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

Inclusion Criteria: * Subjects, \>= 18 years old, must have newly diagnosed or locally recurrent MDM2 or CDK4-amplified adipocytic sarcoma as determined by fluorescence in situ hybridization (FISH), or other clinically appropriate methodology in the opinion of the reviewing pathologist. Histologic or imaging evidence of the presence of a dedifferentiated component must be present * Subjects must have one or more measurable target lesions by Response Evaluation Criteria in Solid Tumors version 1.1 (RECIST v1.1), assessed via CT scan or MRI * At the time of study enrollment, subjects must have a tumor burden that is judged to be surgically resectable * Have plans to undergo neoadjuvant radiation therapy followed by surgical resection. Review and approval of final treatment plans for subjects receiving radiotherapy locally/at an outside institution must be reviewed and approved by a radiation oncologist investigator prior to initiation of radiotherapy * Absolute neutrophil count (ANC) \>= 1500/mm\^3 (\>= 1.5 GI/L) without granulocyte colony-stimulating factor support in the last 14 days (subjects may not have received blood product transfusion or granulocyte colony-stimulating factor \[G-CSF\] within 14 days prior to screening) * White blood cell count \>= 2500/mm\^3 (\>= 2.5 GI/L) (subjects may not have received blood product transfusion or G-CSF within 14 days prior to screening) * Platelets \>= 100,000/mm\^3 (\>= 100 GI/L) (subjects may not have received blood product transf…