• Patients must have histologically confirmed CEA expressing locally advanced rectal cancer (T3, T4 and N0 or N plus \[+\])

• Patients must be 18 years of age or older

• The effects of 64Cu-M5A on the developing fetus are unknown. For this reason, women of child-bearing potential and men must agree to use adequate contraception (hormonal or barrier method of birth control or abstinence} prior to study entry and for six months following duration of study participation. Should a woman become pregnant or suspect that she is pregnant while participating on the trial, she should inform her treating physician immediately

• Patients must have a known site of disease. Patients must be scheduled to undergo neoadjuvant chemotherapy and radiation therapy followed by surgery

• Patients who are planned for total neoadjuvant therapy where additional chemotherapy is planned before neoadjuvant chemoradiotherapy (CRT) are also eligible

• Although not mandated by the protocol, the results of the computed tomography (CT), magnetic resonance imaging (MRI) and fludeoxyglucose F-18 (FDG) PET scans and labs (blood cell count \[CBC\], comprehensive metabolic panel \[CMP\]) that are performed as part of the standard work up should be available and should have been done within 2 months prior to study entry

• All subjects must have the ability to understand and the willingness to sign a written informed consent

• Prior therapy (chemotherapy, immunotherapy, radiotherapy) must be completed at least 2 weeks prior to infusion of radiolabeled antibody