Teaching Case

A 44-year-old male, a stone former, with preserved renal function being evaluated for an incidentally detected renal cyst after removal of double-J stent 6 months ago. He was asymptomatic but found to have microscopic haematuria on recent urinanalysis.

Ultrasound showed an isoechoic partly exophytic anterior mid-region right renal cyst not showing colour flow on Doppler imaging. Computed tomography (CT) urography with initial arterial imaging for the kidneys was performed, including plain, portovenous and delayed imaging. CT imaging showed no definite arterial enhancement or solid component. Hypodense cyst size was approx. 17 x 12 mm, however, it showed slightly more than usual pseudo-enhancement (densities of cyst content on plain, arterial and venous phases measured 20, 31 and 54 HU). MRI was, therefore, suggested that showed minimal faint arterial enhancement and diffusion restriction. Cyst excision was suggested to exclude any cystic renal neoplasm.

It is sometimes challenging to differentiate cystic from solid renal lesions. Pseudo-enhancement of renal cyst is an artefact (beam hardening) on contrast-enhanced imaging in which calculated density of cyst is inaccurately increased [1]. This phenomenon may be more prominent in small cysts (less than 1 cm) and in more central ones (than the peripheral ones) [2]. An increase of attenuation between 10-20 HU is considered indeterminate however increase in attenuation of more than 20 HU is considered enhancement. In our case, excision of cyst was performed and histopathology revealed a renal papillary adenoma. Hyperdense cyst content can be seen in adenoma [3]. Adenomas are benign renal tumors that can be multiple and associated with renal cell carcinomas and may occur in setting of acquired polycystic kidney disease [4]. Unifocal cystic renal cell carcinomas have good prognosis after surgical resection [5]. Radiologists should be aware of subtle or increase in attenuation of cyst on post-contrast images and needs to be differentiated with pseudo-enhancement to identify benign or malignant cystic neoplasm.

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