A substance being studied in the treatment of some types of cancer. M5A is a monoclonal antibody that binds to a protein called CEA on the surface of some tumor cells. It is linked to a radioisotope called yttrium Y 90, which may help kill the cancer cells. Yttrium Y 90 DOTA anti-CEA monoclonal antibody M5A is a type of radioimmunoconjugate and a type of radiopharmaceutical.