A blood treatment method includes the steps of inducing flow of a patient's blood through a blood treatment device inlet and outlet in fluid connection to the circulatory system of the patient. Metastatic deoxyribonucleic acid (DNA) contained within patient blood is destroyed by passing a patient's blood over a bioreactor surface having attached or immobilized deoxyribonuclease 1 (DNase 1) enzyme. The blood treatment device which consists of a bioreactor containing immobilized DNase 1, enables continuous treatment of a patient's blood and increases the effective concentration of DNase 1 in a patient's bloodstream to convert metastasizing cancer DNA in blood into non-oncogenic nucleotide fragments in vivo without adding any chemicals to the blood of the patient.