Dedifferentiated Liposarcoma Presenting as Primary Diffuse Peritoneal Sarcomatosis: A Case Report and Literature Review.

BACKGROUND: Dedifferentiated liposarcoma (DDLPS) typically presents as a solitary retroperitoneal mass; diffuse granular and nodular peritoneal sarcomatosis as the inaugural manifestation is exceptional. We report such a case to highlight a critical diagnostic consideration: distinguishing primary sarcomatous peritoneal dissemination from the more common epithelial peritoneal carcinomatosis. CASE PRESENTATION: A 72-year-old Chinese man with no prior abdominal surgery presented with incidentally discovered multifocal intra-abdominal masses. CT demonstrated multiple peritoneal and retroperitoneal masses with encasement of the jejunum and transmural infiltration of the descending colon, and a retroperitoneal plaque abutting the left kidney. CT-guided biopsy confirmed DDLPS via MDM2 and CDK4 amplification on fluorescence in situ hybridization. Multidisciplinary team consensus directed surgical exploration for impending dual-site bowel obstruction. Laparotomy revealed diffuse granular and nodular peritoneal deposits (Peritoneal Cancer Index 28/39); cytoreductive surgery achieved a completeness of cytoreduction score of 2. Histopathology confirmed FNCLCC Grade III DDLPS. Postoperative next-generation sequencing demonstrated high-level co-amplification of CDK4, MDM2, TSPAN31, CCND2, MDM4, and RAC1, with microsatellite stability and tumor mutational burden of 0 mutations/Mb. CONCLUSION: DDLPS can present with primary diffuse peritoneal sarcomatosis even with a retroperitoneal component. When diffuse peritoneal implants yield an epithelial-marker-negative spindle cell neoplasm on biopsy, sarcoma must be considered in the differential diagnosis and FISH for MDM2/CDK4 should be performed to avoid misdiagnosis as peritoneal carcinomatosis from gastric, colorectal, or ovarian cancer.