Chronic intramuscular abscess masquerading as soft tissue neoplasm on biopsy and hydatid cyst on imaging: a diagnostic challenge and pitfall.

Introduction: Chronic intramuscular abscess is a rare condition that may clinically and radiologically mimic neoplastic or parasitic lesions, creating significant diagnostic challenges. Case presentation: An 18-year-old immunocompetent male presented with a progressively enlarging swelling over the medial aspect of the right thigh for 18 months. An initial incisional biopsy suggested a benign hamartomatous lesion, whereas magnetic resonance imaging (MRI) favored a hydatid cyst. Surgical exploration revealed a thick-walled intramuscular cystic lesion containing dark grayish fluid. Histopathologic examination and cytology confirmed a chronic walled-off intramuscular abscess (chronic pyomyositis). Complete excision was performed. Following histopathologic confirmation, albendazole therapy was discontinued, and the patient was treated with antibiotics. The postoperative course was complicated by hematoma formation requiring evacuation, after which recovery was uneventful. Discussion: Chronic pyomyositis may present without systemic inflammatory signs and can mimic neoplastic or parasitic lesions both clinically and radiologically. In this case, prolonged disease progression, normal laboratory findings, and cystic MRI appearance contributed to diagnostic discordance. The misleading biopsy findings were likely related to the sampling of peripheral reactive fibrous tissue rather than the purulent core. This case highlights the limitations of imaging and biopsy in complex cystic soft tissue lesions. Conclusion: Chronic intramuscular abscess should be considered in the differential diagnosis of cystic soft tissue masses, particularly in endemic regions where hydatid disease is prevalent. Careful clinicoradiologic correlation and definitive histopathologic evaluation remain essential for accurate diagnosis and appropriate management.