Treatment Options > Radiation Oncology > Overview

Radiation Therapy: Which type is right for me?

Eric Shinohara, MD, MSCI
Abramson Cancer Center of the University of Pennsylvania
Last Modified: June 4, 2007

The purpose of this article is to briefly discuss the types of radiation therapy that are in common use today. Radiation therapy can be classified according to the various types of radiation particles or waves that are used to deliver the treatment, such as photons, electrons, and protons. Of these, photons and electrons are widely available and thus most often used. Protons are currently only available at a handful of institutions; however, several new proton therapy facilities, including one at the University of Pennsylvania , are currently under development. Generally, the three major subtypes of radiation therapy have similar biological effects on tumor tissue, meaning that they all have relatively similar potential to eradicate tumors. However, each type does have certain advantages and disadvantages, and we will briefly discuss the situations and rationale behind why your radiation oncologist may pick one treatment modality over another. Your radiation oncologist is an expert at deciding which of these modalities to use to best deliver the radiation in your particular circumstance.

Photon Treatment

Photons can be used in several different types of radiation therapy. This includes orthovoltage radiation therapy, conventional radiation therapy, 3D conformal radiation therapy, Intensity Modulated Radiation Therapy (IMRT), brachytherapy, and stereotactic radiation therapy, also known as “stereotactic radiosurgery”. Photon beams are the same type of beam that are used in diagnostic X-ray machines, such as those used to take chest X-rays. However, in radiotherapy, much higher energy photon beams are used. Conventional radiation therapy, 3D conformal radiation therapy, and IMRT are generally all delivered by machines called linear accelerators, or “linacs” for short.

Conventional Radiation Therapy

In conventional radiation therapy, X-rays films are used to determine how best to position the radiation beams in order to adequately treat tumors. Typically, a machine called a fluoroscopic simulator is used to plan the radiation treatments. The bones seen on the X-ray are used as landmarks to determine where the tumor is and where to position the radiation beams around the patient in order to treat the tumor, but avoid normal organs. Planning can be done rapidly, and the patients can start treatment very quickly, as opposed to other techniques that require more in-depth planning. This type of treatment is excellent for a number of tumors and for the treatment of metastatic lesions.

3D Conformal Radiation Therapy

With improvements in CT imaging quality and availability, presently, most hospitals are using CT imaging to plan treatment for tumors in a process known as 3D conformal radiotherapy. The advantage of CT-guided therapy compared with conventional therapy is that CT-guided therapy allows the tumor and normal organs to be defined in three dimensions. In this type of therapy, a CT scan is obtained of the person in the position that they are to be treated. The tumor is then outlined in three dimensions on the CT scan. Normal organs which are located near the tumor and need to be avoided are also outlined in 3D (figure I). Beams are then arranged to best avoid normal organs while delivering an optimal dose of radiation to the tumor. Computer software is then used to calculate the amount of radiation the tumor and normal tissues receive in order to assure that all parts of the tumor are covered sufficiently, while no organ receives radiation doses that could damage its function. The beams of radiation can then be adjusted based on these calculations to further optimize the dose to the tumor and minimize the dose to normal organs. In addition to optimally positioning the beams, cerrobend blocks can also be used to shape the beam to avoid normal organs in the treatment field. These blocks can either be molded to the shape required, or in some linear accelerators, “leaves” within the linac can be used to form highly tailored beam shapes. Leaves (also known as multileaf collimators, or MLC's) are skinny metal blocks which are able to move quickly and independently to form different, complex patterns (figure II). These leaves are a critical component of IMRT, which we will discuss next.

Intensity Modulated Radiation Therapy (IMRT)

