How External Beam Radiation Therapy Works
It has taken more than half a century of slow, steady advances to reach a point where carefully calibrated doses of external beam radiation can be used to effectively kill prostate cancer cells and do so with minimal damage to the nearby urethra, rectum, and bladder.
Significant improvements have been made in external radiation therapy and the LINAC (linear accelerator) machine that delivers the beams of high-energy photons. Equipment capable of delivering radiation has been made even more powerful, and also more precise, thanks to the development of technologies that combine the latest in computer and imaging capabilities.
This integration of the latest technologies into effective treatment offers major benefits to the prostate cancer patient that will truly improve his quality of life and increase the likelihood of cure. Here is an overview of the various types of radiation therapies in use:
External beam radiation therapy
This treatment involves aiming high-energy beams precisely at the cancer from outside the body in an effort to shrink and ultimately destroy cancer cells.
These days, external beam radiation therapy is often used as a palliative treatment. For a man with prostate cancer that has spread to the bones, radiation therapy can reduce pain and lessen the likelihood of bone fractures. It can also reduce neurological symptoms resulting from spinal cord compression when cancer has spread to the spine.
Radiation oncologists have made a number of improvements in external beam radiation therapy in an attempt to increase cure rates and reduce the risk of complications. These improvements are found in intensity modulated radiation therapy and image-guided radiation therapy.
IMRT (intensity-modulated radiation therapy)
IMRT relies on computer software to determine the orientation, number, and intensity of the radiation beams. What is so innovative about IMRT is that we can modulate the intensity of each beam during the therapy with a multileaf collimator. (This is a shutter-like attachment at the end of a linear accelerator that filters the X-rays; the longer a leaf stays open, the stronger the dose of radiation.) Using IMRT, we can treat specific portions of the prostate with higher—or lower—dosages in an infinite number of patterns.
With over 100 digital CT scans taken in a matter of minutes to create a 3-dimensional picture of the prostate tumor, we can sculpt the dose of radiation even more precisely, delivering extremely high doses of radiation to within a millimeter of a cancer site.
This maximizes the dose delivered to the thickest part of the tumor and minimizes the dose that affects the nearby healthy tissues of the bladder and rectum. The result is a higher cure rate and fewer side effects.
IGRT (image-guided radiation therapy)
We are currently in the era of image-guided radiation therapy, or IGRT, which uses daily computed tomography (CT) scanning to create three- dimensional images that pinpoint the exact size and location of the tumor just before treatment that day. This information is then transmitted to a computer and allows doctors to compare the current image with earlier images taken the day before. During IGRT, doctors continually compare these images to see if the treatment area needs to be adjusted for optimal treatment.
Theodore L. DeWeese, M.D., is a professor in the department of oncology and urology at the Johns Hopkins University School of Medicine in Baltimore. He is the chairman of the department of radiation oncology and molecular radiation science.
His laboratory research efforts primarily focus on the DNA-damage response of cancer cells (primarily prostate cancer) to ionizing radiation, including repair, cell cycle perturbations, and growth factor regulation.