Prostate Cancer
External Beam Radiotherapy (IMRT)
External beam radiotherapy has a very long history in the treatment of prostate cancer, with a documented ability to cure the disease dating back decades (1). With the advent of the original medical linear accelerator (FIGURE 1) in the late 1950s, first used at Stanford University Medical Center, patients gained an effective alternative to radical surgery for prostate cancer. Virtually immediately thereafter, a debate was born regarding the relative effectiveness of radiotherapy versus radical prostectomy, which persists to this day, unresolved (Both techniques continue to improve.).
The original high-energy radiotherapy technique for prostate cancer represented a major step forward in the war against this disease, and served as the foundation upon which the practice of external beam radiotherapy has since evolved. Unfortunately, there were some serious shortcomings in the original technique compared with contemporary methodology that limited its effectiveness.
The original radiotherapy technique was known as “two-dimensional” radiation and relied upon standard X-rays to define the treatment volume. This technique was the standard method in most radiotherapy practices until the 1990s. Essentially, this process used landmarks such as the pubic bone, bladder and rectum (filled with contrast material) to align the radiation beams (FIGURE 2). As the prostate has a predictable relationship with such landmarks, it is possible to estimate its location against such X-ray landmarks, to direct the radiation beams to cover its estimated location. Unfortunately, any individual prostate may or may not exactly conform to “normal” anatomic relationships to these landmarks, and the prostate itself is invisible to standard X-rays.
This result of the above-described uncertainty was that the above-described “two-dimensional” technique had a significant inherent error potential. If small beams were used, this uncertainty error would have a high chance of translating into “geographic miss” of some of the prostate target volume, leading to inadequate coverage of the tumor volume with radiation. If large beams were used, the “geographic miss” scenario would be successfully resolved, but the resulting large volume of irradiated surrounding tissue would result in a higher chance of potentially severe damage to the adjacent bladder and rectum, limiting the dose of radiation that could be used.
As a result of the above problems, the cure rates were relatively lower and the complication rates higher compared with contemporary radiation therapy methods. Because of this, the above-described “two-dimensional” technique is of historical interest, but has been abandoned by contemporary radiotherapy practices.
With the additional passage of time though, there have been extraordinary improvements in the external beam radiotherapy technique, due to the revolution of computing power and software that drive the medical linear accelerator, as well as a fundamental redesign of the linear accelerator machine itself. This has turned the contemporary medical linear accelerator (FIGURE 3) into a high technology, computer-driven, radiotherapy delivery device.
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