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IMRT - Published Article Print E-mail

William Salter, Ph.D

  Intensity Modulated Radiation Therapy (IMRT) is a revolutionary new method of delivering radiation to cancerous tumors. For many years now the benefits of treating cancer with radiation have been recognized. Radiation inflicts damage to cells, and if this damage is directed appropriately it can be used to kill tumor cells. Simply stated, the fundamental premise on which radiation therapy is based is that with care we can do considerably more damage to cancerous tissue than to surrounding healthy tissue. The healthy, non-cancerous cells of our body have an extremely effective repair mechanism that, when functioning correctly, can repair mechanism is fairly dysfunctional. This difference in tolerance to radiation damage is the edge that is exploited in radiation therapy. It is some times referred to as a therapeutic ration.

   A critical role of the radiation oncologist is to understand the relationships between the tolerance of healthy organs and tissues in the vicinity of the tumor and that of the tumor. The radiation oncologist makes the decisions regarding what radiation dose level must be used to kill the tumor and what dose levels the surrounding healthy tissues can tolerate without undue risk of complications. The fractionation, or delivery of this radiation dose in "small pieces" given daily over several days or weeks, allows the healthy, non-cancerous tissues of the body to effect repairs overnight. The tumor cells are not as effective at repairing the radiation damage, so that the net effect is that much more damage is done to the tumor than to the healthy organs and tissues. While the healthy tissues are capable of effecting repair of radiation damage there is a limit, above which the healthy tissue cannot recover. Fortunately this tolerance limit, or level, is generally higher for the healthy tissues than the tumor.

   While the more effective repair mechanism of healthy tissue is a definite advantage to be exploited. The radiation therapy team has a few other techniques to use against the tumor, as well. For one thing, the team typically uses a crossfire technique to hit the tumor with multiple beams of radiation from multiple directions. This has the very desirable effect of "spreading" the dose to healthy tissues and organs around while still delivering a full and lethal dose to the tumor. Typically, two to four beams of radiation are used to hit the tumor from multiple directions. Each beam is customized to match the shape of the tumor as viewed from the direction of that particular beam (beam shaping). The net effect is a concentration of the radiation dose on the tumor and a "sparing" of the surrounding healthy tissues.

   Another technique that the radiation oncology team uses is often referred to as variable weighting of the beams. This simply means that if we use, for instance, three beams of radiation to treat a tumor of the neck we might identify which of these three beams is most effective at delivering dose to the tumor while minimizing the dose delivered to healthy tissues. We might then "weight" this particular beam more heavily, meaning that we might put more than 1/3 of the tumor dose in from this beam. We might, for instance, deliver ? of the tumor dose from this beam and ? of the dose from each of the two remaining beams. This would have the effect of delivering the full dose to the tumor while spreading the dose to surrounding healthy tissues. This procedure might also do an even better job of sparing healthy tissues since the most effective beam would be delivering more of the dose to the tumor. Another way of saying weighting is modulating. This technique is the underlying basis for Intensity Modulating Radiation Therapy (IMRT) which can be even more effective than the sophisticated treatment methods already in routine use today.

   Quite simply, Intensity Modulated Radiation Therapy is a method of delivery that takes the previously discussed techniques of beam shaping, cross fire, and intensity modulation to new extremes. The idea behind IMRT is based on the realization that if spreading the dose to healthy tissues using 2 or 3 or 4 beams is good, then the use of even more beams should be even better.

   IMRT uses literally thousands of small pencil beams of radiation to deliver the required tumor dose. The liner accelerator machine that produces the radiation rotates in an arc about the patient while the thousands of pencil beams are fired as precisely the right time to hit the tumor and miss surrounding critical structures. A computerized treatment planning system calculates which potential pencil beams should be fired and which ones should not (because they would hit sensitive structures). The intensity modulation in IMRT comes from the fact that in addition to selecting which pencil beams to fire and when, the computer planning system also determines how much dose to put in from each beam. This is exactly like the weighting or modulation discussed previously.

   Intensity Modulated Radiation Therapy allows the radiation oncology team to do an even better job of restricting radiation dose to the designated tumor tissues than conventional radiation therapy. The net result is an ability to deliver higher doses to tumors, which can lead to an increased probability of local control. At the same time the dose to healthy tissues can be reduced, leading to a reduction of side effects. While not every tumor geometry will require the use of such sophisticated new technology, it is already making the treatment of previously untreatable tumors, possible.

   In the past, Radiation Oncologists have sometimes been forced to deem a tumor residing in close proximity to the spinal cord as untreatable because to treat the tumor to a lethal dose would also represent considerable risk of damage to the cord, as well. Such situations can certainly exist for tumors of the head and neck. With the advent of IMRT we are now able to treat many such tumors to a lethal dose while maintaining the spinal cord at an acceptably low radiation dose level. While such complications are rare, the significant reduction of radiation dose to healthy tissue afforded by IMRT can also translate into reduced incidence of radiation induced bone necrosis.

   Perhaps most promising for patients suffering from cancers of the head and neck is the potential for reduction of incidence of the more commonly occurring complications of dryness of the mouth (due to mucosa and salivary gland irradiation) and fibrosis of muscle and connective tissues (which can lead to considerable discomfort). In short, IMRT, hold the promise of allowing Radiation Oncologists to prescribe higher doses for control of head and neck tumors, while at the same time reducing the probability of troubling complications.

   The first commercially available IMRT system was introduced by a company called NOMOS, based in Sewickley, Pennsylvania. Many vendors of radiation therapy equipment are now striving to offer such IMRT capability. To date the Nomos Peacock System has been used to treat literally thousands of cancer patients worldwide and other vendors should be delivering such technology in the very near future. The impact of IMRT on the field of Radiation Therapy has been likened to the advent of Cat scans some 25 years ago. The technology holds the promise of changing, for the better, the way therapeutic doses of radiation are delivered from this point forward.

Editor's Note:
   Dr. William Salter holds a PhD. in Medical Physics from the University of Texas Health Science Center at San Antonio. He is currently the Associate Director of Medical Physics at the Cancer Therapy and Research Center in San Antonio and holds a dual appointment as Assistant Professor in the Department of Radiation Oncology and Department of Radiology at the UT Health Science Center.

   Support for People with Oral, and Head, and Neck Cancer, Inc. does not endorse any treatments or products mentioned in this newsletter. Please consult your physician before using any treatments or products.

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