Renal cysts in children and adolescents are due to a variety of causes and can be single or multiple, unilateral or bilateral. Autosomal recessive polycystic kidney disease, which is due to a mutation in the PKHD1 gene on chromosome 6 (fibrocystin protein), is characterized by enlarged, diffusely echogenic kidneys with multiple, bilateral cysts and typically presents in the neonatal period or infancy. In countries with widespread use of prenatal ultrasound, it is increasingly recognized in utero. In contrast, autosomal dominant polycystic kidney disease is due to mutations in genes encoding polycystin-1 (PKD1 on chromosome 16) or polycystin-2 (PKD2 on chromosome 4) and is a late-onset disorder characterized by progressive, bilateral renal enlargement due to multiple cysts. It usually becomes symptomatic after the third decade of life but may also present in adolescents or children. Other diseases associated with renal cysts include, but are not limited to, tuberous sclerosis, von Hippel–Lindau syndrome, Ehlers–Danlos syndrome, Zellweger syndrome, Meckel–Gruber syndrome, Beckwith–Wiedemann syndrome, short rib-polydactyly syndrome, renal dysplasia, and calyceal diverticula [1–5]. Acquired simple cysts, which are the most common cysts encountered in adults, are uncommon in children. Current imaging techniques allow accurate assessment of the risk of malignancy in many cystic renal lesions, precluding the need for fine-needle aspiration (FNA) except in cases with indeterminate findings or for decompression of a benign cyst.

FNA of a renal cyst usually yields clear or blood-stained fluid with macrophages, cyst-lining cells, and occasional inflammatory cells. Renal tubular cells and Liesegang rings are infrequently observed. Benign cysts of various etiologies cannot be distinguished from each other on FNA.

Cyst fluid may benefit from liquid-based cytology to decrease obscuring blood in hemorrhagic cyst contents and to increase the ability to find cyst-lining cells, to exclude malignancy.

Neoplasms that undergo cystic degeneration are the main consideration but usually have greater cellularity and more atypia. Cyst contents with macrophages may also mimic granulomatous infections (mycobacterial or fungal) or the xanthomatous histiocytes of xanthogranulomatous pyelonephritis.

Misinterpretation of reactive atypia or degenerative changes in cyst-lining cells can lead to a false-positive diagnosis of malignancy. Conversely, a hypocellular specimen with few or no malignant cells can result in a false-negative diagnosis in a cystic neoplasm.

Pyelonephritis is caused by a variety of bacterial, viral, fungal, and parasitic organisms. Infection can be blood-borne or ascending and in children less than 6 years of age is associated with vesicoureteral reflux in up to 50 % of cases. The most common causes of pyelonephritis are bacteria, such as E. coli, Proteus mirabilis, Klebsiella spp., Enterobacter spp., Enterococcus faecalis, Staphylococcus spp., and others. Fungal pyelonephritis usually occurs in the context of systemic infection with opportunistic or pathogenic organisms, such as Candida spp, Aspergillus spp, Cryptococcus spp, Histoplasma capsulatum, Coccidioides immitis, and others and, although more common in immunocompromised patients, can also affect otherwise healthy individuals. Viruses, especially BK virus and cytomegalovirus, are associated with clinically significant renal infection in immunocompromised patients, particularly those with renal allografts. Pyelonephritis due to parasitic infiltration of the renal parenchyma is rare but can be seen in schistosomiasis, leishmaniasis, and malaria. In addition, schistosomiasis can cause obstructive uropathy at the level of the bladder and/or ureter with consequent ascending pyelonephritis due to other organisms. Mycobacterial infection of the kidney is usually due to M. tuberculosis. Infection may be due to disseminated infection or be localized within the urinary tract. Xanthogranulomatous pyelonephritis occurs in children, albeit infrequently, and is associated with kidney stones and persistent urinary tract infection [1, 2, 6].

The cytomorphology of these various infections is similar at whatever body site they occur. In the setting of mycobacterial infection, the aspirates show necrotizing granulomatous inflammation (Fig. 7.1a) Immunosuppressed children may present with an FNA comprised almost entirely of necrosis or a more suppurative picture with a moderate number of neutrophils in a necrotic background. A complication of infection, especially bacterial, is a renal abscess, and the FNA yields purulent material with numerous neutrophils, scattered histiocytes, and debris. Xanthogranulomatous pyelonephritis shows vacuolated histiocytes, acute and chronic inflammatory cells, multinucleated giant cells, cholesterol crystals, and necrotic debris (Fig. 7.1b). Rare cases of aspergillosis involving the kidney with fungal balls eliciting ureteral obstruction have been described [6].

A portion of the aspirate should be submitted for microbial cultures and cell block if infection is suspected. Special stains, such as Grocott, AFB, and gram stains can be performed on the cell-block material. In addition, if there is limited inflammatory or cellular material, extra unstained smears can be prepared and used for key special stains, in the event there is scant material on the cell block [7]. In cases in which special stains or culture results are negative but clinical suspicion for mycobacterial infection is high, molecular diagnostic techniques can be performed on the FNA material for identification and subtyping.

In infectious cases dominated by macrophages, the differential diagnosis includes cyst contents.

In case of suspected infection, it is important to preserve fresh material for microbial cultures and antibiotic sensitivities.