Intramuscular Hydatid Cyst Involving the Hamstring Muscles: A Rare Localization.

Background: Echinococcosis is a parasitic zoonosis that predominantly affects the liver and pulmonary system. Isolated involvement of skeletal muscle in the absence of visceral disease is exceptionally rare, accounting for approximately 1-3% of cases. Owing to nonspecific clinical manifestations and overlapping imaging characteristics, muscular hydatid cysts may be misinterpreted as neoplastic lesions or abscesses, thereby complicating the diagnostic process. Case Report: A 64-year-old woman presented with a painful mass in the posterior aspect of her thigh that had gradually increased in size. Magnetic resonance imaging revealed a well-circumscribed cystic lesion containing detached internal membranes, consistent with World Health Organization (WHO) stage 3a cystic echinococcosis (CE3a) and indicative of an active transitional hydatid cyst. Serological testing was strongly positive, and no evidence of hepatic or pulmonary involvement was detected. The patient was treated with albendazole therapy followed by a percutaneous puncture-aspiration-injection-reaspiration (PAIR) procedure, resulting in cyst regression without complications. Conclusion: In endemic regions, hydatid disease should be considered in the differential diagnosis of atypical soft-tissue masses, even in the absence of visceral organ involvement or a clear history of exposure.