Principles of Radiation Therapy

All types of radiation therapy follow these general principles:

1. Precisely locate the target
2. Hold the target still
3. Accurately aim the radiation beam
4. Deliver a radiation dose that damages abnormal cells yet spares normal cells

Precisely locate the target

Any tumor, lesion or malformation to be treated with radiation is called a target. When locating a target, the doctor needs to know several things: its location in the body, its size and shape, and how close it is to important organs and structures. Small targets are harder to locate than large ones. Diagnostic scans such as computerized tomography (CT) and magnetic resonance imaging (MRI) have greatly improved over the years, allowing doctors to locate tumors and diseases earlier, when they are smaller. Also, positron emission tomography (PET) and functional MRI (fMRI) scans provide information about the function of critical areas next to the target.

Determining the exact location and border of a target within normal tissue is not always clear on diagnostic scans. Doctors can use a technique called stereotaxis to precisely locate targets, especially small deep ones. Stereotactic means to locate a structure by use of three dimensional coordinates (x, y, and z axis). First, a stereotactic head or body frame is attached over the target area. Next, a CT or MRI scan is taken and interpreted by computer software. The stereotactic frame shows up on the scan and helps the doctor pinpoint the exact location of the target (Fig. 1). In some cases, stereotactic localization is performed using internal landmarks, such as bones, and a frame is not necessary.

Hold the target still

Once the target is located, the doctor must hold the body as still as possible to accurately aim the radiation only at the target and to avoid healthy tissue. This is especially difficult in areas that are normally moving, such as the lungs and abdominal organs. Immobilization also is important for smaller targets, because a slight shift in position can move the target out of the radiation beam's path. Immobilization devices are used to prevent movement and secure the body area to the treatment table. These devices include molds, masks and stereotactic head or body frames (Fig. 2). Molds and masks are custom-made from plastic to fit your body exactly and are used during each treatment.

1 1 Figure 2: Immobilization devices such as masks (left) or stereotactic head frames (right) attach to the treatment table to hold the head still.

Accurately aim the radiation beam

Multiple radiation beams are aimed so that they all meet at a central point within the target, where they add up to a very high dose of radiation. In order to accurately aim radiation, both you and the machine must be correctly aligned with each other.

Patient Alignment

Depending on the body area to be treated, different techniques may be used to position your body, including: skin markers, laser lights, field lights, infrared cameras and x-ray positioners. Laser lights are used to make sure you are level and straight on the table. Field lights correspond to the skin marks. Infrared cameras use body markers to detect your position and match the markers to the position in the treatment plan. X-ray positioners take stereoscopic x-rays of your anatomy and match them to the position in the treatment plan images (Fig. 3).

Machine Alignment

Several types of machines used to create a radiation beam and aim it at the target. Each machine offers a different level of accuracy and ability to deliver various radiation techniques to treat the target.

A Linear Accelerator (LINAC), the most common type of radiation machine, uses electricity to form a stream of fast-moving subatomic particles (Fig. 4). The radiation beam produced by a LINAC can be shaped and aimed at the target from a variety of directions by rotating the machine and moving the treatment table. The advantage of LINAC-based systems is their versatility.

They:

• are used for both radiotherapy and radiosurgery treatments
• treat any area of the body
• treat large and small tumors
• use highly focused radiation sources
• produce high intensity radiation
• can use techniques such as Intensity Modulated Radiotherapy (IMRT)

The Gamma Knife system uses 201 converging beams of gamma radiation (cobalt-60). All 201 beams meet at a central point within the target, where they add up to a very high dose of radiation. In contrast to LINAC, the Gamma Knife does not move around you. Rather, you are placed in a helmet unit that allows the target to be placed exactly in the center of the converging beams. The features of Gamma Knife systems include:

• used for radiosurgery only
• limited to treating head and neck lesions

Shape the radiation beam to the target

It is crucial that the radiation dose is delivered only to the target. Shaping the beam to match the target minimizes exposure to normal tissue. The problem is that most tumors are irregularly shaped and most radiation beams are round. Beams can be shaped using treatment planning software and hardware.

Treatment Planning Software

High-end computers and software are used to plan the treatment so that all beams meet at a central point within the target, where they add up to a very high dose of radiation. The software uses your CT or MRI images to form a 3D view of your anatomy and the target (Fig. 5). The radiation oncologist uses different settings in the software to create a final radiation prescription specifically for you. The prescription includes:

• correct radiation dose of each beam (measured in rads or Gy)
• correct size and shape of the beams
• number and angle of treatment arcs
• number of treatment sessions

Hardware

Radiation beams can be shaped by attaching blocks or collimators to the radiation machine to block a portion of the beam (similar to placing your finger in the path of a flashlight to cast a shadow). The goal is to shape the beam to the exact contour of the tumor and minimize exposure to normal tissue. Block devices shape the beam in a linear fashion and are only able to squarely shape the beam (Fig. 6). Collimator devices are able to shape the beam into circular or elliptical shapes (Fig. 7). Multileaf collimators can focus and shape the beam in infinite ways and are the most precise method at this time (Fig. 8).

Deliver a radiation dose that damages abnormal cells yet spares normal cells

Radiation works best when given in high rather than low doses; however, normal cells that border the target cannot repair themselves very well after a high-dose exposure. Determining the best radiation dose is a balance between the maximum dose tolerated by normal cells versus the minimum dose necessary to cause tumor cell death. Doctors can take advantage of the body's own healing process by delivering a fraction of the complete dose over multiple sessions. In this method, called fractionated radiotherapy, normal cells are allowed time to repair between each radiation session and are protected from permanent injury or death. The fewer the treatment fractions, the more the radiation affects tumor and normal tissue equally. The greater the number of treatment fractions, the less the risk of injury to normal cells and the fewer the side effects. During fractionated radiotherapy, patients receive treatment daily for 3 to 6 weeks.