UESL is a primary mesenchymal malignant liver neoplasm, and the concept of this neoplasm was first proposed by Stocker and Ishak in 1978 [1]. It accounts for approximately 9–13% of primary liver tumors in children [3]; however, it is extremely rare in adults, accounting for approximately 0.2% of the primary liver tumors [4]. Furthermore, UESL has a worse prognosis in adults, with a 5-year overall survival (OS) rate of 48.2% compared to 84.4% in children [5, 6]. The standard treatment strategy for UESL has not been established, and there is no drug for the treatment of UESL. However, recent studies have shown that margin-negative resection improves the OS [5, 6]. Currently, most patients receive vincristine, actinomycin-D, cyclophosphamide (VAC), or ifosfamide based on the Intergroup Rhabdomyosarcoma Study protocol [7]. To our knowledge, only 16 adult patients over the age of 18 years, including our patient, have survived for more than 48 months [3, 4, 8,9,10,11,12,13,14,15,16,17,18,19] (Table 1). With the exception of one patient who underwent liver transplantation after neoadjuvant chemotherapy [15], all the other patients received initial surgical treatment. We found that 10 patients, including our patient, who survived for more than 48 months had recurrence after the primary surgery. Of the 10 patients who relapsed, eight (80%) presented with hepatic recurrence. Although the high recurrence rate of residual liver tissue indicates that current radical resection and chemotherapy may not be sufficient to achieve complete resolution, long-term survival appeared possible even in cases with recurrence by combining surgery and chemotherapy (and/or radiation therapy). Liver transplantation for UESL has been performed to control hepatic recurrence, and there have been reports of patients who have survived for more than 10 years with the combination of chemotherapy and liver transplantation for unresectable cases [3, 20]. However, the number of such cases is small, and careful consideration and more extensive studies are required to determine the indications for combined chemotherapy and liver transplantation for adult UESL in the future.

UESL is commonly characterized by a lack of differentiation tendency, without specifically oriented differentiation, such as vessels, striated muscle, smooth muscle, fat, and nerves, and there is no specific immunohistochemical pattern [1]. In UESL, pathological findings reveal a proliferation of spindle, oval, or stellate pleomorphic immature cells with poorly defined cell borders embedded in the mucinous stroma; and near the tumor margin, entrapped bile duct-like structures surrounded by tumor cells were observed [1]. It often accompanies multinucleated giant cells and d-PAS-positive granule-containing cells [1].

In our case, immunohistochemical analysis showed diffusely positive expression of vimentin, α1ACT, and α1AT and was focally positive for desmin, α-SMA, glypican-3, and DOG-1. Local expression of myogenic markers and DOG-1 may reflect the local differentiation trend in the undifferentiated tumor. There are no prior reports on the expression of DOG-1 in UESL. DOG-1 is a member of the transmembrane protein 16 family, featured as a calcium-activated chloride channel and expressed in GIST, but the detailed functions are unknown. Recently, the details of DOG-1 have been revealed gradually by reported expression in poorly differentiated tumors [21, 22], such as sarcomatous carcinoma of the liver, in lymph node metastasis of colorectal cancer [23], and as a poor prognostic factor in breast cancer [24]. The clinical significance of DOG-1 positivity in UESL is unclear; however, it may become more evident with the accumulation of cases.

The occurrence of UESL has not been understood, but Mori et al. reported that a hamartoma-like lesion is one form of UESL and that this type of case has a good prognosis [25]. It is yet to be determined whether hamartoma becomes malignant and transforms to UESL or some UESLs present with hamartoma-like lesions. Although hamartoma-like lesions were not observed in our case, further pathological analysis and treatment development may be the key to long-term survival of patients.

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