Radioisotopes have half-lives ranging from a few seconds to billions of years. Many cancers are asymptomatic in their early stages, and can go undetected for 10 years or more [1]. How do these two relate?
Exotic nuclei, a type of radioisotope, are created through reactions that occur when nuclei are accelerated and smashed into other nuclei at production facilities like the Facility for Rare Isotope Beams (FRIB) at Michigan State University [2]. In the system, a heavy ion beam is accelerated as it travels through the Superconducting Radio-Frequency Heavy Ion Linear Accelerator, creating fission fragment ions that are directed onto a target material to induce nuclear reactions that add or remove certain subatomic particles from the beam. What results is the production of exotic nuclei in flight [2].
While the focus of the FRIB is to further the study on exotic nuclei, attention has also been given to the numerous byproducts formed from the subatomic particles in the reaction. Before, the byproduct isotopes were simply transported to a rotating water-filled beam dump to be safely discarded [2]. However, discoveries show that if these particles were to remain in the beam dump, they will continue to undergo multiple nuclear reactions such as fragmentation, fission, and spallation, creating new isotopes [2]. Instead of discarding them, scientists are now considering methods of “recycling” the isotopes (especially radioisotopes) to use in a variety of fields, such as cancer treatment [2].
In a process called “isotope harvesting”, excess ions left over from reactions are transported to various medical labs for clinical diagnostic nuclear medicine scans. Many of these radioisotopes are used in the 1.5 million scans made per year by the 3-dimensional tomographic technique of Positron Emission Tomography (PET) [3]. Used for non-invasive cancer staging, these scans measure the chemical activity of tissues by attaching a radioisotope to chemical substances naturally used by a particular organ or tissue suspected of harboring cancerous cells [3]. Since cancer cells undergo metabolic reactions faster than normal cells, they will utilize more of the radioisotope-tagged substance relative to normal cells. That radioisotope will undergo decay and emit positron particles that are detected by the gamma rays of the PET scanner to image the organ in real time [4].
Unlike PET, however, targeted radiotherapy uses alpha particles instead of positrons [5]. If the goal of curing cancer is by killing more cancerous cells than healthy cells, targeted radiotherapy is much more effective than PET. Alpha particles, which are injected internally in targeted radiotherapy, are much larger and operate over a shorter distance than positrons [5]. When delivered correctly, alpha particles can target the cancer cell specifically, with a concentrated high dose of radiation, all while leaving healthy tissue alone. The same cannot be said of PET that utilizes positrons, which operate externally and affect both healthy and cancerous cells.
Examples of targeted radiotherapy that use isotopes like Ac-255 for prostate cancer [6] and Ra-223 for bone cancer [7] have already shown success. However, both of these isotopes emit multiple alpha particles. In comparison, At-211 is one of the few single alpha-emitting radionuclides, which makes it applicable across a wide range of cancers[8]. Isotopes that emit only one alpha particle are much safer because all of the energy can be directed to the same location [8]. Traditionally, At-211 was written off due to its scarcity as an extremely rare element [9]. But with isotope harvesting, it is possible to create this radioisotope chemical agent for use.
So, how exactly is At-211 “harvested”? In a study on U-238 fragmentation, scientists discovered that a large amount of Rn-211 was produced as a byproduct of the reaction, which eventually decays to At-211 in the water-filled dump storage [2]. This process is significant beyond just simply harvesting At-211 — it also combats the radioisotope’s shorter half-life issue [2].
With a half-life of only 7 hours, much of the At-211 created would decay in the time it takes to transport the sample from the production facility to the medical laboratory [3]. Coincidentally, Rn-211 has a longer half-life of 15 hours, making it suitable for shipment over longer distances [2]. This way, At-211 is preserved for use once the water dump tank reaches the medical facility [2]. In other words, the issue of time wasted on transportation during the 7 hour half-life of At-211 can be solved by harvesting Rn-211, which is allowed to decay in transit.
The At-211 radioisotope and its application to targeted radiotherapy shows us the emerging technology of isotope harvesting across the medical field with the potential of saving hundreds of thousands of lives. Who knows what other isotopes, previously considered impossible to use in cancer treatment, can now be recycled and used? The possibilities are endless, but one thing is for certain: “harvesting” Astatine-211 can take us one step closer to finding the cure for cancer.