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Not All Low Dose Rate Brachytherapy Isotopes Are The Same

Oakland, CA May 28 2013.  Recently there has been a promotion of the idea that a newer isotope Cesium 131 has markedly improved the permanent seed, or low dose rate, brachytherapy for prostate cancer. The claim is that this newer isotope has a higher energy, a shorter half life, and gives a higher biologically effective dose to the tumor. It is true that Cesium 131 has a higher average energy than the other commonly used prostate cancer permanent seed isotopes on the market. The more commonly used isotopes used are Palladium 103 and Iodine 125. Cesium 131 delivers 90% of the prescribed dose to the prostate gland in 33 days compared to 58 days for Palladium 103 and 204 days for Iodine 125. Another claimed benefit to the short half-life of Cesium 131 is what is known as the “biological effective dose” against cancers exhibiting different characteristics – for instance, slow versus fast growing tumors. This all sounds good and it is an improvement over conventional low dose rate permanent seed brachytherapy with Palladium 103 or Iodine 125.

So how does Cesium 131 compare to temporary high dose rate brachytherapy? High dose rate brachytherapy delivers the entire dose in two treatments over eight days compared to 33 days with Cesium 131. Prostate cancer unlike some other cancers can have a slower growth rate which can be closer to normal tissue than the more virulent cancers. Research has shown that larger doses of radiation over a shorter period of time as delivered by high dose rate brachytherapy may be radiobiologically superior to the protracted low doses over a long time that permanent seeds delivers, including Cesium 131. Since high dose rate brachytherapy is a temporary implant there is no radioactive source left in the patient. Cesium 131 is still a permanent seed implant with the same limitations compared to high dose rate brachytherapy. The Cesium 131 seeds are all of uniform strength so it is very difficult to get uniformity of dose with proper spacing of the seeds. The seeds are implanted into deformable tissue and once the seed is implanted it can’t be moved. The dose plan is still done as an estimated pre-plan prior to surgery or on the fly with “real time intra-operative planning” at the time of surgery. In either case the doses are calculated AFTER the implant. In contrast, with high dose rate brachytherapy the implant catheters and treatment positions are known and the dose can be designed in advance of the source delivery. The final treatment plan is completed and approved by the physician before rather than during or after the source is administered.

Other disadvantages of a permanent seed implant are unchanged by the type of isotope. There can be anatomic changes or migration of seeds during the time it takes the permanent seeds to emit the dose, the permanent seeds cannot be placed at the capsule because there is no anchoring tissue, there are prostate size limitations, the bony pubic arch may limit the implant, and so on.
Although Cesium 131 appears to have an advantage over other permanent seed isotopes, in my opinion the overall advantage still goes to temporary interstitial high dose rate brachytherapy.