Primary clear cell sarcoma of the femur: a unique case with RT-PCR and direct sequencing confirmation of EWSR1/ATF1 fusion gene

Primary Clear Cell Sarcoma of Bone: Clinicopathologic Study of a Rare Presentation

Clear cell sarcoma (CCS) is an uncommon malignant mesenchymal neoplasm of young adults with a predilection for tendons and aponeuroses of distal extremities, a distinctive nested growth pattern, melanocytic differentiation, and usually an EWSR1::ATF1 fusion. Distinction from melanoma can be challenging but is critical for clinical management. Rare cases of primary bone CCS have been reported. The purpose of this study was to evaluate the clinicopathologic features of a series of primary bone CCS. Three cases of primary bone CCS were identified out of 140 CCS diagnosed between 2010 and 2021. Two patients were female, and 1 patient was male; ages were 19, 47, and 61 years. All tumors arose in the long bones of the extremities (femur, humerus, fibula). Two tumors also involved regional lymph nodes at presentation. Two showed characteristic histologic features, in the form of nests and fascicles of uniform epithelioid to spindle cells with prominent nucleoli and pale eosinophilic to clear cytoplasm; 1 tumor showed sheet-like growth, unusual focal pleomorphism, and more notable nuclear atypia. By immunohistochemistry, S100 protein was positive in 2/3 cases, SOX10 in 3/3, HMB-45 in 2/3, MiTF in 2/2, and melan A in 1/3. All cases were confirmed to harbor EWSR1 rearrangement and EWSR1::ATF1 fusion or t(12;22). On follow-up, all 3 patients developed metastases and died of disease, 5, 18, and 21 months after diagnosis. In summary, CCS rarely presents in the skeleton. At such locations, distinction from metastatic melanoma is particularly challenging. Clinical and pathologic features are similar to conventional CCS of soft tissue. Primary bone CCS may pursue an aggressive clinical course.

Patellar dislocation following distal femoral replacement after extra-articular knee resection for bone sarcoma: A case report

BACKGROUND

For the treatment of bone sarcoma in the distal femur, wide-margin resection and knee reconstruction with tumor endoprosthesis are standard therapies. Extra-articular knee resection is required in cases of tumor invasion of the knee joint; however, the incidence of complications, such as aseptic loosening, prosthesis infection, and implant failure, is higher than that following intra-articular knee resection. To the best of our knowledge, there are three reports of patellar dislocations after replacement of a tumor endoprosthesis.

CASE SUMMARY

A 36-year-old man with no significant past medical history was admitted to our institution with continuous pain in his left knee for 4 mo. An open biopsy was performed, and the patient was diagnosed with a left distal femoral malignant bone tumor. Extra-articular knee resection and knee reconstruction with a tumor endoprosthesis were performed. Although the alignment of the tumor prosthesis was acceptable, knee instability was noticed postoperatively. The axial radiographic view of the patellar and computed tomography showed lateral patellar dislocation at 4 wk postoperatively. The patient had to undergo a lateral release and proximal realignment. He could perform his daily activities at 9 mo postoperatively. Radiography revealed no patellar re-dislocation.

CONCLUSION

Proximal realignment may be considered during primary surgery if there is an imbalance in the forces controlling the patellar tracking.