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hydatid disease ( hd ) is an endemic parasitic infection that is most commonly caused by the larval stage of echinococcus granulosus ( 1 , 2 ) . cysts are mostly found in the liver ( 55 - 60% ) and lungs ( 30% ) and involvement of the bones is very rare ( 1 - 4% ) ( 3 , 4 ) .
we herein present this case as a reminder that hydatid cyst that is located in the sacrococcygeal region can be confused with chordoma whose most common location in the spine is the sacrococcygeal area .
a 30-year - old woman with a high socio - cultural status presented with the complaints of progressive left lower limb pain . the radiating intermittent pain started 1.year ago and progressively increased in intensity . on admission , her neurological examination revealed hypoesthesia and radicular pain of the left leg on admission .
the pelvic computed tomography ( ct ) was carried out to detect the sacral mass after the lucent bone lesion was spotted on plain pelvic radiography .
there was an expansile lytic mass with cortical bone destruction in the sacrococcygeal region ( figure 1 ) .
an abdominopelvic magnetic resonance imaging ( mri ) was performed , which showed a lobulated and septated cystic mass at the sacrococcygeal region , hyperintense on t2 weighted ( w ) and hypointense on t1w sequences .
there was no contrast enhancement and the cystic mass was expanding the neural foramina between the s4 vertebra and coccyx ( figure 2 -d ) .
the displacement and the compression of the lumbosacral nerve roots was revealed by mri . there were no additional pathological findings on the abdominal mri . she also had a history of liver hd operation 6 years ago and the positive serological test results obtained at the time of pelvic mass imaging were diagnostic for hd .
the cystic mass was diagnosed as hd based on the imaging findings and serological results .
the patient received albendazole ( 10 mg / kg / day ) for 6 months with close observation of the liver enzymes .
there was no change in size at the one year follow - up period and the patient rejected the operation .
hd is a parasitic infection that is most commonly caused by the larval stage of echinococcus granulosus and less frequently echinococcus alveolaris .
it is still an important public health problem and endemic illness in countries such as china , central asia , the middle east , the mediterranean region , eastern africa and parts of south america ( 1 , 2 ) . echinococcal eggs of the adult worm are present in the small intestine of definitive hosts .
when these eggs are ingested by intermediate hosts like sheep , cattle , goat or humans , they penetrate the intestinal mucosa of these hosts and enter the blood and lymphatic circulation .
they are carried to the various organs , most commonly the liver and lungs where they develop into forms of hydatid cysts ( 2 , 5 ) .
hydatid cysts are found in the liver ( 55 - 60% ) , lungs ( 30% ) , kidneys ( 2.5 - 3% ) , bones ( 1 - 4% ) , and in other organs such as the brain ( 1 - 2% ) ( 3 , 4 ) .
the distribution of the disease in the spine is 10% in the cervical area , 50% in the thoracic area , 20% in the lumbar area and 20% in the sacrum ( 6 ) .
spinal hds are classified into five groups as intramedullary hydatid cysts , intradural extramedullary hydatid cysts , extradural intraspinal hydatid cysts , hydatid cysts of the vertebrae and paravertebral hydatid disease ( 7 ) .
chordoma is the most common tumor of the sacrococcygeal region and may mimic cysts on ct or mri ( 8) . wherever cysts are found in the body especially in the endemic countries ,
our case was an example of a very unusual location of hd as a sacrococcygeal cystic mass .
lumbar disc herniation is the most common reason for sciatica , lower back pain and neurological symptoms ( 9 ) .
if there is no disc pathology , the vertebral column and pelvis should be examined to define the reason .
although very rare , in patients presenting with the radiating limb pain and neurological symptoms that are caused by the compression of spinal cord or nerve roots , hd is to be considered in the differential diagnosis in endemic countries ( 6 ) .
the recurrence of hd occurs usually within 2 years after the previous hd operation ( 8 - 22% ) , whereas our case was operated for liver hd 6 years ago ( 5 ) .
we diagnosed our case as hd with the imaging findings and furthermore the history of hd operation and serological test results .
recognition of hd , especially in endemic regions can be helpful for the diagnosis of cystic masses in the whole body .
hydatid disease ( hd ) is a parasitic infection that is most commonly caused by the larval stage of echinococcus granulosus . 
 unusual location for this disease can cause diagnostic and therapeutic problems . 
 we herein report a case of sacrococcygeal hd at an unusual location in a 30-year - old woman . 
 she was evaluated using computed tomography ( ct ) imaging and magnetic resonance imaging ( mri ) after the demonstration of the lucent bone lesion on plain pelvic radiography . 
 there was an expansile lytic mass without contrast enhancement suggesting a cystic mass in the sacrococcygeal region . 
 medical history revealed that she had undergone surgery for liver hd and the serological test results were diagnostic for hd . in light of this , 
 no surgery was carried out for this cystic mass and she was followed with the diagnosis of sacrococcygeal hd .