Rationale: Immunoglobulin (Ig)G4-related pseudotumors of the liver are very rare diseases that are difficult to distinguish from malignant tumors. They can be usually improved by steroid therapy. Actinomycosis is a chronic, suppurative, granulomatous infection, for which immune suppression is a predisposing factor. It can also mimic malignant tumors. Patient concerns: A 67-year-old man presented with mild abdominal discomfort and a 5-kg weight loss for 3 months. Initially, he visited another hospital and was treated with antibiotics under the assumption of a liver abscess. Symptom was not resolved. Diagnoses: He diagnosed as having an IgG4-related pseudotumor of the right lobe of the liver after liver biopsy. Despite 2 months of steroid therapy, the liver mass was aggravated and invaded the right lung, as observed on follow-up computed tomography scan. Interventions: We performed en bloc resection of the tumor under the assumption that it was a malignant tumor. Outcomes: The pathology of the tumor was revealed as actinomycotic colonies and IgG4-positive plasma cells of the liver. He recovered well and was discharged with ursodeoxycholic acid tablet for 14 days. After 3 months, he underwent postoperative followup CT and there was no remarkable finding in remnant left hepatic lobe. Lessons: Hepatic actinomycosis and IgG4-related pseudotumors of the liver are both difficult to diagnose. As in our patient, combined diseases are more difficult to diagnose and to determine the optimal treatment. Since immunosuppression therapy of autoimmune diseases can cause and aggravate infection, management must be approached carefully. We can learn that various possibilities must be considered before diagnosing and treating a hepatic mass.
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