By: G. Lowndes Harrison, MD, FACRO
Radiation therapy traditionally utilizes the delivery of megavoltage radiation directed to cancerous tissues. Early in its use, it was discovered that radiation in large doses would cause severe and often fatal side effects. However, if smaller fractions of radiation were given on a daily basis, many tumors could be controlled while allowing the normal surrounding tissues to remain healthy. Thus the conventional daily-fractionated method of radiation delivery became the standard for over 100 years.
Later it was worked out radiobiologically that daily fractionation took advantage of several innate repair mechanisms known as the “4Rs”: repair, reassortment, repopulation and reoxygenation. Radiobiological calculations predicted for a fraction dose size that would cause cumulative damage to tumor cells while allowing healthy surrounding tissues to survive utilizing these repair mechanisms. That fractionation size was found to be 180-200 RADS or cGy in today’s terminology. Over the past century, clinical trials have set standard fractionation schedules for most common tumor types utilizing several weeks of fractionated radiation therapy.
In the 1960s, Leiksell experimented with and produced what was later termed the Gammaknife, designed to operate against these principles. It was reasoned, that if the energy of the beam could be safely directed to a very small target and guided away from the surrounding healthy tissues, single fractions of very large radiation could be used to ablate tumors while leaving the surrounding tissues unaffected. The Gammaknife utilizes hundreds of small beamlets of radiation (about the size of a pencil) to deliver an intense dose of radiation to a small focus; the dose to the surrounding tissues is relatively low. This intense dose overwhelms all repair mechanisms and the tissue within the target volume dies a necrotic cell death.
The Gammaknife is an extremely successful tool. It has had decades of success treating small intracranial tumors and has a long safety record. The success of the Gammaknife is due to its precision, which improved tremendously in the latter 20th century with the advent of superior imaging techniques. The key to the Gammaknife is the rigid immobilization of the patient prior to planning and treatment. The Gammaknife requires the attachment of a head frame to the patient’s skull prior to imaging for planning and treatment. The patient is subsequently clamped into the treatment machine during therapy.
Many machines have mimicked and advanced the concept behind the Gammaknife allowing for radiosurgery to be delivered to sites outside of the CNS. Machines such as Tomotherapy and Cyberknife have taken advantage of rapid imaging and tumor tracking techniques and no longer require strict patient immobilization. With these machines, the treatment volume adjusts as the tumor moves. These methods expound on the beamlet delivery concept. By utilizing multiple foci, target volumes are no longer limited to a few centimeters. Radiosurgery has successfully treated tumors as large as 7 cm.
The Varian Truebeam linear accelerator has been installed at Gadsden Regional Cancer Center since May of 2016. We are proud to say that we have started our radiosurgery program and have successfully completed radiosurgery to several different types of tumors. The Varian Truebeam is capable of mimicking Gammaknife, Tomotherapy, and Cyberknife but with the distinct advantage of speed. It takes several hours to deliver hundreds of beamlets to a single focus. As the targets grow, more foci are required to deliver the treatment. Sometimes this can take several more hours to complete. The prolonged treatment times can be quite distressing to the patient, and many chronic lung conditions may preclude radiosurgery. By taking advantage of Volumetric Modulated Arc Therapy (VMAT), the Truebeam is capable of delivering radiosurgery to any sized target in less than 15 minutes. This improves patient comfort and treatment accuracy. The Varian Truebeam is the pinnacle of radiosurgery delivery, and Gadsden Regional Cancer Center is one of the few sites in the State of Alabama to have one. There is simply no other machine anywhere that surpasses its accuracy and efficiency.
Needless to say, the Truebeam is capable of delivering conventional radiation with tremendous efficiencies. This has allowed improved patient throughput while minimizing down time. The Truebeam is not the only recent embellishment to the cancer center. Elaborate systems are now in place that track tumor motion and monitor patient movement. New 4-D CT scanning is available that reproduces dynamic images of tumors in real time. These are just a few of the recent additions at GRCC.
The staff of the Gadsden Regional Cancer Center is proud to serve this community with the latest in cancer treatment technology. We look forward to the continuing innovation and enhancement of our program for many years to come.