Doctor of Philosophy (PhD)
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A technique is examined here that utilizes high energy beta decays from a short lived radioisotope to treat medical conditions such as shallow cancerous lesions. A major benefit of beta particle interaction in tissue is a fixed penetration depth for the charged particle, with dose limited to the ultimate range of the beta particle. This method improves on some current techniques of radioactive brachytherapy, where "seeds" are placed inside patients through temporary or permanent implantation in order to kill cancerous cells or inhibit growth of tissue. The use of low energy gamma-rays is the most common method of treatment currently for brachytherapy, with Ir-192 used in most high dose rate procedures. The 73.8 day half-life of Ir-192 means frequent replacement and the requirement to deal with the logistics of constantly decaying, fixed radioactive sources.
This method instead utilizes the short 14.1 second half-life of indium-116 to quickly deliver dose to a treatment area while decaying to a stable ground state. Since the isotope is very short lived, pumping is used to transport the isotope in a room temperature liquid eutectic between the “activation site” where the In-116 is created, and the “application site” where it is allowed to decay over a target area. In-116 is produced at the activation site through neutron capture on the stable isotope In-115. This radioactive In-116 is then pumped through a sealed, closed loop system using a peristaltic to an application site where it is allowed to decay for one minute, enough time to pass through over four 14.1 second half-lives and reach a stable ground state. This applicator is a sealed spreader surface with a thin barrier to allow passage of the decay betas. The loop is then repeated with the In-115 activated again and pumped.
This work examined the feasibility of this method with three types of neutron sources including a fixed Pu-Be source, a Dense Plasma Focus pulsed fusion neutron source and an XRay producing accelerator using photoneutrons from a beryllium target. Radiation transport modeling was used to determine the efficacy of this technique on two higher output neutron sources, including the use of a standard clinical accelerator used for external beam therapy, the Varian Clinac 2200C. Neutron output from the Clinac was modeled based on photonuclear production in the accelerator components when operated at 20MeV. Dose outputs were found to be viable for clinical use when the system is used on a Clinac due to the substantial photoneutron output.
In addition to therapy, this work demonstrated the ability to measure neutron fluence at a remote location by measuring decaying In-116 from the eutectic after irradiation. In particular, this was demonstrated with the pulsed DPF source and compared with existing yield measurement techniques. The yield of neutrons from pulsed sources can be difficult to measure due to the intense and brief burst, preventing the use of normal radiation detectors that measure radiation over time. Pulsed sources instead require activation of materials to create a signature of the yield magnitude which is then counted after the pulse. This method showed excellent agreement with an existing method of beryllium activation detection.
Indium; Medical Physics; Neutrons; Radiation Therapy
Biomechanical Engineering | Biomedical | Biomedical Devices and Instrumentation | Engineering | Mechanical Engineering
O'brien, Robert, "Radiation Therapy and Dosing Material Transport Methodology" (2016). UNLV Theses, Dissertations, Professional Papers, and Capstones. 2891.
Available for download on Saturday, December 15, 2018