Leventouri, Theodora

The purpose of the study is to investigate the effect of preparation temperature on the
crystal structure and magnetic properties of nanophase Fe-substituted Hydroxyapatite. Multisubstituted
Hydroxyapatite (HAp), Ca5(PO4)3OH, is the main mineral phase in physiological
apatite, ~70 wt% in bones and dentin, and ~96 wt% in enamel. Iron is one of the minor
substitution elements (0.01-0.1 wt% in bone and 0.003 wt% in enamel) replacing Ca in the HAp
structure. Research interest on Fe-HAp is related to the fact that Fe for Ca substitution reduces
the solubility of HAp therefore it functions as a cavities preventive agent. In contrast, Fe
overload causes a decrease in bone mechanical strength. Furthermore, Fe-HAp can find
applications in hyperthermia based anti-cancer treatments and in magnetic resonance imaging
(MRI) contrast agents. Two sets of Ca5-xFex(PO4)3OH samples (x=0,0.05,0.1,0.2,0.3) were
synthesized by a chemical precipitation method at physiological temperature (370C) and 800C.
The samples were calcinated at 6500C and deuterated at 6000C. The samples were characterized
by x-ray powder diffraction (XRD), neutron powder diffraction (NPD) and SQUID
Magnetometry. A single-phase system was detected for nominal iron content x≤0.1 by XRD
phase identification in both sets of samples, while hematite (α-Fe2O3) and/or maghemite (γ-
Fe2O3) develops starting at x=0.1. According to the magnetic measurements, sample with x=0
showed diamagnetic behavior while samples with Fe showed paramagnetic behavior. Combined
Rietveld refinements of XRD and NPD patterns will provide accurate information on the effect
of processing temperature on the crystallographic parameters of the nano-materials.

Synthetic hydroxyapatite HAp bears poor mechanical properties that limit its
applicability in orthopedics. We study the possibility of overcoming such limitations by
incorporating functionalized single walled carbon nanotube fSWCNT and polymerized є-
caprolactam. Sonication method was used to disperse fSWCNT in the HAp. A simple hot
blending method was used to incorporate HAp/fSWCNT powder with melted polymerized
є-caprolactam. The fracture toughness of the composite material was tested in
compliance with ASTM D-5045 standard. A critical stress intensity factor K1C of the composite
material was found to be 3.55 MPa.m1/2, which is a value comparable to the one for cortical
bone.

The goal of this study was to improve dosimetry for pelvic, lung, head and neck, and other cancers sites with aspherical planning target volumes (PTV) using a new algorithm for collimator optimization for intensity modulated radiation therapy (IMRT) that minimizes the x-jaw gap (CAX) and the area of the jaws (CAA) for each treatment field.
A retroactive study on the effects of collimator optimization of 20 patients was performed by comparing metric results for new collimator optimization techniques in Eclipse version 11.0. Keeping all other parameters equal, multiple plans are created using four collimator techniques: CA0, all fields have collimators set to 0°, CAE, using the Eclipse collimator optimization, CAA, minimizing the area of the jaws around the PTV, and CAX, minimizing the x-jaw gap. The minimum area and the minimum x-jaw angles are found by evaluating each field beam’s eye view of the PTV with ImageJ and finding the desired parameters with a custom script. The evaluation of the plans included the monitor units (MU), the maximum dose of the plan, the maximum dose to organs at risk (OAR), the conformity index (CI) and the number of fields that are calculated to split.
Compared to the CA0 plans, the monitor units decreased on average by 6% for the CAX method with a p-value of 0.01 from an ANOVA test. The average maximum dose remained within 1.1% difference between all four methods with the lowest given by CAX. The maximum dose to the most at risk organ was best spared by the CAA method, which decreased by 0.62% compared to the CA0. Minimizing the x-jaws significantly reduced the number of split fields from 61 to 37.
In every metric tested the CAX optimization produced comparable or superior results compared to the other three techniques. For aspherical PTVs, CAX on average reduced the number of split fields, lowered the maximum dose, minimized the dose to the surrounding OAR, and decreased the monitor units. This is achieved while maintaining the same control of the PTV.

