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GPS for the Body: Treating a Moving Target

Eric M. Horwitz, MD
Published: Wednesday, Feb 25, 2009
Prostate cancer is second only to lung cancer as the leading cause of cancer death for men, with the American Cancer Society estimating that 28,660 men died from prostate cancer in 2008. However, many experts agree that prostate cancer is treatable and highly curable if the disease is detected early.



A common treatment option for prostate cancer is radiation therapy, which is as effective in treating the disease as surgical removal of the prostate. Although men who opt for treatment with radiation instead of surgery can experience side effects—including fatigue and urinary and bowel frequency and urgency—they do not have to worry about urinary incontinence, bleeding, and other risks associated with surgery. Also, radiation treatments do not require hospitalization. In fact, most men are able to enjoy their normal daily activities during treatment.

One of the biggest challenges doctors face in delivering high doses of radiation to the prostate is controlling for natural internal organ motion. Clinical studies have documented that prostate organ motion is both unpredictable and variable, based on breathing and bladder and bowel filling. Each day, the prostate can shift more than 1cm in some directions. During treatment, it may shift several millimeters as the patient breathes or coughs. Conventional methods to localize the prostate, including daily external ultrasound and CT imaging, do not allow for continuous real-time monitoring of the tumor’s position and cannot predict organ motion.

Organ movement is a natural and commonly occurring bodily function. However, if the target (the prostate) can be effectively tracked during treatment, the radiation can be delivered more effectively. Knowing the exact location of the target, in real time, maximizes treatment benefits and minimizes potential damage to nearby tissues or organs, which limits side effects. Tracking the target allows the physician to confidently treat a smaller volume with higher doses of radiation.

Several leading hospitals throughout the country, including Fox Chase Cancer Center in Philadelphia, have adopted a revolutionary technology called GPS for the Body. Also known as the Calypso 4D Localization System, GPS for the Body specifically tracks the prostate motion in prostate cancer patients, enabling physicians to detect the slightest organ movement in real time, as well as the direction of the radiation if the prostate moves more than expected. The prostate motion is tracked based on the signals from three electromagnetic Beacon transponders, each the size of a grain of rice, which are permanently implanted within the patient’s prostate gland. The transponders send benign radio waves that allow physicians to precisely pinpoint the location of the prostate, even as it moves during treatment. If necessary, the physician can pause treatment and re-adjust the patient, thereby minimizing or avoiding potential damage to the nearby rectum and bladder.

Current methods for aligning treatment involve imaging the tumor—using implanted gold markers and X-ray images or ultrasound scans—before each radiation session. The Calypso system provides continuous, real-time positioning, akin to having a video versus having a snapshot of the tumor.

Patients who have undergone radiation using the Calypso System, and their physicians, find comfort in knowing that the chances of eradicating early-stage prostate cancer is potentially increased when radiation is delivered with extra precision. Patients are relieved that side effects may be minimized and quality-of-life factors, such as bladder control and sexual function, may be preserved, enabling a faster recovery.


Dr. Horwitz is the Acting Chairman of Radiation Oncology at Fox Chase Cancer Center in Philadelphia.


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