RecruitingDiagnostic testA Study of Response to Standard Treatment Before Surgery in People With Rectal CancerThe purpose of the study is to learn more about how the body responds to standard treatment (chemoradiation and chemotherapy). The study will use the results of testing down on participants' blook, tissue, and scans to learn more about how people with rectal cancer respond to chemoradiation and chemotherapy treatment and if it is useful for predicting whether a person's cancer get better, gets worse, or stats the same after treatment.