Systemic means that a drug (in the case of cancer, a chemotherapeutic agent) is introduced into the bloodstream, allowing it to act on the entire organism. A chemotherapeutic agent must therefore be able to distinguish between healthy cells and cancer cells, in the same way that an antibiotic must be able to differentiate between the normal cells of the body and harmful bacteria.
Rapid cell division often characterizes cancer cells. There are, however, also healthy cells that undergo frequent cell division, such as the hair follicles, digestive tract, and the bone marrow. Although a course of chemotherapy may destroy up to 99.9% of the cancer cells, it is almost inevitable that severe side effects will befall these healthy cells. Moreover, cancer cells may very quickly become resistant to the chemotherapeutic agent by mutating, or, in other words, undergoing a change in genetic material.
Systemic therapy is used on diffuse tumors, such as leukemia (blood cancer). They also include other metastases that cannot be diagnosed and localized due to their microscopic size. Chemotherapy is also a suitable treatment method for solid tumors that cannot be treated via surgery or radiation.