be used with caution in patients with severe renal dysfunction (eGFR < 30 mL/min/1.73 m²) after assessing the risk-benefit ratio (5).

An alternative method could have been to do diffusion-weighted imaging (DWI) along with the other non-contrast MRI sequences to serve as the gold standard. The role of DWI in renal imaging offers promising results (6–8) without any additional cost/significant time. This is a non-contrast technique and displays tissue contrast based on the Brownian motion of water molecules in the tissues. The apparent diffusion coefficient (ADC) values of CKD pseudotumors has been reported to be higher than that of RCCs and, more importantly, higher than the surrounding diseased parenchyma (6). Thus, absence of restricted diffusion may point away from malignancy in such cases, since solid RCCs usually produce some degree of diffusion restriction. In a recent meta-analysis, DWI has shown moderate accuracy in differentiating benign and malignant renal lesions (9). ADC values can even predict tumor aggressiveness and proliferation potential based on the lesion cellularity (10). MRI has the advantage of not being operator-dependent and is not plagued by poor image quality in obese patients as could be an issue with ultrasonography. In addition, the reader in the study was highly experienced in CEUS, which may not be available in all centers.

Lastly, of the 32 suspected pseudotumors, nine were found to be neoplasms on gold standard. The authors have reported the accuracy of CEUS for diagnosing the same. However, whether one or a combination of all the CEUS parameters were used as the criteria is not clear.