51-year-old male patient complained of fullness and occasional pain in the right hypochondrium for a few months. Ultrasonography was performed outside which revealed a complex cystic mass in the right kidney for which further evaluation with CT was advised.
Plain and contrast-enhanced CT of the abdomen, pelvis and delayed urography was performed. The findings were:
- A large thick-walled cystic lesion was seen arising from the interpolar region of the right kidney. The lesion was partly exophytic and indented the liver parenchyma. The lesion showed few thin internal membranes as well as daughter cysts. It measured approximately 10 x 8 x 13.5 cm (AP, Trans, CC)
There was no evidence of calcification/enhancing mural nodule within.
- Compression of the right renal pelvis was seen due to mass effect from the lesion.
Hydatid disease is a zoonotic disease caused by the larval stage of Echinococcus tapeworm of which the two main types are E. granulosus and E. multilocularis 
The cysts usually have three components (from outside to inside):
• Pericyst: composed of inflammatory tissue of host origin
• Ectocyst (laminated membrane): which is acellular and allows passage of nutrients.
• Germinal layer: which produces the scolices (the larval stage) and the laminated membrane
Daughter vesicles appear as a bunch of grapes and as small spheres that contain the protoscolices and are formed from rests of the germinal layer 
Liver and lungs are commonly involved in hydatid disease. Renal involvement occurs only in about 3% of cases and is mostly asymptomatic. The common complaints of patients are flank mass, pain, and dysuria. Cysts are solitary at times and may reach up to 10 cm before any clinical symptoms appear . Cysts may rupture into calyces system resulting in acute pain and hydatiduria.
Renal hydatid cysts are usually solitary and three types have been described:
Type 1 cysts are unilocular, without internal architecture;
Type 2 cysts contain multiple daughter cysts; and
Type 3 cysts are completely calcified and represent the death of the parasite.
USG and CT scan features of renal hydatid cysts are similar to those of cysts in other locations and can range from purely cystic lesions to completely solid appearances.
USG findings: Type 1 unilocular hydatid cyst mimics simple renal cyst. Type 2 is a multiseptated daughter cysts which mimics polycystic kidney disease; however, it shows thick bilayered walls. USG is most useful for demonstrating the hydatid sand (echogenic foci in purely cystic lesion which move as patient rolls resulting in snowstorm appearance). Separation of endocyst and pericyst results in floating membranes appearance. Multiple daughter cysts separated by fluid matrix which contains membranes of broken daughter cysts, scolices and hydatid sand resulting in spoke wheel appearance. Bright echogenic focus with strong posterior acoustic shadowing due to calcification is seen in Type 3 cyst.
CT findings are similar to USG, however, CT is best for detecting calcification of the cyst and revealing the internal cystic structure posterior to calcification and internal membranes. Several small daughter cysts may be seen, which appear hypodense compared to the maternal matrix . The wall and internal septae may enhance.
Ring-like calcification of the cyst wall may point towards a diagnosis of hydatid cyst. Calcification is more common in hydatid disease of the liver, spleen, and kidney and can become quite large in compressible organs.
Treatment is surgical: partial or complete nephrectomy.
Differential Diagnosis List
Renal hydatid cyst
Renal cyst (Bosniak 2)