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How Radiation Therapy Works:

Radiation therapy works through the localized release of ionizing radiation, which damages the DNA of cells either directly, or more commonly, indirectly, through the formation of free radicals and reactive oxygen species.

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Radiation Therapy for Cancer: Questions and Answers http://www.cancer.gov/cancertopics/factsheet/therapy/radiation

A happy patient

The major mechanism of radiotherapy-induced tumor shrinkage is mitotic cell death, in which the double and single-stranded breaks in a cell's DNA prevent that cell from replicating. Radiation therapy can also cause direct cell killing through a mechanism called apoptotic cell death.

Palliative vs. Curative Intent:

Clinically, radiation therapy is categorized as palliative or curative intent.

PALLIATIVE: The goals of Palliative Radiation Therapy are to relieve pain or improve function and/or quality of life in a patient with disseminated disease or other life limiting condition.

CURATIVE INTENT: The goal of Curative Intent Radiation Therapy, as the name implies, is to cure the animal's cancer by eliminating both gross and microscopic disease.

Terminology:

A radiation therapy protocol is described by its fractionation schedule and the number of Gray per fraction.

Gray (Gy) refers to the dose unit of radiation therapy.

specific for ionizing radiation
energy deposited / tissue mass
Joules/kg

Fractionation schedule refers to the number of treatments a patient will receive over the entire course of radiation therapy.

The number of fractions and Gray per fraction may vary depending on the protocol (curative intent vs. palliative ), tumor type, tumor location and other patient factors.

Side Effects of Radiation:

The side effects of Radiation Therapy are categorized as Acute Effects or Late Effects. These occur only in the irradiated field.