Synchronous Retroperitoneal Pleomorphic Liposarcoma and Colonic Mucinous Adenocarcinoma.

Introduction: Multiple primary malignancies (MPM), defined as two or more distinct primary tumors in 1 patient, are uncommon, with synchronous presentations (diagnosed within 6 months) being rarer. The coexistence of retroperitoneal liposarcoma and a synchronous gastrointestinal malignancy, particularly colorectal adenocarcinoma, is an exceptionally rare clinical entity, posing significant diagnostic and therapeutic challenges. Case Presentation: A 63-year-old male presented with fatigue, anorexia, and weight loss. The initial workup revealed microcytic anemia and positive fecal occult blood. While initial endoscopic biopsies were inconclusive, a contrast-enhanced CT scan identified two distinct masses: a large (20 cm) heterogeneous retroperitoneal mass suggestive of liposarcoma and a thickened right colic flexure. The patient underwent a combined en bloc surgical resection, including an extended right hemicolectomy and resection of the retroperitoneal mass with the right kidney. Histopathology confirmed two high-grade primary malignancies: a retroperitoneal pleomorphic liposarcoma (stage IIIb) and a colonic mucinous adenocarcinoma (stage pT3 pN2b). Adjuvant chemotherapy (CAPOX regimen) was initiated for high-risk colon cancer. Conclusion: This case highlights the necessity of a high index of suspicion for synchronous malignancies in patients with non-specific constitutional symptoms. Comprehensive cross-sectional imaging is crucial for accurate staging. Management requires a multidisciplinary approach to plan complex surgical interventions and tailor adjuvant therapy, often prioritizing the more aggressive or higher stage malignancy. This report underscores the complexity of MPM and the need for individualized patient care.