Evaluation of Abemaciclib and Radiation Therapy Before Surgery for the Treatment of High-Risk Adipocytic Retroperitoneal Sarcoma

This study is testing whether adding a drug called abemaciclib (a targeted cancer therapy) to radiation therapy before surgery can shrink a rare and aggressive type of fat-tissue cancer called high-risk dedifferentiated retroperitoneal sarcoma. This cancer grows behind the abdominal organs and is hard to treat. The goal is to make surgery more effective by treating the tumor first. **You may be eligible if...** - You are 18 or older with a newly diagnosed or locally recurrent retroperitoneal fat-tissue sarcoma (adipocytic sarcoma) with specific genetic features (MDM2 or CDK4 amplification) - Your tumor is considered removable with surgery - You are otherwise in good health with adequate blood counts, kidney, and liver function **You may NOT be eligible if...** - You have already received chemotherapy or radiation for this cancer - Your cancer has spread to distant parts of the body - You have a recent blood clot history (within 1 year) - You have a history of serious heart rhythm problems or sudden cardiac arrest - You are pregnant or breastfeeding Talk to your doctor to see if this trial is right for you.