An undifferentiated (embryonal) liver sarcoma (ULS) originates from a primitive mesenchymal cell, with a predilection for childhood and very rare occurrence in adults. We report a case of a ULS that was incidentally found in a 53-year-old female. Our case was initially interpreted as a large hydatid cyst, which was later suspected to be a neoplastic lesion because its size was increasing and a solid portion was newly detected after shrinkage of the cyst following drainage. The patient underwent successful right hepatic lobectomy with complete resection, and is currently disease-free without adjuvant therapy. Although it is difficult to diagnose a hepatic cyst as a ULS due to its rare occurrence in adulthood and lack of specific findings, its possibility should be considered, especially when its size is increasing, because early diagnosis and curative resection are necessary for a favorable outcome.
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