primary peritoneal hydatidosis is a rare condition , even in areas where hydatid disease is endemic . although liver and lung are the organs most commonly involved , peritoneal echinococcosis , either primary or secondary , represents an uncommon but significant manifestation of the disease . we report a case of extrahepatic primary  peritoneal hydatid in a patient , who presented with a lump in the lower abdomen . surgical excision and postoperative treatment with albendazole were effective in controlling the disease and preventing a recurrence . a 45-year - old female presented with lump in lower abdomen gradually increasing in size since last 6  months , associated with dull aching pain . ultrasonography of abdomen revealed a large hypoechoic mass with echogenic septations in the lower abdomen ( fig.1 ) . contrast - enhanced computed tomography ( cect ) of abdomen and pelvis revealed a huge cyst , 30    25  cm in size ( fig.2 ) in lower abdomen . initial differential diagnosis of ( 1 ) hydatid cyst of peritoneum , ( 2 ) ovarian cyst was made based on findings of usg and ct abdomen pelvis . , there was a large cyst ( 50    25  cm ) arising from pelvic peritoneum ( fig.3a and b ) . histopathological examination showed cyst wall with a laminated membrane and inner germinal layer containing degenerated eosinophilic protoplasmic mass suggestive of hydatid cyst . hydatid disease is a parasitic disease caused by infection with larva of the cestode echinococcus granulosus . the most common site involved is the liver ( 5975% ) , followed by  lung ( 27% ) , kidney ( 3% ) , bone ( 14% ) , and brain ( 12% ) . other sites such as the heart , spleen , pancreas , omentum , ovaries , parametrium , pelvis , thyroid , orbit , retroperitoneum , and muscles are very rarely affected 1 . primary peritoneal hydatidosis is rare and has been reported to occur in 2% of all abdominal hydatids 2 . intraperitoneal hydatid cysts are usually secondary to the rupture ( spontaneous or accidental at surgery ) of a primary hepatic , splenic , or mesenteric cyst 1 . a solitary cyst in the pelvic cavity is considered as primary when no other cysts are present . in such a case , the hydatid embryo gains access to the pelvis by hematogenous or lymphatic route 3 . pelvic hydatid cysts usually present as a nonspecific mass with pressure effects on adjacent organs such as the rectum and urinary bladder . rarely , they can cause obstructed labor , obstructive uropathy , and renal failure . sometimes , they can rupture spontaneously 3 . ultrasound is the preferred first - line imaging but cect gives more precise information regarding the morphology ( size , location , state of surrounding structures , and number ) of the cyst . drug treatment with albendazole has been found to be successful in a proportion of cases , but drug therapy is generally not used as the primary treatment except in cases where the patient is not fit for surgery or the cyst size is smaller or deeply located . combination of preoperative albendazole therapy , surgery and postoperative albendazole therapy is a useful regime . the incidence of primary hydatid cyst of peritoneum is 2% . it can present as a palpable abdominal lump . clinically and radiologically it is worth keeping peritoneal hydatid disease as a differential in ultrasonographically diagnosed pelvic cystic lesions .
key clinical messagethe incidence of primary hydatid cyst of peritoneum is 2% . it can present as a palpable abdominal lump . 
 the differential diagnosis is cystic ovarian tumor clinically and radiologically . 
 peritoneal hydatid disease should be included in differential diagnosis of sonographically diagnosed pelvic cystic lesion .