The Next Generation of Radiotherapy
The Medical Physics Department at the Cross Cancer Institute, of the Alberta Health Services, (AHS-Cancer Care) has built a prototype Medical Linear Accelerator (Linac) coupled to a Magnetic Resonance Imaging System (MR or MRI) for real-time guidance of radiotherapy treatments.
This project “Advanced Real Time Adaptive RadioTherapy” (ART)2 is focused on developing the World’s first Image Guided Radiotherapy (IGRT) system that will utilize MR guidance. The goal of (ART)2 is “Seeing, Adapting, Treating”
Radiotherapy is a proven method of treating and curing cancer. Current radiotherapy machines use X-ray or CT based imaging to verify the position of the tumor prior to treatment. The images provided by MR imaging are far superior to those from CT when trying to distinguish tumors from the soft tissue surrounding them. However, it has always been thought to be impossible to connect a Linac to an MR scanner. The group led by Dr Gino Fallone and Brad Murray hold patents that solve these problems allowing the integration of an MR scanner and a Linac.
Typical Images of the Prostate
Real Time Imaging
Current Image Guidance systems for radiotherapy rely on CT imaging or 2D X-ray imaging. These systems are plagued by two issues.
- The CT scans currently take a couple of minutes to measure and calculate, and after reviewing the images, the patient is re-positioned and treated based on these images. These systems cannot provide any real time feedback during treatment to see if there is any change in position during treatment.
- The X-ray systems are capable of capturing 2D images of the treatment field; however, due to the fundamental physics limitations these “Portal Images” have intrinsically low contrast and have limited usefulness.
There are alternative methods of image guidance where gold seed markers, or active beacons, are permanently implanted into the body. However, these systems do not have the ability to view the size and shape of the tumor to detect any changes. Thus none of the current systems are suited for good image guidance or adaptive therapy.
MR guidance is the solution to the above problems. The implementation being proposed at the Cross Cancer Institute will solve these problems by integrating an MR imaging system along with the linear accelerator. Not only will the system be able to generate high quality 3D images, but it will be able to take streaming images during treatment. This will provide the clinician with 3D MR scans daily, so they can easily quickly and accurately determine if the tumor volume and surrounding structures have changed size and shape thus requiring the plan to be modified or adapted to the new anatomical shape. As well it will allow multiple images to be taken per second, which will be quick enough to track the tumor position in real time during treatment. The radiation beam will be able to follow the motion of the tumor for those cases such as lung tumors where there is significant motion during each treatment.
The Cross Cancer Institute (CCI) has a long standing
project called IGAR
(Image Guided Adaptive Radiotherapy). IGAR has four
pillars, one of which is (ART)2. The
(ART)2 program was initiated in 2005 to
develop the next generation of radiotherapy treatment
machine. AHS-ACB holds the patents (pending) on the
technology necessary to integrate a linear accelerator
to a Magnetic Resonance Imaging system. The first
prototype is now built and is being tested. The present
prototype, composed of a biplanar 0.2 T MR system with
a 27 cm square opening coupled to a 6 MV linac in a
custom configuration, is being used for proof of
principle. The fundamental proof of principle has been
achieved, as we have integrated these two devices and
been able to produce radiation and images. This
prototype will continue to be developed and tested to
show that real time imaging will be possible. Shielding
for both magnetic and radio frequency interference has
been achieved for operation. Future testing will also
include rotating the system to prove that the imaging
system will work at all treatment angles. We have done
significant computer simulations to understand and test
the system. Simulations of the effect of the rotating
magnet show that we can predict the change in the
Larmor Frequency of the MRI system due to the
earth’s magnetic field. We have also simulated
the radiation transport through the patient when
immersed in a magnetic field. Our simulations have
shown small and predictable differences between the
dose deposition in the presence of the magnetic field
compared to those with no magnetic field which can be
corrected in various ways.