A Case Report: Retroperitoneal dedifferentiated liposarcoma with heterologous myogenic differentiation and testicular metastasis

Dedifferentiated liposarcoma (DDLPS) is the most common retroperitoneal soft tissue sarcoma, characterized by MDM2 and CDK4 gene co-amplification. However, DDLPS with heterologous myogenic differentiation is extremely rare and poses a significant diagnostic challenge due to its expression of myogenic markers such as desmin, myogenin, and MyoD1, which can easily lead to misclassification as primary rhabdomyosarcoma. Testicular metastasis of DDLPS is even rarer, with only isolated case reports documented in the literature. We report a case of a 46-year-old male who presented with a large retroperitoneal mass without testicular involvement on initial imaging. He underwent en bloc resection of the retroperitoneal tumor including right nephrectomy and right hemicolectomy. Ten days postoperatively, he developed a right scrotal mass, prompting radical orchiectomy. Initial pathological evaluation suggested rhabdomyosarcoma based on desmin and MyoD1 positivity. However, subsequent expert pathological consultation with fluorescence in situ hybridization (FISH) analysis revealed MDM2 and CDK4 gene amplification in both specimens, establishing the final diagnosis of DDLPS with prominent heterologous myogenic differentiation. Postoperative chest CT demonstrated early pulmonary metastases, underscoring the aggressive nature of this tumor. This case highlights the critical importance of MDM2 / CDK4 FISH testing in accurately diagnosing DDLPS with heterologous differentiation, particularly when myogenic markers are expressed, to avoid misdiagnosis and guide appropriate management.