Case Report: Pediatric mediastinal actinomycosis mimicking lymphoma diagnosed by tissue metagenomic next-generation...

Mediastinal actinomycosis is rare in children, and when it presents as a mass-like lesion, its clinical and imaging features overlap substantially with lymphoma, making differential diagnosis extremely challenging. We report a 2-year-1-month-old girl admitted with fever and cough. Contrast-enhanced chest computed tomography (CT) showed multiple enlarged mediastinal and bilateral hilar lymph nodes coalescing into a mass-like lesion with heterogeneous enhancement and small hypoenhancing foci, encasement of adjacent mediastinal vessels, and compression of the left main bronchus and the origin of the lingular bronchus. Magnetic resonance imaging (MRI) demonstrated heterogeneous signal intensity and enhancement; the radiologic differential included lymphoproliferative and granulomatous disease. Bone marrow biopsy, leukemia immunophenotyping, and tumor markers did not support malignancy. Ultrasound-guided biopsy of the mediastinal lesion revealed necrotizing granulomatous inflammation. Metagenomic next-generation sequencing (mNGS) of unstained tissue sections detected Actinomyces oris with mixed oropharyngeal flora, while Mycobacterium tuberculosis complex, fungi, viruses, and atypical pathogens were not detected. Pulmonary inflammation improved with antimicrobial therapy; however, repeat CT on January 28, 2026 showed little change in the mediastinal-hilar lesions. Because lymphoma could not be excluded, thoracoscopic partial resection was performed at another hospital, and postoperative pathology again showed granulomatous inflammation with caseous necrosis and negative acid-fast staining. Oral amoxicillin-clavulanate was continued postoperatively, in line with the principle of 2-6 weeks of intravenous therapy followed by 6-12 months of oral antibiotics for thoracic actinomycosis, with duration individualized to residual disease, imaging response, and drug tolerance. Follow-up ultrasonography on April 13, 2026 demonstrated reduction of the residual lesion, and the patient remained asymptomatic. This case highlights that pediatric mediastinal actinomycosis can mimic lymphoma and that integrated assessment of deep-tissue pathology, mNGS, serial imaging, and treatment response can guide diagnostic and therapeutic decision-making, preventing misdiagnosis and mistreatment.