Modality	Description	Indications/uses	Administration
External beam radiation therapy
Three-dimensional conformal radiation therapy	CT or MRI is used to target tumors while minimizing radiation exposure of healthy tissues	Most solid tumors	Daily outpatient treatments (as short as one to two minutes each), administered Monday through Friday for two to seven weeks; overlying skin may be marked with freckle-size tattoos or colored ink marks to guide the radiation beam; a mesh face mask or body mold may be used to immobilize the patient
Four-dimensional radiation therapy	Computer-assisted tracking or gating of CT images of moving targets	Tumors that are susceptible to movement, most commonly in the lung, liver, pancreas, or breast	Similar to three-dimensional conformal therapy; for gating, patients may be asked to hold their breath while the radiation beam is activated
Intensity-modulated radiation therapy	The radiation beam is divided into components (“beamlets”), which permits sparing of normal tissues	Tumors surrounding or adjacent to normal critical structures, most commonly head and neck or prostate cancers	Similar to three-dimensional conformal therapy, although individual treatments may last more than 30 minutes
Stereotactic radiosurgery (e.g., Gamma Knife)	Multiple radiation beams converge on target tumor, delivering high-dose radiation to the tumor, but little to surrounding tissues	Intracranial lesions, such as brain metastases, meningiomas, acoustic neuromas, arteriovenous malformations, and trigeminal neuralgia	Single treatment; to ensure proper patient positioning and immobility, a positioning frame is secured to the patient's skull, then attached to the radiation source; treatment lasts 45 to 60 minutes
Stereotactic body radiation therapy (e.g., Cyberknife)	High-dose radiation delivered using robotic guidance	Treatment of spine tumors, localized lung cancer, and other tumors in patients who are not candidates for surgery	Most commonly delivered as three to five fractions; during treatment, a robotic arm containing the radiation source (a linear accelerator) rotates around the patient to deliver radiation from multiple positions; each treatment lasts up to two hours; positioning may be accomplished using fiducial markers placed beforehand or using a rigid body frame
Internal radiation therapy
Temporary brachytherapy implant	A radiation source is placed within or near the tumor target and is subsequently removed	Cervical cancer, sarcoma, vaginal cancer, oral cavity cancers	Catheters (smaller) or applicators (larger) are placed in body cavities or tissues; subsequently, the radiation source is placed within these devices; the patient may be hospitalized in a private room during treatment (radiation source is left in place throughout treatment), or the patient may undergo outpatient treatment for up to several weeks (radiation source is removed between treatments)
Permanent brachytherapy implant	A low-dose rate (i.e., long half-life) radiation source is placed within or near the tumor target	Prostate cancer	Radioactive seed implants are inserted into target tissue through a catheter under local or general anesthesia; initially, the patient may be required to limit social contacts after placement for up to one month; implants are never removed, but radiation dissipates within six months
Systemic radiation therapy	Systemically administered radioisotopes target tumor cells	Iodine-131 for thyroid cancer; strontium-89 and samarium-153 for painful bony metastases; yttrium-90 ibritumomab tiuxetan (Zevalin) and iodine-131 tositumomab (Bexxar) for non-Hodgkin lymphoma	Administered intravenously or orally; inpatient or outpatient, depending on specific treatment; patients are required to follow radiation precautions (careful disposal of body fluids, including urine, sweat, and tears; hand washing; condom use) for one week after treatment