The purpose of this research is to accurately match the calculation environment, i.e. the treatment planning system (TPS) with the measurement environment (using a 2-D diode array) for lung Stereotactic Body Radiation Therapy (SBRT) patient-specific quality assurance (QA). Furthermore, a new phantom was studied in which the 2-D array and heterogeneities were incorporated into the patient-specific QA process for lung SBRT.
Dual source dual energy computerized tomography (DSCT) and single energy computerized tomography (SECT) were used to model phantoms incorporating a 2-D diode array into the TPS. A water-equivalent and a heterogeneous phantom (simulating the thoracic region of a patient) were studied. Monte Carlo and pencil beam dose distributions were compared to the measured distributions. Composite and individual fields were analyzed for normally incident and planned gantry angle deliveries. The distributions were compared using γ-analysis for criteria 3% 3mm, 2% 2mm, and 1% 1mm.
The Monte Carlo calculations for the DSCT modeled phantoms (incorporating the array) showed an increase in the passing percentage magnitude for 46.4 % of the fields at 3% 3mm, 85.7% at 2% 2mm, and 92.9% at 1% 1mm. The Monte Carlo calculations gave no agreement for the same γ-analysis criteria using the SECT.
Pencil beam calculations resulted in lower passing percentages when the diode array was incorporated in the TPS. The DSCT modeled phantoms (incorporating the array) exhibited decrease in the passing percentage magnitude for 85.7% of the fields at 3% 3mm, 82.1% at 2% 2mm, and 71.4% at 1% 1mm. In SECT modeled phantoms (incorporating the array), a decrease in passing percentage magnitude were found for 92.9% of the fields at 3% 3mm, 89.3% at 2% 2mm, and 82.1% at 1% 1mm.
In conclusion, this study demonstrates that including the diode array in the TPS results in increased passing percentages when using a DSCT system with a Monte Carlo algorithm for patient-specific lung SBRT QA. Furthermore, as recommended by task groups (e.g. TG 65, TG 101, TG 244) of the American Association of Physicists in Medicine (AAPM), pencil beam algorithms should be avoided in the presence of heterogeneous materials, including a diode array.

Hydroxyapatite (HAp) is an ideal bioactive material that is used in orthopedics.
Chemical composition and crystal structure properties of HAp are similar to the natural
bone hence it promotes bone growth. However, its mechanical properties of synthetic
HAp are not sufficient for major load-bearing bone replacement.
The potential of improving the mechanical properties of synthetic hydroxyapatite
(HAp) by incorporating carboxyl functionalized single walled carbon nanotubes
(CfSWCNT) and polymerized ɛ-caprolactam (nylon) is studied. The fracture toughness,
tensile strength, Young’s modulus, stiffness and fracture energy were studied for a series
of HAp samples with CfSWCNT concentrations varying from 0 to 1.5 wt. % without, and
with nylon addition. X-ray diffraction (XRD), Scanning Electron Microscopy (SEM),
Transmission Electron Microscopy (TEM) and Differential Scanning Calorimetry (DSC)
were used to characterize the samples. The fracture toughness and tensile test was
performed under the standard protocol of ASTM D5045 and ASTM D638-02a respectively. Reproducible maximum values of (3.60 ± 0.3) MPa.m1/2 for fracture
toughness and 65.38 MPa for tensile strength were measured for samples containing 1 wt.
% CfSWCNT and nylon. The Young’s modulus, stiffness and fracture energy of the
samples are 10.65 GPa, 1482.12 N/mm, and 644 J/m2 respectively. These values are
comparable to those of the cortical bone. Further increase of the CfSWCNT content
results to a decreased fracture toughness and tensile strength and formation of a
secondary phase.

The effect of processing temperature on the crystal structure properties of the Fe-substituted Hydroxyapatite (Fe-HAp) was studied by using the Rietveld refinement method of powder x-ray (XRD) and neutron diffraction (NPD) patterns. Superconducting QUantum Interference Device (SQUID) magnetometry, transmission electron microscopy (TEM) and x-ray fluorescence spectroscopy (XRF) were used to study the magnetic properties, particle morphology and chemical composition of the prepared samples. Two sets of samples of chemical formula Ca5-xFex(PO4)3OH were prepared with x = 0, 0.05, 0.1, 0.2 and 0.3 by using processing temperatures of 37°C and 80°C, following a two-step co-precipitation method. A single phase HAp was identified in samples with x = 0 and 0.05. Processing temperature affects the type and percentage of secondary phases: hematite was detected in samples prepared at 37°C with x ≥ 0.1, hematite and maghemite were detected in samples prepared at 80°C with x = 0.2 and 0.3. Rietveld refinements of NPD and XRD patterns showed that the a lattice constants are greater in Fe-substituted samples prepared at 37°C, whereas the c lattice constants are greater in the 80°C samples for x ≥ 0.05. Fe preferentially substitutes at the Ca2 site in the 80°C samples, whereas Ca1 is the preferred substitution site in the 37°C samples. Fe substitution results to a decrease of the lattice constants at both preparation temperatures. The ratios Fe/(Fe + Ca) of the refined atomic fractions of the samples prepared at 80°C are greater than those of the 37°C samples. Further, more secondary phases form in samples prepared at 37°C compared to 80°C samples. The magnetic measurements reveal that pure HAp is diamagnetic, whereas samples with x = 0.05 and 0.1 are paramagnetic. Samples with x = 0.3 showed superparamagnetic behavior based on ZFC and FC measurements. Similar hysteresis loops in samples x = 0.2 and 0.3 indicate that the samples with x = 0.2 may show superparamagnetic properties. For x = 0.2 and 0.3, the samples prepared at 80°C showed higher magnetization compared to the 37°C samples, because of the maghemite secondary phase. Based on the TEM images, Fe substituted HAp nanoparticles prepared at 37°C are mainly spherically shaped, and the 80°C particles are mainly elongated. Increase of the Fe concentration favors formation of elongated particles and larger spherical particles. The XRF measurements confirm the Fe for Ca substitution in the HAp structure based on the decrease of the Ca/P and the increase of the Fe/(Fe + Ca) atomic ratios with the Fe concentration.

