Download Radiotherapy treatment planning based solely on magnetic

Department of Radiation sciences
Umeå University
Radiotherapy treatment planning based
solely on magnetic resonance imaging
Thesis paper for Master of Science in Medical Radiation Physics
By Joakim Jonsson
Regional University Hospital of Umeå
Umeå University
Department of Radiation Sciences, Radiation Physics
Thesis paper for Master of Science in Medical Radiation Physics
Date 2008-01-26
Author Joakim Jonsson
Supervisor Magnus Karlsson
Division Department of Radiation Sciences, Radiation Physics
Radiotherapy treatment planning based solely on magnetic resonance imaging
Abstract
Radiotherapy treatment planning is today based on computer tomographic (CT) images, a
modality that employs ionizing radiation to form a 3D image of the patient. Since the image
displays how the radiation interacts with the material, i.e. how much of the radiation that is
attenuated, it is a very good base for dose calculation which is the important quantity in treatment
planning. The drawback of CT images is the poor soft tissue contrast, since the attenuation in soft
tissue is fairly constant regardless if the radiation passes through muscle, fat, brain or a tumor.
For this reason, magnetic resonance (MR) imaging has become more interesting for treatment
planning since it has superior soft tissue contrast, making it possible to delineate tumors and
other soft tissue organs with greater accuracy.
The MR images does not offer the attenuation information provided in CT images, making it
impossible to perform dose calculations. To get around this problem, different parts of the
anatomy can be delineated and manually assigned mass density values, making dose
calculations possible. Also, the MR images have an inherent geometric distortion caused mainly
by inhomogeneities in the magnetic fields. These can be corrected for by software, but the
accuracy needed to be assessed before the clinical use of the method could be put into place.
The geometric distortion was investigated by comparing CT images to MR images of the same
patients and also by comparing the length of a measuring stick to the imaged length. The results
show that the geometric distortion after the software correction was very small and should not
cause significant error to the treatment plans.
To assess the dosimetric accuracy of the manually assigned densities, treatment plans were
generated on normal CT images, CT images with assigned mass density and MR images with
assigned mass density and were compared. 40 patients were investigated in 4 different
anatomical regions, 10 in each. For the CT images with manually assigned densities, the mean
difference of the different anatomical regions was within ± 0.3 %, and no single patient exceeded
2.0 % difference compared to a normal CT based treatment plan.
.
MR images with assigned mass densities served as base for treatment planning in 20 patients
with cancer in the thoracic or pelvic region. In the prostate region, a total mean error of -0.8 %
was present. This error can be reduced by better routines in the MR imaging. In the thoracic
patients, the mean error was 0.0 % based on MR imageswith assigned mass densities.
From a dosimetric standpoint, it is very feasible to base treatment plans on MR images. There are
however some practical problems that need to be addressed before this routine can be clinically
implemented.