External Beam Radiation Therapy

External Beam Radiation Therapy

External beam radiation therapy

External-beam radiation therapy is most often delivered in the form of photon beams (either x-rays or gamma rays). A photon is the basic unit of light and other forms of electromagnetic radiation. It can be thought of as a bundle of energy. External beam radiation therapy consists of several different methods of delivering radiation with photons from an external source into the patient’s body to treat the cancer. These methods are described in greater detail below. 

Three-dimensional conformal radiation therapy (3-D CRT)

One of the more common types of external-beam radiation therapy is called 3-dimensional conformal radiation therapy (3D-CRT). 3D-CRT uses very sophisticated computer software and advanced treatment machines to deliver radiation to very precisely shaped target areas.  3D reconstructions of the patient’s anatomy and the target treatments areas are outlined by the radiation oncologist.  Once complete, the treatment team develops 3D reconstructions of the radiation.

Intensity modulated radiation therapy (IMRT)

IMRT uses dozens of small radiation beam-shaping devices, called multi-leaf collimators, to deliver a radiation. The collimators can be stationary or can move during treatment, allowing the intensity of the radiation beams to change during treatment sessions. This kind of dose modulation allows different areas of a tumor or nearby tissues to receive different doses of radiation. Unlike other types of radiation therapy, IMRT is planned in reverse (called inverse treatment planning). In inverse treatment planning, the radiation oncologist chooses the radiation doses to different areas of the tumor and surrounding tissue, and then a high-powered computer program calculates the required number of beams and angles of the radiation treatment to achieve this dose. In contrast, during traditional (forward) treatment planning such as 3D-CRT, the radiation oncologist chooses the number and angles of the radiation beams in advance and computers calculate how much dose will be delivered from each of the planned beams. The goal of IMRT is to increase the radiation dose to the areas that need it and reduce radiation exposure to specific sensitive areas of surrounding normal tissue. Compared with 3D-CRT, IMRT can reduce the risk of some side effects, such as damage to the salivary glands (which can cause dry mouth, or xerostomia), when the head and neck are treated with radiation therapy. However, with IMRT, a larger volume of normal tissue overall is exposed to a low dose of radiation. Also, whether IMRT leads to improved control of tumor growth and better survival compared to 3D-CRT is not yet known.  Depending on your diagnosis, 3D-CRT may be a better delivery method than IMRT for your treatment.

Stereotactic radiosurgery (SRS)

Stereotactic radiosurgery (SRS) delivers a single high dose of precisely targeted radiation therapy to a tumor in the brain. The targeted lesion can be malignant such as a metastatic tumor or benign such as an arteriovenous malformation (AVM).  This method requires high precision and significant quality assurance due to the single high dose of radiation.  This results in a longer time period to deliver the treatment.  Often this type of treatment is delivered with a special machine that only delivers this type of treatment.

Stereotactic body radiation therapy (SBRT)

Stereotactic body radiation therapy (SBRT) delivers radiation therapy most often in 1-5 sessions, using smaller radiation fields and higher doses than in most cases. By definition, SBRT treats tumors that lie outside the brain. Because these tumors can sometimes move with the normal motion of the body and therefore cannot be targeted as accurately as tumors within the brain, SBRT is usually given in more than one dose. SBRT can be used to treat small, isolated tumors, including cancers in the lung, bone and liver.

Volumetric modulated arc therapy (VMAT)

VMAT (Volumetric Modulated Arc Therapy) establishes new standards for radiation therapy treatment speed and dose reduction to the patient. It is similar to IMRT but improves the speed of treatment delivery by using single or multiple radiation IMRT beams in uninterrupted arc(s) around the patient. Doctors can use VMAT with complete or partial arc(s) to reduce treatment times. 3D volume imaging technology integrated into the treatment systems increases the precision of VMAT. This enables physicians to visualize the tumor target at the time of treatment and to guide therapy that both maximizes the radiation dose to the target and minimizes exposure to surrounding healthy tissues.


Intra-Operative Radiation Therapy (IORT)

Intraoperative radiation therapy (IORT) is a limited form of radiation therapy that delivers a concentrated dose of radiation to a tumor bed during surgery. This advanced technology may help kill microscopic disease, reduce radiation treatment times or provide an added radiation "boost". This form of radiation therapy can be given as external beam therapy (photon or electron) or as brachytherapy. 

  • IORT is helpful when vital normal organs are too close to the tumor because it allows your radiation oncologist to avoid exposing those organs to radiation. 

  • IORT may help some patients finish treatment and get back to their lives quicker by reducing the need for additional radiation therapy, which is typically given over five to six weeks. The IORT treatment itself takes about four to five minutes. 

  • Patients who must receive additional radiation therapy following surgery can receive a boost of radiation during IORT. After they have recovered from the surgical procedure, they can continue with their radiation treatments, with typically fewer complications.


Electron beam radiation therapy

In conventional radiation therapy, sometimes radiation is delivered with electrons instead of photons. This often referred to as electron beam radiation therapy. Electrons are negatively charged particles and due to their low energy they do not penetrate deeply into the body. Therefore this type of radiation is most often used to treat skin cancer or tumors very close to the surface of the body.  Electrons are not used for tumors deep within the body.