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Primary hydatid cyst of the thigh
K. Atmatzidis, I. Koutelidakis, B. Papaziogas, A. Alexandrakis, G. Chatzimavroudis, M. Grigoriou, P. Tsiaousis (Chirurgia, 101 (4): 419-421)

Introduction
Echinococcosis is an endemic parasitic disease of the Mediterranean region and may develop in almost every organ of the body. The liver is the most frequent location (75%) followed by the lung (15%), while all other reported locations (spleen, kidney, heart, muscles, bones, central nervous system etc) account for the remaining 10% [1, 2]. The development of hydatid cysts in unsual locations may cause significant difficulties in the differential diagnosis from other cystic or solid tumors [3, 4].
Localization of a primary hydatid cyst in the musculoskeletal system is found in approximately 0,5-4% of the patients suffering from echinococcosis [5].
We present a case of a 73-year old woman with an echinococcal cyst, which developed within the muscles of the posterior thigh.
Case report
A 73 year-old woman was admitted to our department complaining about a palpable mass in the posterior surface of the left thigh. The patient mentioned that she firstly discovered this mass several years ago, but she seeked no medical advice, since it remained unchanged in size and was indolent. However, the mass grew significantly in size during the last six months and became slightly painful on palpation. She also reported a mild restriction of the mobility of the left knee.
The clinical examination revealed the presence of a palpable, slightly painful mass in the posterior surface of the left thigh, which seemed to be firmly attached to the surrounding muscles.
Ultrasonography of the thigh showed the presence of a cystic formation between the muscles of the thigh, which was in close proximity to the left lateral circumflex femoral artery and left femoral vein (Fig. 1).
The CT of the left thigh confirmed the presence of a large cystic mass (6,8 x 4,2 cm), which was septated and adherent to the surrounding muscles (Fig. 2). The detection of antiechinococcal antibodies with ELISA was negative.
A 99mTc-MDP scintigramm showed that the cyst had no connection to the femur or the knee joint (Fig. 3).
In view of these findings, we decided to explore the region surgically and remove the cyst. An incision was performed at the external lateral side of the posterior thigh, 3 cm above the knee joint. Direct under the skin, we found a white colored mass, which was firmly
associated with the posterior thigh muscles (biceps femoris, semimebranosous and semitendineous). During the preparation of the mass from the surrounding muscles, the cyst opened inadvertently, revealing that the cyst contained clear fluid and doughter cysts. The contents of the cyst were fully aspirated and sent out for biopsy along with a small piece of the wall of the cyst. The field was washed repetitively with hypertone saline solution. The biopsy confirmed the diagnosis of echinococossis. The cyst was removed with a small part of the adjacent fibrotic muscle layers. A closed drainage was placed between the muscles and the skin was closed with interrupted sutures.
The postoperative course of the patient was uneventful. The drain was removed after 48 hours.
After the mobilization of the patient, we performed a CT-scan of the thorax and abdomen, which showed no evidence of other echinococcal cysts, so we considered our finding as a primary echinococcal cyst of the thigh.
The patient received postoperatively adjuvant antielminthic treatment with albendazole 10mg/kg for six months. At follow-up, one year after the operation, the patient remains in good general condition. The ultrasonography and CT revealed no signs of recurrence of the cyst in the thigh or any other part of the body.

