ABSTRACT
In radiation therapy, the
majority ofMost radiation therapy researches have has
focused on reducing unnecessary radiationdosage
into
normal tissues and critical organs around the target tumor volume. Proton beam therapy
is considered asto be one of
the most promising radiation therapy methods, withgiven
its physical characteristics in the dose distribution, delivering most of the
dose just before protons come to rest, at the so-named called
Bragg peak;. thatThis
is
to say that, proton therapy allows for a
very high radiation dose to the tumor volume, effectively sparing adjacent
critical organs. To maximize these benefits and minimize the risk of
misapplication of the Bragg peak, which is to say, to
guarantee both successful treatment and patient
safety, it is essential to accurately determine
the distal dose edge and effectively monitoring
the in vivo dose distribution. and precisely determining
the distal dose edge is very important for the safety of patient and successful
treatment.
The present dissertation suggestsrecommends
the prompt gamma measurement method tofor
monitoring of the in
vivo proton dose distributions during the treatment,.
and
the present study also verifiesAs a key part of the
process of establishing the utility of that method,
the verification of the clear relationship
between the prompt gamma distribution and the proton dose distribution,
was
accomplished withby
means of
First, the physical properties of prompt gammas were
investigated basedon the basis of
on
cross-section data and mainly generated mainly from the proton-induced
nuclear interactions, and then emitted isotropically
in
less than 10-9 sec withat
the
energies of up to 10 MeV.
in
less than 10-9 sec. The sSimulation
results offor the
prompt gamma yield for the major elements of the human body showed that within the optimal
energy range of 4-10 MeV, the
highest number of prompt gammas areis generated
from oxygen, whilewhereas forover
the entire energy range, it is from calcium.
generates
the more prompt gammas than other elements.
Secondly, to seedetermine[JH1]
the relationship between the proton dose distribution and the prompt gamma
distribution, the present study developed a proof-of-principle measurement
system (the PGS system) employing thea
scanning process. It isFirst-time
experimentally verifiedverification
for the
first time thatwas achieved, not only of the fact that the prompt gammas can be measured during the treatment,
but also that itstheir
distribution has a clear relationship with proton dose distribution for
the therapeutic energy range of proton beams.
Thirdly, for clinical application, a small
array-type prompt gamma measurement system, which could be for
used
without the problematic scanning process, iswas
designed, and its optimal dimensions tofor
effectively reductione of
background gammas arewere
determined (by ;:
that
is, the 3 mm scintillation thickness;
of
3 mm, the 2 mm slit width; of 2
mm, the 2 mm collimation-plate thickness;
of
2 mm, and the150 mm collimation-plate length.
of
150 mm are determined to be the optimal dimensions of the measurement system. To
speed
upaccelerate the simulations,
the present study employeds a
parameterized source term whichthat improveds
the calculation speed up toby a factor of
300.
times.
Finally, the performance of anthe
array-type measurement system for the clinical
application arewas examinedevaluated
with thea test
measurement system which is composedcomprising
of
a multi-slit collimation system, a CsI(Tl)
scintillation detector, and a precise motion system. A methodology to
determine the distal dose edge
from the prompt gamma distribution is proposed in the present study. Additionally,
the phantom effect on the prompt gamma distribution and the
analysis of background gammas arewas studied
by
6.1 Conclusions
In this dissertation, thean
in vivo dose verification method based on a prompt
gamma measurement during proton therapy was suggestedrecommended,
and its feasibility was verified with
First, the physical characteristics of the prompt
gammas generated by the proton-induced nuclear
interactions were investigated withusing
cross-section data and withat 4.44 MeV through the inelastic interactions of 16O(p,x)12C,
12C(p,x)11B, and 12C(p,x)12C, and thenafter
which they it isare emitted isotropically in less than a nanosecond. The simulation
study foron the
prompt gamma yield with the major elements (carbon, oxygen, and calcium) and
organs of athe human
body showeds that
calcium emits more than two times the number of prompt
gammas more than two times thanthat
carbon or oxygen do, when considering the entire energy
range of 0-10 MeV prompt gammas energy
range per one incident proton particle; however, ifwhen
we considered only the optimal energy window of 4-10
MeV, the prompt gamma yield of oxygen was higher than
that of calcium or carbon.
To experimentally measure the prompt gamma distribution and
to seedetermine
its relationship with the proton dose distribution, a proof-of-principle
measurement system (the PGS system) was developed based on a scanning process.
The PGS system was designed with the emphasies
on the suppression of fast neutrons with three layers (the paraffin, B4C, and lead layers) and the
selectively measurement of the gammas
passing through only[JH2] the
collimation holeslit[JH3] .
For the therapeutic proton beams, it iswas
experimentally proved that the location of the distal dose edge in the water
phantom couldcan be
determined by measuring the prompt gamma distribution with the PGS system outside
the water phantom with the PGS system.