IMRT is basically just another way to deliver the same photons to treat a tumor, but has the potential to lower the high doses of radiation that normal structures can experience, compared to the modalities above. The process of planning IMRT also begins with a CT scan of the person in the position that they are to be treated. Similar to 3D conformal therapy, the tumor and normal organs are outlined on the CT with 3-dimensional information (figure III). Multiple beams are positioned at various points around the person to optimally deliver the radiation. However, in IMRT, these beams are divided into a grid- like pattern, separating the one big beam into numerous smaller “beamlets.” Special software is used to determine the best pattern of beamlets to use from each larger beam, in order to deliver the optimal amount of radiation to the tumor while sparing normal organs as much as possible. To deliver these patterns, the linac's leaves, or MLC's, form numerous different shapes, often 50 or more, during the course of a radiation treatment. The advantage of delivering radiation as beamlets to form these patterns is that very precise control of the radiation is obtained, which can be utilized when a tumor is in a difficult position to treat. For example, if a tumor is directly adjacent to a normal organ or wrapped around a normal organ, IMRT can shape the radiation such that is avoids as much of the normal organ as possible, but still delivers a large dose to the tumor (figure IV). This is why IMRT is commonly used in cancers of the head and neck where many critical structures, that may be near the tumor, such as the spinal cord, must be avoided. The downsides of IMRT are that it can take longer to both plan the treatment course and deliver the daily treatment than 3D conformal therapy due to the numerous shapes the leaves are required to form. Also, because so many small beamlets are being used, the dose of radiation going to the tumor may not be as even as is usually seen with 3D conformal therapy. Furthermore, one of the disadvantages of using a greater number of beams to shape the radiation is that while normal organs are spared high doses of radiation, a larger volume of normal organs receives a low dose of radiation. Finally, some tumors can move, such as in the lung, where breathing motion can cause the tumor to shift several centimeters. In such cases, a larger margin may be desired on the tumor, and the sharp edge that IMRT provides, may not be of much benefit. Due to the fact that IMRT is often significantly more time-consuming and complex than standard radiation, and thus more costly as well, some insurance companies may not initially approve payment for this treatment.

Stereotactic Radiotherapy and Radiosurgery

Stereotactic radiotherapy involves delivering a high dose of radiation very precisely to a tumor. Stereotactic radiosurgery can be done with photons or protons (which will be discussed later). Stereotactic radiotherapy delivers radiation from numerous different angles to focus the radiation at one small point, like a magnifying glass. By using a large number of unique beam angles to deliver the radiation, stereotactic radiotherapy minimizes the effects on the normal tissue, which the radiation passes through, but delivers a large dose of radiation to a single point where all of the beams converge. However, since the dose of radiation to that single point is so high, very precise targeting of the tumor is required. Due to these constraints, the most common use of radiosurgery involves tumors of the brain. The brain does not move and hence does not have the problems with motion that other tumors sites can have, and the skull serves as a stable landmark for the location of the tumor. Generally, a head frame or halo needs to be attached to the skull using small screws. This allows the head to be positioned with sub-millimeter accuracy in the treatment machine and allows the precise delivery of the stereotactic radiation. With the frame on, the person undergoes an MRI scan to localize the tumor and the frame serves as a stable landmark for the location of the tumor. The MRI is then used to plan the radiation treatment using specialized software. The tumor and normal structures are outlined on the MRI and a treatment plan is constructed to avoid critical brain structures while giving optimal dose to the tumor. Because the MRI was taken with the frame on, the tumor location within the frame should be the same on the treatment machine. The frame is then attached to the treatment machine and radiation can be delivered with sub-millimeter accuracy. Several different machines can be used to deliver stereotactic radiotherapy, including gamma knife machines and specialized linacs. Stereotactic radiotherapy is limited by the need for precise immobilization of the tumor, such as with a head frame and by the size of the lesion which can be treated. Due to the small focal spot of highly intense radiation used in stereotactic radiation, only lesions of about 3 centimeters or smaller are treatable with this technique.

Brachytherapy

Brachytherapy involves the use of a radioactive source, generally one which predominantly emits photons. The source is either implanted into the tumor (interstitial brachytherapy) or placed near the cancer, generally in a body cavity (intracavitary brachytherapy). Prostate seeds are an example of interstitial brachytherapy, where radioactive seeds are placed directly into the prostate using needles. Uterine cancer treated with a removable implant placed in the uterine cavity through the vagina is an example of intracavity brachytherapy. The advantage of brachytherapy is that since the source of the radiation can be placed in, or adjacent to the tumor, the amount of normal tissue affected by the radiation can be minimized. This is because the dose of radiation released from the source is very high near the source itself, but the dose falls off rapidly within a few centimeters. This limits brachytherapy to cancers in locations where a radioactive source can be inserted safely, but still treat the tumor effectively. Brachytherapy is not effective for treating large areas or deep tumors unless the source can be implanted correctly.