Treating Prostate Cancer Safely and Accurately with Radioactive Seed Implants

Pictured: Michael J. Zelefsky

Chief of the Brachytherapy Service Michael J. Zelefsky

The use of radioactive seed implants to treat cancer, a process known as brachytherapy, stretches back to the 1950s, when some of the pioneering advances in the field were developed at Memorial Hospital. Today, Memorial Sloan Kettering Cancer Center clinicians continue to lead the way in advancing the use of brachytherapy, including the development of a more refined approach for treating men with prostate cancer. Using real-time, computer-based treatment planning and intraoperative CT scanning, our doctors are able to optimize the radiation dose delivered to the prostate through seed implants while limiting radiation exposure to important nearby structures — thereby minimizing the likelihood of side effects. This approach allows our treatment team to use CT scanning to assess the accuracy of seed placement during the procedure, a significant improvement when compared to standard prostate brachytherapy assessment, which is typically done hours or even weeks later.

Brachytherapy, which comes from the Greek word brachy, meaning “short distance,” is a form of radiation therapy in which radioactive seeds are placed inside or next to a tumor. This distinguishes it from external beam radiation therapy, which uses a machine outside of the body to deliver high-energy x-rays directed at the cancer.

Brachytherapy for Prostate Cancer

In low-dose-rate brachytherapy, the most common type of brachytherapy used to treat prostate cancer, ultrasound images of the prostate and the surrounding structures — combined with the results of complex mathematical computerized computations run by medical physicists — are used to determine the most effective placement of the seeds. The radioactive seeds are then permanently inserted into the prostate during an outpatient procedure, which allows the man being treated to return home the same day.

The seeds, which are about the size of a grain of rice and are made of titanium, are implanted in the prostate through long, thin needles inserted in the perineum, the area between the anus and the testicles. Approximately 80 seeds are inserted during an average procedure, which usually takes less than two hours and is performed with the patient under either spinal or general anesthesia. Over several months, the seeds gradually lose their radioactivity. During that time, the patient is allowed to go about his normal routine.

In standard brachytherapy for prostate cancer, doctors use ultrasound images taken several weeks before the procedure as a map for seed placement. This has the potential to cause problems if swelling has occurred since the original ultrasound or if the prostate’s shape or geometry has changed. Additionally, the position that the patient is placed in when undergoing the procedure can cause changes in the prostate’s shape and relative position, which can result in less than optimal seed placement.

Continued Refinements for Improved Accuracy

Memorial Sloan Kettering doctors and physicists have helped to develop a refined approach, known as intraoperative computer-based conformal optimization. Here, the treatment team — including radiation oncologists, medical physicists, and radiation therapists — visualize the prostate using CT scans to assess optimal seed placement while in the operating room.

Pioneered at Memorial Sloan Kettering in 1998, the use of sophisticated computer programs in the operating room to help target where and how many seeds to place within the prostate gland allows the optimal radiation dose to be delivered to the prostate, sparing as much normal tissues as possible from radiation exposure. Planning is done in the operating room during the actual procedure instead of weeks before the procedure, as had been previously done.

“Using on-site computers and ultrasound images, our medical physicists employ a sophisticated computer program developed at Memorial Sloan Kettering that can examine within minutes millions of configurations of seed-coordinate placement possibilities,” says Marco Zaider, PhD, Head of Brachytherapy Physics at Memorial Sloan Kettering. “The program selects the placement plan that will deliver the most effective dose to the prostate while keeping the dose delivered to the rectum and urethra as low as possible.”

As an extra measure of safety, the calculations are checked by a separate program run by a second medical physicist, also present during the procedure. “This completes the quality assessment and planning stage of the implant,” adds Dr. Zaider. “Next, we have to make sure that what we’ve planned is actually achieved, and this brings us to the newest tool in our arsenal, intraoperative CT scanning, which, for all intents and purposes, amounts to a shift in paradigm.”

Introducing a CT scan unit into the operating room represents the next step in image guidance, enhancing the precision and safety of seed implants by allowing the treatment team to confirm the placement of the seeds during the procedure. This approach combines quality control and an attention to detail that is not available for patients receiving standard brachytherapy, in which routine assessments of seed placement are normally not done until hours or even a full month after the procedure.

“Our goal going forward was to further minimize side effects and to try to improve accuracy in seed placement, which led to the use of intraoperative CT scans,” says Michael Zelefsky, Chief of the Brachytherapy Service at Memorial Sloan Kettering. “With this addition, we are working on ongoing enhancements and significant improvements in the actual delivery of the seeds.”

“This image-guided approach that we are using is beginning to revolutionize the way that prostate cancer brachytherapy is being done,” adds Dr. Zelefsky. “We are now able to assess the quality of the implant during the procedure, as the seeds are being implanted. This allows us the opportunity to make corrections, if necessary, and to optimize the quality of the implant.”

As Dr. Zelefsky explains, sometimes there may be slight changes in the patient’s anatomy during the procedure, which may cause some of the seeds to receive less than optimal placement. “This new refined approach to brachytherapy for prostate cancer has given us much greater confidence in knowing that we are able to deliver the radiation dose exactly where we intended to place it,” he says.

Success Rates and Side Effects

Treatment success rates for prostate cancer brachytherapy have been excellent, especially for men diagnosed with an early stage of the disease, for which survival rates are generally comparable to those produced by prostate surgery. In addition, the new intraoperative computer-based conformal optimization approach, and the enhanced precision it allows, reduces the likelihood of both urinary and rectal side effects when compared to external beam radiation. Another benefit of treating prostate cancer with brachytherapy is that the time commitment required to perform it is dramatically less than that required for external beam radiation, which is usually delivered in about 50 sessions over the course of ten weeks.

“We are one of a few institutions in the world using this particular intraoperative CT scanning device on a regular basis to help us to optimize the quality and accuracy of the seed implant procedure,” Dr. Zelefsky notes. “We think it may be appropriate for many forms of brachytherapy, and we are exploring the use of these procedures to treat a variety of other cancers.”