My Question is Why?

By Dr. Ronald M. Benoit
Department of Urology, University of Pittsburgh School of Medicine

Why would a clinician be hesitant to use Cesium-131 to treat prostate cancer? I would venture to say the likely answer is the data that speaks to its benefits has not yet diffused through the system.

Dr. Ronald M. Benoit Department of Urology, University of Pittsburgh School of Medicine

Here at the University of Pittsburgh School of Medicine we began using Cesium-131 in 2006. At that time, we were using primarily Iodine-125 and occasionally Palladium-103 as our isotopes. We were pleased with our cancer control with Iodine-125, but the extended duration of lower urinary tract symptoms associated with this isotope was very bothersome to our patients. We are an academic institution, and our brachytherapy team has always been interested in new and innovative approaches. Cesium-131 was proposed to have a shorter duration of the bothersome voiding symptoms after brachytherapy than Iodine-125 and Palladium-103 due to its shorter half-life. Our brachytherapy team was of the mind that the prolonged duration of these voiding symptoms was the main drawback to prostate brachytherapy.

We found the technique of implantation of the Cesium-131 isotope very similar to Iodine-125 given its similar energy level, and we were able to achieve excellent dosimetry with Cesium-131 in our very first cases with this isotope. Most importantly, we did find a significant difference in the duration of lower urinary tract symptoms in our patients who were implanted with the Cesium-131 isotope. Most of our patients had their lower urinary tract systems return to baseline within three to six months. With Iodine-125, our patients generally took 12 to 18 months to return to their baseline.  Our oncological control with Cesium-131 was equal to or better than our results with Iodine-125, and we achieved a significant decrease in the duration of bothersome lower urinary tract symptoms associated with prostate brachytherapy.

We at the University of Pittsburgh School of Medicine have the largest Cesium-131 prostate brachytherapy database in the world. We have been accumulating data on urinary symptoms, data on bowel symptoms, and PSA outcomes since we first started using the Cesium-131 isotope in 2006. Our follow-up on these patients now reaches 15 years. Our data clearly demonstrates a return to baseline voiding symptoms by three to six months in the large majority of our patients.

I believe the shorter duration of lower urinary tract symptoms after Cesium-131 prostate brachytherapy when compared to I-125 prostate brachytherapy is due to the half-life of the isotope. The half-life of Cesium-131 is 9.7 days, while the half-life of Iodine-125 is 60 days. The longer half-life of Iodine-125 leads to a longer period of inflammation and therefore a longer duration of lower urinary tract symptoms. The obvious question is why does Palladium-103, with its half-life of 17 days, have a duration of voiding symptoms similar to Iodine-125? In my view, this fact is due to the lower energy level of Palladium-103. The lower energy level of this isotope requires placement of a larger number of seeds to achieve adequate dosimetry. The large number of seeds leads to prolonged inflammation and a longer duration of voiding symptoms despite its relatively short half-life.

I have been involved in prostate brachytherapy since 1992. In my experience, and I believe our data supports my contention, Cesium-131 is the best isotope for low dose rate brachytherapy.  We believe Cesium-131 delivers the best of both worlds. We have achieved excellent oncologic control while our patients have a much shorter duration of their voiding symptoms after the procedure when compared to Iodine-125 and Palladium-103.

The landscape of the treatment has changed since we first began treating patients. The majority of patients we are now treating have high-risk prostate cancer. When we first started using Cesium-131, most of our patients had low and favorable intermediate risk disease and underwent prostate brachytherapy as monotherapy. Given that the majority of patients we are now treating have unfavorable intermediate and high-risk disease, most of our prostate brachytherapy patients are now undergoing prostate brachytherapy as part of combination therapy. We have certainly found that the benefits of Cesium-131 are present with combination therapy as well as monotherapy.

I would encourage practitioners of prostate brachytherapy to strongly consider using Cesium-131. For those currently using Iodine-125, I think they will find the technique of implantation of Cesium-131 is very similar to Iodine-125 and there is no learning curve. Cesium-131 will provide excellent cancer control, and a significantly shorter duration of the lower urinary tract symptoms associated with prostate brachytherapy. The data from the ASCENDE trial has clearly demonstrated that a brachytherapy boost significantly improves oncologic control in men with unfavorable intermediate and high-risk prostate cancer. I do strongly believe our data from the University of Pittsburgh demonstrates Cesium-131 should be isotope of choice for low dose rate prostate brachytherapy.

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