Primary giant intramuscular hydatid cyst of the thigh: A rare multistage presentation in a non-endemic region

Background: Musculoskeletal hydatid disease is an uncommon manifestation of Echinococcus granulosus infection, accounting for less than 1% of cases even in endemic regions. Primary skeletal-muscle localization is exceptional. Its atypical presentation may mimic soft-tissue tumors, abscesses, hematomas, or cystic neoplasms, leading to diagnostic delays and potentially hazardous procedures. Case summary: A 55-year-old man presented with a three-month history of painless, progressively enlarging swelling of the right thigh. Ultrasound, contrast-enhanced computed tomography (CT), and magnetic resonance imaging (MRI) revealed a large multiloculated intramuscular cystic lesion in the medial/posteromedial right thigh involving the adductor magnus and gracilis muscles and measuring 9 × 8.5 × 18 cm. MRI demonstrated CE2/CE3b features, with daughter cysts and floating membranes. Serology for echinococcosis was positive. Preoperative contrast-enhanced CT of the chest, abdomen, and pelvis excluded hepatic, pulmonary, abdominal, pelvic, or other visceral hydatid localizations, supporting a primary intramuscular presentation. The patient underwent complete cystopericystectomy. The cyst was removed intact, and brownish fluid containing membrane fragments was released only after specimen excision. Histopathology confirmed hydatid disease. Albendazole 400 mg twice daily was administered postoperatively for 3 months. At 12-month follow-up, contrast-enhanced CT of the chest, abdomen, pelvis, and thigh showed no local recurrence or newly developed visceral disease. Conclusion: Primary intramuscular hydatid cyst should be included in the differential diagnosis of multiloculated soft-tissue masses, even in non-endemic settings. Multimodal imaging and serology are essential to avoid biopsy-related complications, to establish the primary nature of the lesion, and to plan radical excision safely. Core tip: Primary intramuscular hydatid cysts are rare soft-tissue lesions that may be mistaken for benign or malignant tumors. This case, supported by extensive imaging and histopathology and contextualized within recent literature, is notable for its giant size, multistage CE2/CE3b imaging pattern, involvement of the adductor magnus and gracilis muscles, absence of visceral disease on staging CT, and successful complete cystopericystectomy followed by postoperative albendazole and CT-confirmed disease-free follow-up at 12 months. It emphasizes the importance of accurate preoperative diagnosis to guide surgical management and prevent complications such as cyst rupture or anaphylaxis.