Chemoradiotherapy

The major modalities for the treatment of cancer patients are surgery, radiotherapy, and chemotherapy, all of which aim to eliminate all cancer cells. Chemo-radiotherapy is the combination of chemotherapy and radiation therapy administered to cancer patients. Adjuvant chemotherapy refers to the application of chemotherapy after radiation or surgery to control micrometastatic tumor and to try to increase the cure rate for breast and colorectal cancer. Neoadjuvant chemotherapy, on the other hand, is the use of chemotherapeutic agents prior to radiotherapy or surgery, causing initial shrinkage of tumors.

The first clinical use of chemotherapy was documented in the early 1940s when the use of nitrogen mustard obtained a brief remission in a patient with lymphoma. Since then, many different types of drugs or biological agents that are cytotoxic (cell killing), hormonal, monoclonal antibodies, or small molecules have been developed to treat cancer patients either as single drugs or in combination with others. The goal of these drugs is to kill cells. Ideally, the chemotherapeutic agents would target and eradicate only growing tumor cells, but in reality, healthy cells are often killed, which leads to side effects such as hair loss and vomiting. The efficacy of chemotherapy is sometimes hindered when tumor cells become resistant to the chemotherapeutic agents due to cellular mechanisms such as decreased uptake and increased efflux of drugs.

Radiation therapy refers to cancer treatment using ionization radiation such as X-rays or gamma (γ) rays that can be delivered by various methods such as external beam radiotherapy with a linear accelerator. Brachytherapy is another type of clinical radiotherapy that gives powerful localized radiation doses from wires or seeds containing radioisotopes that give out radioactive decay while being implanted within an [Page 378]organ or tissue. Radiotherapy can provide high curative rates in cancers such as Hodgkin's disease and testicular cancer. While being absorbed by the cells, radiation causes random injury to the DNA. Efficacy is determined by the extent of cellular damage that is beyond repair. In general, cell killing is achieved when the normal cells are more effective in repairing themselves, causing more death in the cancer cell population. The dose of radiation is quantified as the amount of energy absorbed per unit mass and the standard unit is the gray (Gy), which is defined as one joule per kilogram. When delivered to a tumor, the dose of radiation is limited to the damage to the surrounding healthy cells. It is also dependent on several factors such as the goal of the therapy (curative versus palliative), relative sensitivity of the specific cancer cells to radiation, the volume of cancer, and condition of the patient.