| Abstract |
Nowadays, a continuous improvement on modern radiotherapy techniques is
observed. Therefore, 3D dose verification and accurate determination of the tumor
target volume are essential. The aim of the current study was twofold: (a) to assure
the radiotherapy dosimetric results by the utilization of MRI polymer gel dosimetry
methodologies and (b) to optimize the dosimetric radiotherapy results in conformal
radiotherapy by the utilization of Magnetic Resonance Imaging in special phantoms.
Nowadays, polymer gel dosimeters have been proven a valuable tool for the
beam-dose characteristics measurements and 3D radiation dose verification in
radiotherapy. The overall polymer gel dosimetry procedure resides in the field of
chemical dosimetry and consists of: (a) the preparation of a certain chemical
composition polymer gel, (b) the irradiation of the gel, (c) the readout process and
(d) the mathematical analysis method used for the estimation of the final irradiated
doses. A typical gel dosimeter consists of water, a gelatin agent, monomers and a
cross-linker co-monomer. Upon irradiation, the monomers are polymerized
(radiation-induced polymerization). The extent of polymerization is related to the
absorbed dose. The changes in the polymer gel can be visualized utilizing different
imaging modalities such as Magnetic Resonance Imaging (MRI), Optical Computed
Tomography (CT), x-ray CT and Ultrasound. The appropriate processing of the results
for each modality generates a relation between the absorbed dose and the contrast
parameter basis of each method. The estimation of the uncertainty in each
implementation step is going to increase the reliability of this dosimetric procedure.
The increased sophistication of modern radiotherapy planning techniques
such as conformal (CFRT) necessitates improved means of defining target volumes
for treatment. This step remains the most crucial and difficult part of the
radiotherapy planning process, otherwise a geographical miss of the tumor or
systematic error will be perpetuated throughout therapy. MRI is being increasingly
used in oncology for staging, assessing tumor response and evaluating disease
recurrence. Similarly, the improvement characterization of soft tissues and
visualization of tumor extent using MRI can be used to benefit the radiotherapy
treatment planning (RTP) process from delineation and treatment response.
Unfortunately, there are some limitations in MRI methodologies for the unique
utilization of MRI in radiotherapy treatment planning. For this reason last years, coregistration
images of MRI and CT are used for the radiotherapy treatment planning.
The current study can be divided into three parts.
The first part was focused on the optimization of the chemical formula, the
preparation methodology of the N-vinylpyrrolidone polymer gels for the utilization in
3D verification of modern radiotherapy techniques. Also, the optimum time intervals
between ‘gel manufacture’ – ‘irradiation’ – ‘MR scanning’ were assessed, based on
the optimum sensitivity of the dosimetric system. Amongst these intervals, beam
profile measurements have been performed in order to assess the effect of the gel
system dose characteristics in clinical beam measurements. It has been shown that
the increase of the N-vinylpyrrolidone concentration enhanced the dosimetric
characteristics at low level doses. Unfortunately, this increase in concentration
resulted in a dose range response restriction. Regarding the influence of preparation
conditions in dosimetric characteristics of these gels, it was revealed that under
anoxic preparation conditions the dose sensitivity and the dose resolution at one day
post-irradiation were improved as compared to the same gel manufacturing under
normal atmospheric conditions. The temporal stability was also improved under
anoxic preparation conditions for the time period of one month post-irradiation.
These results validated the importance of anoxic and normal atmospheric
manufacturing conditions in the dosimetric performance of normoxic Nvinylpyrrolidone-
based polymer gels. Moreover, it was presented that the dosimetric
characteristics of the evaluated polymer gel system were rapidly deteriorated if the
irradiation process took place for a period more than 1 week after gel
manufacturing. On the other hand, it seemed that the time between irradiation and
MR scanning didn’t affect the gel system dose characteristics. It has been confirmed
that the %dose profiles measured with the presented polymer gel system within the
mentioned ‘gel manufacture – irradiation – MR scanning’ time periods were rather
reliable and valuable. Polymer gels seemed to h ave a c ertain r ole for modern
radiotherapy techniques treatment plan validation and relative dose distribution
measurements. The presented [VIPET / MR-PHAPS / weighted-linear-regression]
polymer gel system accompanied with the derived characteristics and practical
limitations of use, could be conveniently and reliably used in clinical practice.
The second part was focused on the optimization of the readout process and
the mathematical analysis method used for the estimation of the final irradiation
doses utilizing of the proposed polymer gel system. Nowadays, different imaging
modalities applied for the readout and analysis processes are used. These include
Magnetic Resonance Imaging (MRI), Optical Computed Tomography (opt-CT), x-ray
CT and Ultrasound. Amongst them, MRI is an established methodology for the
readout process in polymer gel dosimetry. The last ten years the multiple echo spin
echo (MESE) pulse sequence is well-established in MRI polymer gel dosimetry. The
main drawback of this pulse sequence is the relatively long acquisition time. In order
to overcome this problem a new Multi- Echo Half Fourier Acquisition Single Shot
Turbo Spin Echo sequence (MEHASTE) was presented. Three fitting regression
algorithms were utilized for the assessment of the dosimetric characteristics with
MEHASTE sequence. It was finally presented that the most accurate algorithm was
the one that minifies the effect on the image background and noise. This algorithm
was utilized for the comparison of the dosimetric characteristics of the polymer gel
system using the new pulse sequence (MEHASTE) and with the standard one (MESE).
These results revealed that the two methods did not exhibit large deviations and
MEHASTE could be an alternative faster and qualitative method for MRI polymer gel
dosimetry. Furthermore in this part, the dosimetric results between the treatment
planning calculations and polymer gel dosimeter measurements were compared,
using the new pulse sequence (MEHASTE) and the most accurate fitting regression
algorithm. The two comparison methods were the isodose lines and the gamma
index factor. The results between the qualitative (isodose lines) and the quantitative
(gamma index factor) comparison method didn’t deviate.
In the third part of the current study, the co-registration procedure of MRI
and CT images was presented and their implementation in radiotherapy treatment
planning was assessed. It seemed that the resulted co-registration images can
present the anatomical characteristics of hard as well as soft tissues. Moreover, the
dose maps of the polymer gel dosimeters were co-registrated with the CT and MRI
images of the selected series of phantoms. This procedure revealed the ability of the
fused images to present the dosimetric results co-registrated with the anatomical
information. Finally, it was confirmed that the complementary role of MRI in
radiotherapy treatment planning is crucial for the precise definition of tumor
volumes in RTP.
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