For clinical purposes, a small array-type prompt gamma
measurement system, which that could
be used without the problematic scanning process, was
designed, and its optimal dimensions tofor
effectively reductione of
the
background gammas were determined withby
the Monte Carlo method; to be that
is, the3 mm scintillation thickness,
of
3 mm, 2 mm the slit
width, of 2 mm, the2 mm
collimation-plate thickness, of 2 mm, and
the150 mm
collimation-plate length. of 150 mm
were determined to be the optimal dimensions of the measurement system. For theTo
reducetion of the
computational burden in the optimization process, a
parameterized source
term offor the
secondary particles[JH4]
was employed, andwhich reduced
the computation time was reduced by a factor of up to 300
for each simulation case. The simulation results also confirmed
that anthe
array-type prompt gamma measurement system, in its optimal
dimensions, wcould
accurately determinelocate the location
of the distal dose edge, (e.g.,
within a few millimeters of error) for the entire range of therapeutic proton
beam energies.
Finally, the performance of anthe
array-type measurement system was experimentally examined with the test
measurement system withand the
therapeutic proton beams. ThroughBy means of thea
methodology based on the sigmoidal curve fitting, it
was proved that with therapeutic proton beams within the 80 220 MeV energy range, the distal dose
edge could be quantitatively determined within about 4 mm. with
therapeutic proton beams of 80 220 MeV energy range. It is aAlso, it
was revealed that the higher-energy of proton
beams show the more neutron capture gammas and the
moregreater beam dispersion, which
are considered asto be the
main problems in the prompt
gamma measurement. The change[JH5]
of the
prompt gamma distribution due to the phantom variation was studied
by the
OurThe
results showed that the phantom size, shape, and
location do not significantly affect on the prompt
gamma distribution. Although the phantom material variations showed
some alteration in the prompt gamma distribution, the location of the distal
dose edge did not change.
6.2 Future Work
Even though the results of the present
study suggestedsuggested a
methodology for the measurement of the prompt
gamma distributions based onusing
a small array-type measurement system, further studies are necessary forinvolving
clinical use of the methodology in proton therapy are necessary.
First, it is required that a full-scale array-type measurement system be
constructed and evaluated with many different types of
therapeutic proton beams. For more accurate prediction of the distal dose edge,
the level of the background gammas should be as low as possible. It is expected
that the full-scale array-type measurement system will yield better results, along
with a lower level of the background gammas. Note that the
current single-detector system, usesusing
only one detector in a collimation slit, and
therefore allows neutrons and photons to enter the measurement
system through the other neighboring collimation slits
without any interruption. In the full-scale
measurement system, the neighboring collimation slits will be blocked with CsI(Tl) detectors, which will not
only will provide some degree of the shielding
effect
from the incoming neutrons and photons, but also will enable
the
employment of the anti-coincidence counting. The
anti-coincidence counting technique can be used to remove the scattering[JH6]
gammas coming from[JH7]
the neighboring scintillators. The prompt gammas also can
also
be exclusively measured exclusively
by employingusing time
gating, which is thea process of
the
selective measurement considering the time difference between the generation
of prompt gammas (within 1 nanosecond) and the generation
of the
background capture gammas (within hundreds of microseconds).
Although ourthe test
measurement system showed that the distal dose edge of the pencil beam (a
single proton spot) could be predicted from a measured (longitudinal)
distribution of prompt gammas, in order to achieve clinical applicability, it
is also required that the two-dimensional (2D) distribution (not only the
longitudinal but also the lateral distribution) of the proton dose covering the
arbitrary shape of the tumor volume be determined. In the development
of the 2D prompt gamma measurement system for the 2D
proton dose distribution, it may be considered necessary that
(1) the dimensions of the detector sensors arebe
properly determined for both the effective measurement and the spatial
resolution, (2) the energy calibration isbe
performed
automatically performed for a multi-channel
measurement system of sufficient precision[JH8] ,
which could bewould[JH9]
consisted of more than 64 channels, with a
sufficient precision, and (3) the advanced
radiation detection techniques arebe
employed to reduce the background gammas; (for example, the particle
tracking technology maymight not
only reduce the gammas from unwanted directions,
but could also increase the detection
efficiency). In fact, physicians real-time
visualization of proton dose
distributions delivered to patients it maycould
be an epoch-making discovery. that the proton dose
distribution delivered to the patient is visualized in real-time for the
medical staff and the patient; mMoreover,
the combination of this real-time dose confirmation
technology with the cutting-edge technology
such as the Image Guided[JH10] Radiation Therapy (IGRT)
and the Adaptive Radiation Therapy (ART) technologies
would provide greatsignificant additional
benefits forto the patients
treated with proton beams.