The goal of any treatment is to kill the cancer cells
 This treatment option works by "poisoning" the cancer cells. This is a treatment that impacts the whole body. Specifically chemotherapy kills any fast-growing cells in the body which means it targets cancer and healthy cells that grow quickly. For example, it targets hair follicle cells or the digestive system hence some of the side effects are hair loss and nausea.
Radiotherapy works by "burning " the cancer cells. Unlike chemotherapy this treatment is very targeted to the tumor. It is used to treat cancer in one specific location in the body.
There are four stages of cancer. In late stages of cancer (third and fourth), the cancer has already spread to multiple parts of the body. Therefore chemotherapy is highly recommended as it will target all those areas. You can not receive whole body radiation because it damages the organs of the body.