Figure 1
Figure 2
Figure 3

Discussion
The incidence of musculoskeletal hydatidosis is not clear, but is usually estimated at approximately 0.7-5.4% of patients suffering from echinococcosis [2, 3]. These cysts are usually associated with involvement of other organs, especially the liver, and are considered as secondary lesions. The majority of the reported cases of musculoskeletal hydatid cysts belong to this category. The development of primary hydatid cyst of the musculoskeletal system, in the absence of liver or lung echinococcosis, is extremely rare [4, 5].
It has been suggested that muscles provide a poor environ-ment for the development of echinococcal cysts due to the presence of lactic acid [6].
Muscles, which have been described as sites of hydatid cysts include muscles of the anterior abdominal wall, muscles of the chest wall, supraspinatus, pectoralis major, biceps brachii or sartorius muscles [7, 8].
Hydatid cysts of the thigh have to be differentiated from other benign or malignant conditions such as abscesses, chronic hematomas, synovial cysts and cystic or solid tumors of the muscles or bones. Furthermore, it has to be differentia-ted from hydatidosis of the subcutaneous tissue or the large bones (femur). The latter tends to develop in the epiphyses of the femur, which the vascular supply is more adequate [7, 9].
The diagnosis of echinococcal cysts should be considered in cases of small growing soft tissue mass, especially if the patient lives in an endemic region or has a history of liver or lung echinococcosis. However, the diagnosis is only rarely set preoperatively, since the clinical and radiological findings are not always typical [9, 10, 11]. The clinical signs are usually not specific, since the cysts generally grow slowly [11]. Large cysts may become symptomatic through pressure or displacement of vessels or nerves. Contamination of the cyst may resemble a common absence of the soft tissues [11, 12, 13].
Calcifications of the cyst wall, presence of septa or daughter cysts within the cysts are the most reliable findings in the x-rays, ultrasonography and computed tomography. The detection of antiechinococcal antibodies with the use of ELISA is the most reliable method to assess the activity of the infection. However, the sensitivity of ELISA is very low in cases of primary extrahepatic involvement (absence of hepatic hydatidosis) and does not exceed 25-30%. Thus, negative serological test may be misleading, as in our case [12, 13, 14].
MRI - tomography can supply significant information about the nature of the cyst and differentiate it from other cystic lesions of the muscles, bones, joints or the subcutaneous tissue. Garcia-Diez reported on the typical MRI - signs of hydatid cysts of the musculoskeletal systems, which included multivesicular lesions with or without hpointense peripheral ring. The intensity of T2-weighted images depends on the presence or absence of daughter cysts. Hypointensity of the daughter cysts compared to the matrix of the mother cyst is considered a sign of death of the parasite [15-19].
Complete surgical excision of the cyst is the treatment of choice. If it is technically difficult to remove the cyst completely (large, complicated cysts or cysts firmly adherent to surrounding muscles) intraoperative drainage and irrigation with scolecidal agents is an alternative. In any case, intra-operative spillage of the parasite has to be avoided in order to prevent recurrence of the disease. In cases of intraoperative opening of the cyst, adjucant antielminthic therapy with albentazole may decrease significantly the recurrence rates [7-12].
If antibody titers are elevated before surgery, resection of the cysts results in gradual decrease over a period of 1-2 years. If titers do not decrease of if they tend to increase after initial regression there is evidence of recurrence [9,10, 11,17].
In conclusion, hydatid disease should be considered in the differential diagnosis of any cystic mass detected in the thigh, especially if occurs in regions where the disease in endemic.

References
1. Hamamci, E.O., Besim, H., Korkmaz, A. - Unusual locations of hydatid disease and surgical approach. ANZ J. Surg., 2004, 74:356.
2. Gossios, K.J., Kontoyiannis, D.S., Dascalogian-naki, M., Gourtsoyiannis, N.C. - Uncommon locations of hydatid disease: CT appearances. Eur. Radiol., 1997, 7:1303.
3. Di Gesu, G., Picone, A., La Bianca, A., Massaro, M., Vetri, G. - Muscular and subcutaneous hydatidosis. Minerva Med., 1987, 78:835.
4. Prousalidis, J., Tzardinoglou, K., Sgouradis, L., Katsohis, C., Aletras, H. - Uncommon sites of hydatid disease. World J. Surg., 1998, 22:17.
5. Hammami, T., Noomane, F., Ketata, M., Ganneme, Y., Nasr, M., Zouari, K., Hamdi, A. - Hydatid cyst of the thigh: three cases. Rev. Chir. Orthop. Reparatrice Appar Mot., 2002, 88:193.
6. Palmerio, G., Listorto, G., Ranalli, B., Ulacco, E., D'Annunzio, A. - Echinococcal cysts with primary muscle localization: comments on a clinical case. G. Chir., 1993, 14:113.
7. Koc, Z., Agildere, A.M., Yalcin, O., Pourbagher, A., Pourbagher, M. - Primary hydatid cyst in the anterior thigh: Sonographic findings. J. Clin. Ultrasound., 2004, 32:358.
8. Buyukbebeci, O., Gulec, A., Gokalp, A., Karakurum, G. - Combined treatment of primary hydatid disease of the vastus lateralis muscle. Orthopedics, 2004, 27:621.
9. Alexiadis, G., Lambropoulou, M., Deftereos, S., Papadopoulos, N., Manavis, J. - Primary muscular hydatidosis. US, CT and MR findings. Acta Radiol., 2002, 43:428.
10. Memis, A., Arkun, R., Bilgen, I., Ustun, E.E. - Primary soft tissue hydatid disease: report of two cases with MRI characteristics. Eur. Radiol., 1999, 9:1101.
11. Ben M'Rad, S., Mathlouthi, A., Merai, S., Ghrairi, H., Mestiri, I., Ben Miled M'Rad, K., Djenayah, F. - Multiple hydatid cysts of the thigh: the role of magnetic resonance imaging. J. Radiol., 1998, 79:877.
12. Martin, J., Marco, V., Zidan, A., Marco, C. - Hydatid disease of the soft tissues of the lower limb: findings in three cases. Skeletal Radiol., 1993, 22:511.


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