CS-131: One of the Most Significant Advancements in Brachytherapy

In today’s never ending search for the permanent cure for the various types of cancer, many treatments have been developed. Some work, some don’t, while others need more improvement before they can fully do the job. Brachytherapy, a form of radiation therapy where radioactive seeds – or isotopes – are placed near or inside the tumor to emit radiation, is one of the most intriguing both in the medical and investor community.

The isotopes used most commonly in brachytherapy include Cesium-137, Cobalt-60, Iridium-192, Ruthenium-106, Iodine-125, and Palladium-103, with the latter two used particularly for prostate cancer treatments. The isotope Cesium-131 (CS-131), however, is considered to be one of the most significant advancements in brachytherapy.  A number of studies comparing CS-131 to other isotopes commonly used in internal radiation therapy, including a 2009 study published in Cancer, Biotherapy and Radiopharmaceutical Journal and a similar study performed by the IsoRay Inc. advisory board, concluded the isotope’s radioactive properties make it a safer, yet stronger, treatment option.

Focus was put on the isotope “energy,” the capability to penetrate tissues and deliver doses uniformly throughout the prostate, and “half-life,” the speed of radiation emission. The medical community tends to favor isotopes with higher average energy and shorter the half-life (at a certain dose), as these seeds more fervently attack cancer cells in less time, thus exposing the patient to less radiation.

A comparison of CS-131 with Palladium-103 (Pd-103) and Iodine-125 (I-125) concluded CS-131 to have the highest average energy and the shortest half-life. Using a 90 percent dose, the 30.4 KeV energy CS-131 was more uniformly delivered throughout the prostate than I-125 (energy of 28.5 KeV) and Pd-103 (20.8 KeV). Cs-131 provides more homogeneity, delivering the needed dose without causing damage to the other organs in close proximity of the prostate.

The study also determined CS-131 to possess the shortest half-life of the leading prostate brachytherapy isotopes. A 90 percent dose takes approximately 33 days to be fully delivered to the prostate. A 90 percent dose delivery of Pd-103 takes 58 days; it’s even slower for I-125, reaching up to 204 days. This is explained by the marked differences in the three isotopes’ half-life: Pd-103 and I-125 have 17 days and 60 days, respectively; meanwhile, Cs-131 has a half-life of only 9.7 days. Iridium-192 has a half-life of 74 days, while other isotopes have a half-life spanning years. This means that, with Cs-131, the patient has to deal with the discomfort and irritation of radiation therapy for a shorter time.

IsoRay Inc, a public company based in Richland, Washington, is the exclusive manufacturer and distributor of Cs-131. The isotope is FDA-cleared for the treatment of prostate, lung, ocular melanoma, brain, breast, colorectal, and head and neck cancer, among other cancers in the body. In December 2011, WestPark Capital announced a $2.6 million IsoRay offering. The company has focused recent funding toward opportunities in breast, pancreatic and gynecologic cancers and the rollout of the world’s first balloon catheter device used in the treatment of brain cancer.

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