Radiation therapy

Radiation therapy (or radiotherapy) is the medical use of ionizing radiation as part of cancer treatment to control malignant cells (not to be confused with radiology, the use of radiation in medical imaging and diagnosis). Although radiotherapy is often used as part of curative therapy, it is occasionally used as a palliative treatment, where cure is not possible and the aim is for symptomatic relief. Other rare uses are to wipe out the immune system prior to transplant to reduce the incidence of tissue rejection, called total body irradiation (TBI); to calm hyperactive muscles—such as might cause twitchy eyes—with mild superficial treatments; and to form scar tissue around a stent to reinforce the vascular wall.

Intensity-Modulated Radiation Therapy

Intensity-Modulated Radiation Therapy (IMRT) machines are a specialized case of 3D conformal therapy that employs a device called a multi-leaf collimator (MLC) to shape the beam intensity. This permits greater sparing of any radiosensitive organs that the beam passes through or near, while still providing the prescription dose to the target volume. IMRT employs inverse planning in which the physician specifies the target volume (sometimes called the planning target volume, or PTV) in the treatment planning system. The radiation oncologist will also specify the radiation tolerances of the neighboring organs in a dose-volume form (e.g. no more than 30% of the rectum can receive 70 Gy). The treatment planning system then calculates how to deliver a plan that meets these constraints. This plan is then exported to the linear accelerator (treatment machine) equiped with an MLC.

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