The purpose of this study is to investigate the changes of the pericardial dose at different respiratory phases and statuses in accelerated partial breast irradiation (APBI) using Cyberknife M6™ multileaf collimators (CK-MLC). Anonymous 6 female patient files with respiration gated four-dimensional computed tomography (4DCT) sets, and 6 left breast cancer cases with CT images in free-breathing (FB) and deep inhalation breath-hold (BH) were selected. One CT image set from each patient was planned for APBI in Accuray Multiplan™ 5.2, and respectively compared its pericardial dose with those from CT sets of other respiratory phases. All the comparable CT images were fused in the planning system according to the left chest wall, among which the lung gap anterior to the pericardium varies by the lung expansion. For the purpose of this study, the tumor volume was outlined in the media-lower quadrant of the left breast where this lung gap is relatively small. All the plans in this study met the requirements set by the National Surgical Adjuvant Breast and Bowel Project/Radiation Therapy Oncology Group (NSABP/RTOG), specifically protocol B-39/RTOG 0413. From the comparisons in this investigation, the mean relative pericardial dose of the BH CT group showed significant or 45% (p < 0.01) lower value than that of FB CT group. However, in FB 4DCT group, 3 of 6 cases indicated a meaningful reduction (p < 0.05) in 100% inhalation phase when compared with the mean dose over other phases. The inconsistent pericardial doses were displayed in FB 4DCT group due to minimal changes in the anterior lung gap of the pericardium, when the diaphragmatic breathing was dominant in those patients.

Intensity Modulated Radiation Therapy (IMRT) is a well-known type of external beam radiation therapy. The advancement in technology has had an inevitable influence in radiation oncology as well that has led to a newer and faster dose delivery technique called Volumetric Modulated Arc Therapy (VMAT). Since the presence of the VMAT modality in clinics in the late 2000, there have been many studies in order to compare the results of the VMAT modality with the current popular modality IMRT for various tumor sites in the body such as brain, prostate, head and neck, cervix and anal carcinoma. This is the first study to compare VMAT with IMRT for breast cancer. The results show that the RapidArc technique in Eclipse version 11 does not improve all aspects of the treatment plans for the breast cases automatically and easily, but it needs to be manipulated by extra techniques to create acceptable plans thus further research is needed.

Purpose/ Objectives: The latest publications indicate that the Ray Tracing algorithm significantly overestimates the dose delivered as compared to the Monte Carlo MC algorithm. The purpose of this study is to quantify this overestimation and to identify significant correlations between the RT and MC calculated dose distributions.
<br>Materials/Methods: Preliminary results are based on 50 preexisting RT algorithm dose optimization and calculation treatment plans prepared on the Multiplan treatment planning system Accuray Inc., Sunnyvale, CA. The analysis will be expanded to include 100 plans. These plans are recalculated using the MC algorithm, with high resolution and 1 uncertainty. The geometry and number of beams for a given plan, as well as the number of monitor units, is constant for the calculations for both algorithms and normalized differences are compared.
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<br>Results: MC calculated doses were significantly smaller than RT doses. The D95 of the PTV was 27 lower for the MC calculation. The GTV and PTV mean coverage were 13 and 39 less for MC calculation. The first parameter of conformality, as defined as the ratio of the Prescription Isodose Volume to the PTV Volume was on average 1.18 for RT and 0.62 for MC. Maximum doses delivered to OARs was reduced in the MC plans. The correlation of the ratio of air in PTV to the PTV with the difference in PTV coverage had a coefficient of -0.54.
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<br>Conclusions: The preliminary results confirm that the RT algorithm significantly overestimates the dosages delivered confirming previous analyses.

The effect of silicon (Si) substitution on the structural properties of hydroxyapatite (HAp) is investigated with powder x-ray and neutron diffraction methods. For this purpose, a series of samples of pure hydroxyapatite and 0.4 wt % Si substituted hydroxyapatite were prepared following a precipitation method. Phase identification from x-ray powder diffraction measurements showed a single hydroxyapatite phase in all the samples. Powder neutron diffraction patterns from room temperature down to 15 K are analyzed using the Rietveld method. The refined lattice constants, interatomic distances and isotropic atomic displacement parameters as a function of temperature for the pure and substituted hydroxyapatite are compared.