Diagnostic Pitfalls in Actinomycosis: Case Report of a Deep-Seated Thigh Mass Mimicking Malignancy

Abstract While typically associated with cervicofacial and thoracoabdominal infections, Actinomycosis, a chronic bacterial infection, can manifest in various ways, occasionally mimicking malignancy. This case report describes a 60-year-old woman presenting with an 8-month history of a progressively enlarging left thigh mass and weight loss. Imaging studies (MRI and FDG-PET) revealed a concerning soft tissue mass with features suggestive of malignancy. Initial core needle biopsy results pointed towards a myofibroblastic tumor. However, wide surgical excision and subsequent histopathological examination with special stains identified sulphur granules with Actinomycotic colonies and filamentous structures, confirming the diagnosis of Actinomycosis involving the left thigh muscle. This case emphasizes the importance of considering Actinomycosis in the differential diagnosis of soft tissue masses and maintaining a high index of suspicion for Actinomycosis, even in atypical presentations, is crucial for optimal patient care. Keywords: histopathological examination, mimick, malignancy, soft tissue mass, actinomycosis.