Linear accelerators have reached maturity in the medical universe. Used for nearly half a century in the clinical setting, the basic mechanism has remained essentially unchanged. A linear accelerator, or linac, sends a beam of high energy photons or electrons into the tumor target. Unlike a cobalt-60 machine, there is no radioactive material to handle and dispose of, and the beam is much more accurate.

Because of their safety and accuracy, linear accelerators have become the primary radiation beam delivery systems for the treatment of cancer. According to a market report from and IMV Medical Information Division, as of mid-2000, there were approximately 2,890 linear accelerators (plus a small number of cobalt-60 units) installed at 1,810 external beam radiation therapy sites. Most of these sites are in hospitals.

While there are no dramatic advancements in the actual beam technology on the horizon, physicians and researchers are at work to solve problems such as creating an up-to-the-minute image of the target tumor, accurately delivering an appropriate radiation dose while sparing surrounding tissue and organs, and improving patient placement on the couch. Solutions to all of these problem areas will mean better treatment delivered more efficiently and more comfortably.

Problems and solutions
Currently, linear accelerators are manufactured by a range of companies, among them Siemens Medical Solutions, (Iselin, N.J.), Varian Medical Systems (Palo Alto, Calif.) and Elekta (Stockholm, Sweden). While these companies define areas for improvement in much the same ways, they are attacking these areas differently, according to company strengths.

IMRT today
Many of the improvements coming in the field are the result of the development of intensity modulated radiation therapy (IMRT). (Also see “The Economics of IMRT” on page RT-30.) IMRT makes use of multileaf collimation to more precisely modulate the radiation and shape the beam. It does this by moving pairs of leaves in various thicknesses in and out of the radiation beam in a predetermined pattern, providing contour to the beam and helping to direct radiation away from radiosensitive tissues and organs. Typically located on a moveable arm, this mechanism can be relocated at various positions around the patient.

Please refer to the October 2002 issue for the complete story. For information on article reprints, contact Martin St. Denis