Indications for radiation therapy

Radiation therapy is one of three major modalities available to treat malignant disease (the other two major modalities being chemotherapy and surgery). The decision to use radiation therapy is guided by specific indications of disease. For example, malignant cells are usually killed by ionizing radiation. However, some tumours can sustain higher amounts of damage before being eradicated. In addition, the tolerance of some normal tissues to ionizing radiation will limit the total dose that can be delivered safely. Thus, the type of tumour and its location influence whether and which form of radiation therapy will be effective. Radiation therapy most often is used to treat tumours that cannot be removed surgically (e.g., brain tumours). However, it may be added to a patient’s treatment regimen following surgery when there is a high likelihood of the tumour’s recurring. Chemotherapy often is combined with radiation therapy to prevent recurrence as well as to treat or prevent the development of metastatic disease (the spread of cancer to other parts of the body).

Toxicities of radiation therapy

Radiation therapy is a local (targeted) treatment, and, hence, the side effects observed generally are limited to the tissues exposed directly to the radiation beams. Side effects may be temporary (acute) or long-lasting (chronic). For example, the rapidly dividing cells in the skin, in the mucosal lining of the oral pharynx (the region that forms the back of the mouth cavity and upper part of the throat) and the gastrointestinal tract, and in the bone marrow are susceptible to immediate, though temporary, damage as a result of radiation therapy. If those tissues are exposed to radiation therapy as part of tumour-directed treatment, skin reactions, inflammation of the mucosa (mucositis), and lowered blood-cell counts will result.

The development of long-term side effects of radiation therapy depends on which tissues are exposed to radiation beams, the age of the patient, and the dose of radiation. Long-term effects are usually permanent. In a young child, bone growth is affected when the growth plate is within the field of radiation therapy. Hormone deficiencies can occur at any age when the pituitary gland or thyroid gland is exposed to high doses of radiation. When children with brain tumours are treated with radiation therapy, neurological function and cognitive ability may be affected, resulting in learning difficulties, which can be severe if the child is very young at the time of treatment.

Secondary malignancies induced by radiation are one of the most-devastating consequences of radiation therapy. Examples of such malignancies include thyroid cancer, breast cancer, lung cancer, gastric and colorectal cancers, and sarcomas of bone and soft tissue. In order for a malignancy to be considered radiotherapy-induced, it must be of a different histology than the patient’s initial tumour, it must occur within the radiotherapy field of treatment, and it must occur after a latency period considered sufficient for induction of a radiation-induced cancer (generally five or more years).

Targeted radiation therapy

Along with advances in technology to eradicate malignant diseases, advances in cancer treatment have focused increasingly on the unique genetic make-up of tumours. Such research is expected to facilitate the development of targeted and individualized radiation therapy and lead to a better understanding of the development of some cancers.