Not Your Every Day Hydatid Cyst

Clinical History

A 53-year-old male presented with a mildly painful right thigh swelling for the past three months. There was no history of trauma, surgery or additional diseases.
On physical examination, a palpable mass was noted with an approximated length of 6cm and there were no signs of inflammation of the skin or lymphadenopathies.

Imaging Findings

Initially, a US was performed revealing a large multilocular cystic mass, with some echogenic debris inside. It was localized in the medial side of the right thigh and showed no significant Doppler signal (figure 1).
On MRI, a well-defined multicystic encapsulated mass was found on the right adductor muscles. Measuring 6, 9 x 6, 7 x 18, 2 cm (antero-posterior x transversal x longitudinal), it showed homogeneous low signal characteristics on T1-weighted images and very high signal on T2-weighted images, separated by thick septa, with no solid component (figure 2). On T1-weigthed images and fat suppressed T1-weighted images, the lesion had no loss of signal intensity (figure 3). Dynamic contrast-enhanced MR images only showed enhancement of the peripheral capsule (figure 4).
Adjacent to the lesion, smaller rounded cysts were found, compatible with daughter cysts (figure 5). No apparent relation with neurovascular or bone structures were found.

Discussion

Hydatid disease (HD) is an endemic infestation caused by the tapeworms of the echinococcus, humans being accidental hosts in their life cycle. Found in the intestine of carnivores, when humans are infected, the development of a hydatid form occurs, especially in the liver and lungs [1].

HD can be either primary or secondary. In the secondary form, there is an actual or previous primary location of a hydatid cyst in the liver, lung or spleen. Primary muscular localization of HD represents 0,9% of cases [2], not only because the sole implantation at this localization involves passage through the filters of the liver and lung but also due to muscle contractility and presence of lactic acid. The muscle volume and its rich blood supply could explain predilection for proximal muscles of the lower limbs [3].

The symptoms include a slow-growing soft tissue mass, which can be associated with pain or inflammatory signs.

Imaging modalities play a major role in the diagnosis of these lesions. The first, most affordable and noninvasive exam to perform is Ultrasound, where the presence of cystic formations with daughter cysts, detached membranes and double-line sign are the characteristic features of these lesions. The sensitivity of US is 95% and can reach 100% if vesicular fibrils are present [4].

Gharbi classification based on their ultrasound features, divides them into 5 types [5]:
• I–Unilocular unechoic cystic lesion with double line sign
• II–Fluid collection with a split wall (water lily sign)
• III–Fluid collection with multiple septa/and/or daughter cyst (honeycomb sign)
• IV–Heterogeneous cyst contents with internal echoes
• V–Cyst with reflecting calcified thick wall

Computed tomography appearance can range from a unilocular cyst to a complex or a solid lesion, mimicking a tumour. On CT one can depict wall calcifications, the presence of daughter cysts and detached membranes [6].

Magnetic resonance imaging T2 sequences show a multilocular cystic formation with low intensity rim, representing the pericyst, which is rich in collagen and is generated by the parasite. A low-intensity rim (rim sign), seen on both T2 and T1 sequences, is a characteristic sign in muscular HD. A feature that is rarely found in HCs located in other regions of the body [7].

Complete surgical removal of the cyst remains the primary treatment option [8].

HD imaging features contribute to the pre-operative diagnosis and can also prevent the use of aggressive diagnostic tools such as fine needle aspiration or core needle biopsy, thus reducing the risk of anaphylactic shock or dissemination of viable parasites [9].

Differential Diagnosis List

Final Diagnosis

Primary Muscle Hydatid Disease

URL:	https://eurorad.org/case/14455
DOI:	10.1594/EURORAD/CASE.14455
ISSN:	1563-4086