Spontaneous regression and subsequent metachronous development of dedifferentiated liposarcoma: a case report

Dedifferentiated liposarcoma (DDLPS) is a high-grade liposarcoma, and spontaneous regression of the primary tumor is extremely uncommon. We describe a patient with primary DDLPS of the lower abdominal wall that regressed without treatment and subsequently developed a second DDLPS at a different site. A 66-year-old man presented with a painful mass in the left lower abdomen. MRI demonstrated an infiltrative soft tissue tumor without a definite fatty component. Repeated biopsies showed spindle cell proliferation with inflammatory myofibroblastic features, and MDM2 amplification was confirmed by fluorescence in situ hybridization, leading to the diagnosis of DDLPS. Although wide resection was planned, the tumor gradually decreased in size after biopsy and almost completely disappeared during follow-up. Nine years later, a new tumor developed in the right lower abdominal wall. Wide resection was performed, and pathological examination demonstrated high-grade DDLPS with adjacent well-differentiated liposarcoma component. Review of the imaging and pathological findings suggested that the initial lesion had undergone spontaneous regression and that the second lesion represented metachronous dedifferentiation arising in a background of well-differentiated liposarcoma. This case illustrates an unusual clinical course of DDLPS. These findings suggest that spontaneous regression of the dedifferentiated component may occur and that metachronous dedifferentiation can subsequently develop at a different site during long-term follow-up.