A 58-year-old male noticing a slowly growing lump in his right thigh. He has no pain, fever or other symptoms.
Ultrasound imaging shows a huge elongated mass involving the posteromedial thigh muscles, constituted by multiple ovoid cystic lesions, the largest with a large axis measuring 7 cm, some of which have echogenic content. Some of the lesions content innumerable clusters of anechoic little cysts corresponding to daughter cysts. Detached membranes and the double-line sign were also present. No hyperechogenic images suggesting calcifications were demonstrated.
MRI confirmed these findings as well as the intramuscular location of the cystic lesions, with no evidence of bone involvement or vascular and nervous infiltration. The 28 x 10 x 10 cm mass involved most of the muscles of the posterior compartment of the thigh.
Some of the cysts have a content showing intermediate signal intensity both in T1 and T2-weighted sequences, suggesting inflammatory debris. Enhancement of the septa and membranes after the administration of Gd contrast was demonstrated.
Hydatid disease (HD) is a an anthropozoonosis caused by tapeworms of the genus Echinococcus, endemic in many countries. The liver is the first and most affected organ but almost any anatomic location can be involved 
HD primary soft tissue involvement is very rare, eliciting a diagnostic challenge. Clinical presentation varies from a slow-growing lump with variable pain to a sudden onset of symptoms due to cyst rupture [1-3] .
It is not always possible to establish the link of contact with animals in HD, as it happened in this case (the patient denied any contact with animals), but musculoskeletal HD should always be suspected in patients originating from endemic areas , such as Spain, as in this case.
Preoperative diagnosis is key in the management of HD, as treatment strategies are based on the imaging features. If a cyst is less than 5 cm in diameter and unilocular, antihelminthic albendazole is administered. Surgical management in conjunction with adjuvant therapy (albendazole) is preferred when the cyst is larger than 5 cm. As effective antihelminthic agents were developed, the percutaneous minimally invasive management is preferred, involving puncture, aspiration, injection, and reaspiration (PAIR) .
Ultrasound is widely used in the diagnosis of HD, and as a guide during interventional procedures. Daughter cysts, detached membranes, and double-line sign are the most characteristic features [1, 4].
CT is better in detecting wall calcifications and bone involvement, but HD CT findings are rarely typical. The multivesicular lesion is characteristic of hydatid cyst and reflects multiple daughter cysts within the parent cyst. A solid or complex lesion is the result of inflammatory changes and may mimic a tumour [1, 4].
MRI findings of musculoskeletal HD have been described in a few cases. It can depict daughter cysts, detached membranes, and the double-line sign. MRI is an important imaging modality in the differential with other soft-tissue masses. [1, 4].
PAIR treatment was offered to the patient, with successful results. Under US guidance, a percutaneous aspiration of the cyst contents was performed, followed by injection of a hypertonic saline solution and then reaspiration of the contents. Oral pre- and postprocedural antihelminthic treatment was administered.
Take Home Message / Teaching Points
Primary soft tissue HD is sometimes difficult to diagnose preoperatively. It should be considered in the differential diagnosis of an intramuscular cystic lesion in regions where HD is endemic.
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