Catálogo de publicaciones - libros

We provide a new viewpoint for a fundamental but little investigated problem in positron emission tomography (PET): non-collinearity of annihilation radiation from the human body. The cause of the angular deviation from 180 deg is described as well as how to evaluate it under a spatially distributed radiation source and a limited acquisition time. An elegant conversion from the photopeak spectrum into the angular distribution is done based on the conservation laws of momentum and energy to avoid problems in the direct measurement. A healthy volunteer study using FDG and a Ge semiconductor detector reveals the distribution as a Gaussian function with the FWHM of 0.54 deg. Finally, we calculate the physical limit of FDG-PET spatial resolution.

It is mandatory to assess / evaluate performance parameters of all medical imaging systems employed in diagnostic imaging modalities before their use in patient studies. For such evaluations a number of commercially available test phantoms exist, e.g., in nuclear medicine tomography - PET/SPECT phantoms. A cylindrical PET/SPECT phantom (R A Carlson) which consists of hot, cold and linearity inserts has been modified. An additional pair of holes 30 mm diameter (hot regions in a cold background) — including “V” shaped eight pairs of various sized holes — near the edges to the hot regions insert opposite to each other at 180° from the centre of the insert is drilled. For cold regions insert, solid plastic rods 5 cm long (each) with same diameters and exactly the same arrangement as hot region holes pairs is designed. Both inserts have been attached with each other by aligning hot and cold regions for same cross sectional views. The data acquired by this arrangement provides a simple way for measurement and comparison of the performance of a system i.e., image resolution in terms of positions of hot and cold regions in the phantom, lesion detectability. Also the accuracy of scatter and attenuation correction techniques in nuclear medicine tomography may be tested.

Purpose : Effective cancer brachytherapy requires a treatment plan that delivers high dose to tumor while minimizing dose to critical normal tissue. Current planning systems generate accurately physical dose distributions but do not take biological effects into account. In an effort to incorporate modeling of biologic dose effects into paradigms based on physical dose distributions, this study was undertaken with specific aims of (1) developing optimization schemes to produce high dose rate (HDR) remote afterloading treatment plans using both biological model and physical dose based objectives and (2) determining the feasibility of performing HDR treatment planning based on biological models. Materials/Methods : The physical dose objective function is designed to minimize the difference between the prescription dose and the dose delivered to each volume element in tumor. The biological-model-based objective function is designed to maximize the probability of uncomplicated tumor control. Tumor volumes and critical structures are delineated from patients’ CT images. Adaptive simulated annealing (ASA) is used to optimize the objective functions. Plan evaluation indices, such as coverage index, homogeneity index, and overdose index, are used for plan comparisons. Target volumes from 3 gynecological cancer cases are planned to receive 30Gy from HDR treatments. