A 71-year-old man presented with fever, tachycardia, hypoxia, elevated white blood cell count, lactic acidosis and sepsis. Non-contrast CT demonstrated right-sided hydronephrosis with suspicion of obstructive uropathy and infected urolithiasis. Patient is not a surgical candidate given his respiratory status, morbid obesity, type 2 diabetes mellitus and hypertension. Percutaneous nephrostomy is requested.
Antegrade percutaneous nephrostogram was obtained via an enlarged lower pole calyx of the right kidney (Figure 1). (1A-B) Initial injections show significantly enlargement of lower third calyces, but the renal pelvis, ureteropelvic junction (UPJ) and proximal ureter are not opacified. On later rotational views (1C), some contrast is seen passing into a non-distended ureter. The renal pelvis remains non- well visualized or opacified.
Pre-contrast CT images in axial (Figure 2A) and coronal (2B) planes do not show an obstructive radiodense stone. The pigtail of the percutaneous nephrostomy tube is in an enlarged lower pole calyx and not in the renal pelvis. Axial (2C) and coronal (2D) images 10 minutes after intravenous contrast show contrast filling enlarged calyces in the right kidney (contrast in the lower pole calyx presumably already excreted into the nephrostomy tube bag). The renal pelvis is not opacified due to extrinsic compression by a large parapelvic cyst(s).
Background Parapelvic (or peripelvic) cysts are relatively uncommon, often found by chance, comprising 1-2% of all renal cysts and observed in >1% of routine autopsies [1-3]. A commonly held pathophysiologic explanation is the dilatation of lymphatic structures obstructed by adjacent chronic inflammation [1-3], although embryonic remnants have also been invoked . They do not communicate with the renal collecting system . They are structurally similar to simple renal cysts  and morphologically can be simple or multiloculated, single or multiple .
Clinical Perspective Although usually asymptomatic, they can be associated with lumbar pain (98% of symptomatic cases), infection (9%), haematuria (5%) [1-3], or less commonly stones, vascular compression, renin-associated hypertension and spontaneous haemorrhage [1-2].
Imaging Perspective Parapelvic cysts are central in location and can misleadingly suggest hydronephrosis on ultrasound evaluation. Both intravenous pyelograms and antegrade percutaneous nephrostograms can show stretching and extrinsic compression of calyces similar to that seen with renal sinus lipomatosis [1-2]. Definite differential diagnosis relies on computed tomography with intravenous contrast, which fills the renal calyces and pelvis but not the cyst(s), the latter compressing the former in symptomatic cases . Stones can be found within the parapelvic cysts themselves .
Outcome Unlike simple renal cortical cysts, sclerotherapy is contraindicated for the management of parapelvic cysts due to the risk of retroperitoneal extravasation, perirenal inflammation with associated pain, fever, and ureteropelvic junction obstruction [2, 4]. Surgical approach has evolved from open marsupialization to laparoscopy (renal-sparing cyst excision, unroofing, decortication, or ablation). Less invasive treatment options include antegrade percutaneous nephroscopy or retrograde ureteroscopy (rigid or flexible) [2-3, 5]. In retrograde ureteroscopy, the scope guides the incision (1 cm) into the cyst wall, examines its cavity (+/- stone removal and/or incision of septa if present), and guides internal drainage leaving an internal ureteral stent in place for 1-2 months. This results in complete cyst disappearance in 33-79% of cases and >50% diameter reduction of cysts in 62-14%, with cyst recurrence in 0-4% [2-3]. Percutaneous cyst ablation has results similar to laparoscopy .
Take-Home Message / Teaching Points During percutaneous antegrade nephrostogram, the discrepancy between enlarged calyces and a thin, barely or non-visible renal pelvis (“vanishing pelvis”) combined with the difficulty in advancing the wire into the renal pelvis in the absence of a visible obstructing filling defect are key hints to the diagnosis. These observations should prompt the interventionalist to obtain a contrast-enhanced CT scan with an excretory phase.
 Dubilier W Jr, Evans JA. Peripelvic cysts of the kidney. Radiology. 1958 Sep;71(3):404-8. (PMID: 13579239)
 Mao X, Xu G, Wu H, Xiao J. Ureteroscopic management of asymptomatic and symptomatic simple parapelvic renal cysts. BMC Urol. 2015 Jun 6;15:48. (PMID: 26048408)
 Zhao Q, Huang S, Li Q, Xu L, Wei X, Huang S, Li S, Liu Z. Treatment of Parapelvic Cyst by Internal Drainage Technology Using Ureteroscope and Holmium Laser. West Indian Med J. 2015 Jun;64(3). (PMID: 26426175)
 Pabon-Ramos WM, Dariushnia SR, Walker TG, Janne d'Othée BJ, Ganguli S, Midia M, Siddiqi N, Kalva SP, Nikolic B; Society of Interventional Radiology Standards of Practice Committee. Quality Improvement Guidelines for Percutaneous Nephrostomy. J Vasc Interv Radiol. 2016 Mar;27(3):410-4. (PMID: 26803576)
 Kiryluk K, Gupta M. A large obstructive parapelvic cyst: challenging diagnosis and management. Kidney Int. 2007 May;71(9):955. (PMID: 17457336)