Intraoperative Real-Time Planning of Radiation Seed Treatment Effective Against Early-Stage Prostate Cancer

Brachytherapy

For patients receiving brachytherapy, physicians use ultrasound images of the prostate to determine the most effective placement of the seeds, which are permanently implanted in the prostate through thin needles. The seeds, which are about the size of sesame seeds, cause little or no discomfort, and their radioactivity diminishes over time. In traditional brachytherapy, the ultrasound images are taken days before the seed implantation, which means that any subsequent changes in prostate size and shape that occur prior to implantation are not taken into account.

In 1997, Memorial Sloan-Kettering physicians helped to develop a refined technique known as intraoperative computer-based conformal optimization, which allows for more precise radiation dosing. In this technique, radiation therapists determine optimal seed placement in the operating room on the day of treatment using an on-site computer and real-time ultrasound images. A sophisticated software program developed at Memorial Sloan-Kettering processes as many as seven million seed-coordinate placement possibilities, selecting the placement that will deliver the maximum dose to the prostate and the lowest dose to tissues in the rectum and urethra.

"The key to brachytherapy is finding the ideal balance between delivering the optimal dose to the tumor while minimizing the radiation exposure to the surrounding healthy tissues," says Michael J. Zelefsky, lead author of both studies and Chief of Memorial Sloan-Kettering's Brachytherapy Service. "Our sophisticated computer programs and intraoperative planning allow us to achieve that balance."

First Study

In the first study, conducted at Memorial Sloan-Kettering and published in the January 2007 issue of the International Journal of Radiation Oncology*Biology*Physics, 367 men with prostate cancer were treated with permanent seed implantation using intraoperative real-time treatment planning techniques, receiving a follow-up examination an average of five years after treatment.

The study's investigators found that 96 percent of the men involved received 100 percent of the prescribed radiation dose to their prostate, as evaluated by post-implantation CT scan, with limited doses being received by the urethra and rectum.

The men originally diagnosed with favorable-risk prostate cancer had a 96 percent five-year PSA-relapse-free survival rate, meaning that 96 percent of these men had not experienced a diagnosed recurrence of their prostate cancer five years after treatment. (Patients with favorable-risk prostate cancer are defined as men with a PSA less than or equal to ten, a Gleason grade less than or equal to six, and a clinical tumor stage between T1c and T2.)

Of the men in the study diagnosed with intermediate-risk prostate cancer, there was an 89 percent five-year PSA-relapse-free survival rate, which means that 89 percent of them had not experienced a diagnosed recurrence of their prostate cancer five years after treatment. (Patients with intermediate-risk prostate cancer are defined as men with a PSA of ten to 20, a Gleason grade of seven, or a clinical tumor stage of T2b.)

"Using our intraoperative conformal techniques, we were able to deliver the radiation with exquisite precision to the prostate by placing the seeds in a very accurate fashion," Dr. Zelefsky says. "This has yielded significantly fewer side effects than what previous treatments had produced."

Second Study

In the second study, published in the August 2007 issue of Radiotherapy and Oncology, 562 men with clinically localized prostate cancer were treated at Memorial Sloan-Kettering with permanent seed implantation using an intraoperative real-time conformal planning technique, which utilized transrectal ultrasound and planning optimization software. Dose delivery levels were confirmed by CT scans three hours after the seed implantation procedure.

The study's authors found that real-time intraoperative planning achieved a 90 percent success rate in attaining the planned radiation dose to be delivered to the prostate, while at the same time maintaining planned constraints for the amount of radiation received by the urethra and the rectum. "These techniques allow us to comfortably achieve dose thresholds that are critical for treatment success," Dr. Zelefsky explains.

The study notes that further enhancements in image guidance for optimal seed implantation are needed to guarantee an ideal dose distribution for all patients. Accordingly, Dr. Zelefsky reports that investigators at Memorial Sloan-Kettering continue to work to develop further advances in the brachytherapy procedure, employing even more precise image modalities and increasingly sophisticated computer programs.

"This will allow doctors to hone in more precisely on the tumor," says Dr. Zelefsky, "while sparing even more of the surrounding tissue." In the future, he explains that better imaging studies will hopefully allow doctors to pinpoint the tumor regions within the prostate and only target these areas for patients with very early stages of disease. "These approaches may potentially eliminate side effects associated with therapy and improve quality of life for patients we treat."