DOS 525 - Week 1 Discussion
Initial Post: Devices and Practices for Simulation and Verification
Radioactive decay can take several forms, including alpha particle decay, beta particle decay, electron capture, and internal conversion.1:18-22 Each of these decay modes produces an new daughter atom with a different nucleus configuration, as well as a release of energy in the form of kinetic energy of the resulting fragments or a photon, called a gamma ray.
An atomic nucleus is made up of positively charged protons and unchanged neutrons bound together by weak and strong nuclear forces.1:5-6 The electrostatic repulsion between the tightly packed protons is counteracted by the weak and strong nuclear forces, but occasionally an event will occur, most commonly in heavy nuclei, that involves two protons and two neutrons (essentially a helium nucleus) being ejected from the original nucleus. The difference in mass between the original nucleus and the two new nuclei is converted into kinetic energy which is shared between the two fragments, though due to the much smaller size of the alpha particle, it usually takes most of the kinetic energy. Because they are large particles, alpha particles have an extremely high linear energy transfer rate, and deposit their energy very quickly in a very localized area as they travel.1:348-349
Alpha emitters do not usually spring to mind when considering options as viable brachytherapy sources because alpha particles usually have a very short range of less than 100 micrometers.2 Most brachytherapy applications use gamma-emitting isotopes that generate between 20 keV and 1 MeV that can penetrate anywhere from a few millimeters to a few centimeters into surrounding tissue.3 This difference in ranges means that it is impossible to create an implantable alpha emitter source, because even the thinnest housings would be thicker than the range of the particle.
Radium-226 was once used as a brachytherapy source, not because it is an alpha emitter, but because its various decay products and their own child products would pass through a series of transformations emitting at least 49 gamma rays ranging in energy from 0.184 to 2.45 MeV.1:309 While radium-226 has fallen out of favor, radium-223 is just now beginning to generate interest specifically for its alpha emitting properties.2 An injectable compound of radium-223 chloride can be administered to prostate patients with castrate-resistant bone metastases, and the radium-223 will be selectively incorporated into the new tumorous bone growth, where it will decay, causing significant local damage to the cancerous osteogenic cells, while the extremely short range of the alpha particles spares surrounding tissues.
A company attempting to commercialize this technique received FDA clearance in 2013 to market the injectable radium-223 chloride as a product called Xofigo.4 I find this to be a very interesting idea, but I am curious to learn more about the unintended consequences of free-floating radium in the blood stream. Yet another new thing to learn about!
- Khan FM, Gibbons JP. The Physics of Radiation Therapy. 5th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2012.
- Nilsson S, Franzén L, Parker C, et al. Bone-targeted radium-223 in symptomatic, hormone-refractory prostate cancer: a randomised, multicentre, placebo-controlled phase II study. Lancet Oncol. 2007;8(7):587-94.
- Khan FM, Gerbi BJ. Treatment Planning in Radiation Oncology. 3rd ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2011:298-308.
- Yao S. FDA approves new drug for advanced prostate cancer. FDA Website. http://www.fda.gov/NewsEvents/Newsroom/PressAnnouncements/ucm352363.htm. Published May 15, 2013. Updated May 17, 2013. Accessed April 8, 2015.