This article was written by student Sharbacha Edward, the second place winner of the 2019 Annual Report Science Writing Contest. Edward is a PhD student with the Program in Medical Physics and her advisor is Stephen Kry, PhD.
“But… if we can’t see the radiation, how do we know that it’s hitting my tumor?” Mariana* thought to herself, as she laid on the treatment table and watched this huge machine rotate across her chest. Since being diagnosed with lung cancer three months prior, it had been an emotional roller coaster ride, which she hoped would end in cancer remission after this round of radiation therapy.
Like Mariana, over half of all diagnosed cancer patients receive some form of radiation therapy. The radiation is delivered using machines called linear accelerators (LINAC), which have the capability to produce high-energy photons and electrons. The radiation oncology teams at cancer centers like MD Anderson use these high-energy particles as weapons against cancer cells and tumors. They are able to direct the radiation so that it causes maximum damage to the tumor while simultaneously leaving healthy tissue unharmed.
However, Mariana does have a legitimate concern. Radiation is not visible to the naked eye. So how do technicians ensure that it goes in the right location? How do doctors and researchers know that the photons pulverized the tumor but not the delicate lung tissue that surrounds it? With practice of course!
The Imaging and Radiation Oncology Core (IROC), a subsidiary of MD Anderson, has developed and built a number of dummy patients called phantoms, which are used to test the accuracy and precision of radiation therapy around the world. These tests are done before cancer centers can enroll patients in clinical trials, or as a check of their radiation systems and clinical processes. There are different phantoms that mimic different disease sites, including head, spine, lung, and even the prostate. The phantoms are made up of materials that represent the human anatomy they simulate, such as dense polymers for bone and lighter cork for lung tissue. They also have tumors in different locations to mimic typical cancer occurrence.
When a cancer center receives an IROC phantom, their goal is to treat the phantom like they would a patient. They take computed tomography (CT) images, create a treatment plan, and deliver radiation to the phantom’s tumor, while aiming to spare the healthy surrounding tissue. In order to determine whether this is done successfully, IROC places tiny dose measurement devices, called thermoluminescent dosimeters (TLD) inside the tumor and sensitive organs (e.g., heart and spinal cord), before the phantom is shipped to a center for radiation treatment. When the phantom is returned after treatment, IROC personnel read the radiation dose recorded by the TLDs. These doses are then compared to the doses which were calculated by the treatment plan created for that phantom, and a dose agreement within ±7% constitutes a successful phantom treatment. This is a test of a cancer center’s ability to accurately create a plan to deliver radiation and then successfully deliver that plan to the exact spot inside the phantom that they intended to. This is a team effort, and so
the entire process from start to end, involving all members of the radiation oncology team, is being tested on these dummy patients in order to perfect it for real patient treatment.
IROC has operated this phantom program for almost two decades, and has collected thousands of phantom results from institutions all over the U.S. and the world. The failure rate, on average, for all phantoms is 15%. This means that 15% of the time, the phantom is incorrectly treated by a cancer center. These results raise very real concerns about the accuracy of actual patient treatments. If a cancer center cannot accurately direct radiation to the tumor in a phantom, how do they handle tumors in actual patients?
This is a major problem, and the first step to solving it is knowing exactly what causes clinics to perform poorly when administering radiation to cancer patients. This is the crux and focus of our research. Whether deemed a pass or fail, all these phantom tests provide invaluable data for us to study. We aim to use phantom test data to investigate trends, patterns and pitfalls in order to gain knowledge about the areas of the treatment process that need improvement. We are evaluating potential causes
of error, such as dose calculation inaccuracies, treatment complexity, and LINAC calibration problems. Uncovering the root problems that cause the radiation oncology team to fail these phantom tests will equip us with the knowledge required to help centers improve their radiation treatment process, and thereby improve the success of their patient treatments.
So, although Mariana cannot see the radiation that is being directed at her tumor, we can see it through our phantoms and through our research. As the machine moves its way back across from her left side to the right, we are continuously working to ensure that every photon that enters her body will attack the tumor head on!
Through this work, patient treatments will improve, and subsequently so will patient outcomes and survival not just in the U.S., but around the world.
*Mariana is a fictitious name and no patient information was used for this story.