Lung cancer is the leading cause of cancer mortality in the United States. The five-year survival rate is only 18.6 percent, according to the American Lung Association1, lower than that for many other types of cancer. For locally advanced, inoperable lung cancer, radiation therapy, known as radiotherapy, combined with chemotherapy is the treatment of choice. Research efforts have explored whether advanced imaging techniques can help guide radiotherapy, making it more precise, and thus more effective, by identifying the location of tumors more accurately.
Currently, radiotherapy treatment for lung cancer is planned using computed tomography (CT) scans of patients, which measures their breathing patterns. On these images, tumors can be hard to spot because of the way the lungs move during respiration.
One advanced imaging technique that researchers have been exploring is the use of magnetic resonance imaging (MRI) in the radiotherapy treatment pathway for lung cancer. MRI has excellent soft-tissue visualization and motion resolving capabilities. The lungs are one of the most challenging organs to image with MRI because of the presence of air combined with motion from breathing and heartbeat. Because the majority of lung volume consists of air, there are few protons for the MRI scanner to detect. If researchers were able to overcome these challenges, MRI could be used in conjunction with CT and PET scans to improve imaging accuracy, and potentially even radiotherapy efficacy.2
New scanners are emerging that combine imaging with radiotherapy to first accurately locate tumors, then deliver precise doses of radiation directly to those tumors. The potential is to provide additional benefit to patients by increasing personalization of treatment.2
A new technique using MRI
A study published in May 2018 in Radiotherapy and Oncology by physicists from the Institute of Cancer Research, London and the Royal Marsden National Health Service Foundation Trust demonstrates a new way to use MRI, a method the authors have dubbed super-resolution T2-weighted four-dimensional (4D) MRI.3 This technique combines traditional, two-dimensional MRI images to create super-resolution videos that show the lungs expanding and contracting. The hope is that this will allow clinicians to deliver more precise, and thus effective, radiotherapy for patients with diseases like lung cancer because it will allow them to predict the location of a tumor more effectively, even as the lungs move.
The technique yields images of the lungs that are five times more detailed than other types of MRI methods. Getting this level of detail can often involve long scan times that are uncomfortable for patients, but the new technique also allows physicians to obtain the images quickly.
In the study, the authors scanned eight healthy volunteers using standard T2-weighted MRI methods, producing images in both the sagittal and axial planes. Image slices were then sorted slice by slice and stitched together by a computer, creating accurate, high-resolution videos of the study subjects breathing in and out.
The super-resolution video images that resulted showed fewer artifacts or visual errors that result from missing data and can obscure important details, compared to other techniques. Yet the motion of the lungs that was easily visualized at low resolution was retained in the super-resolution version of the images.
This new technique can also produce detailed images of “slices” of the body at any time or position, by combining multiple MRIs of the body from different orientations.
Potential for Treatment
Accurately pinpointing the location of tumors in lung cancer patients can increase the efficacy of radiotherapy by aiding oncologists in applying it more precisely. For radiotherapy systems that image and treat tumors simultaneously, the ability to visualize tumors in moving lungs with this level of high resolution is key to their successful use. And as tumors shrink during the course of radiotherapy treatment, super-resolution T2-weighted 4D MRI may be able to provide up-to-date, accurate images that allow for the continued precise targeting of radiotherapy to areas of the tumor.
There are still more steps researchers must take before this new technique reaches the clinic. This study looked at healthy subjects, so studies are needed to validate the method’s usability with lung cancer patients.
- Lung cancer fact sheet. American Lung Association. http://www.lung.org/lung-health-and-diseases/lung-disease-lookup/lung-cancer/resource-library/lung-cancer-fact-sheet.html. Accessed September 7, 2018.
- Magnetic Resonance Imaging in Precision Radiation Therapy for Lung Cancer. Translational Lung Cancer Research. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5709138/ Last accessed September 7, 2018.
- Super-resolution T2-weighted 4D MRI for Image Guided Radiotherapy. Radiotherapy & Oncology. https://www.thegreenjournal.com/article/S0167-8140(18)30274-3/fulltext#s0060 Last accessed September 7, 2018.