Primary Sclerosing Epithelioid Fibrosarcoma of the Kidney: A Case Report and Review of the Literature

Rare case report: sclerosing epithelioid fibrosarcoma with FUS-CREB3L1 gene fusion

Sclerosing epithelioid fibrosarcoma (SEF) is a rare soft tissue malignancy frequently misdiagnosed due to its overlapping immunohistochemical and molecular features with low-grade fibromyxoid sarcoma (LGFMS). We present the case of a 60-year-old male who initially presented with a mass in the left thigh four years ago, which significantly increased in size over the past year. MRI of the femur revealed a large, well-circumscribed mass in the mid and lower left thigh. Surgical excision of the tumor and associated thrombus in the ipsilateral blood vessel was performed. Histomorphological analysis confirmed a pure SEF with no myxoid stroma, aiding in its differentiation from LGFMS. Immunohistochemical staining revealed diffuse and strong MUC4 positivity, while next-generation sequencing (NGS) demonstrated molecular characteristics consistent with LGFMS, specifically FUS-CREB3L1 gene fusion. This case underscores the asynchrony between the pathological morphology and molecular features of soft tissue tumors during their development and differentiation. Although histologically low-grade, SEF typically exhibits a high rate of local recurrence and distant metastasis. We diagnosed this case as SEF and recommended an aggressive clinical treatment regimen. This report aims to raise awareness of the diagnostic challenges associated with SEF and LGFMS.

Sclerosing Epithelioid Fibrosarcoma

Sclerosing epithelioid fibrosarcoma (SEF) is a distinctive sarcoma that may arise in nearly any soft tissue site or bone. While there has been past controversy as to whether it is related to low-grade fibromyxoid sarcoma (LGFMS), it has been shown to behave far more aggressively than LGFMS. SEF has a propensity to metastasize to the lungs and bone and arise within the abdominal cavity. Histologically, it is characterized by uniform nuclei embedded in a densely collagenous stroma simulating osteoid. By immunohistochemistry, it is often strongly positive for MUC4. The majority (75%) have EWSR1 gene rearrangement, most commonly with CREB3L1 as a fusion partner, although a variety of FUS/EWSR1 and CREB3L1/CREB3L2/CREB3L3 fusions have been described in addition to others. SEF is currently recalcitrant to nearly all chemotherapy and radiation therapy.

Sclerosing epithelioid fibrosarcoma of the kidney

A woman in her 20s presented with headache and back pain and was found to have a left renal mass with bony metastases. She underwent nephrectomy, and on histopathology was initially diagnosed with stage 4 clear cell sarcoma of the kidney. She underwent palliative radiation and chemotherapy; however, the disease progressed and she came to our centre. We started her on second-line chemotherapy and submitted her tissue blocks for review. Due to her age and lack of sclerotic stroma in the tissue, we had our doubts about the diagnosis and hence, tissue sample was submitted for next-generation sequencing (NGS). NGS detected an EWSR1::CREBL1 fusion, clinching the final diagnosis of sclerosing epithelioid fibrosarcoma of the kidney, a singular diagnosis rarely reported in the literature. Currently, the patient is post her third line of chemotherapy, is on maintenance, and is doing well and has resumed her daily activities.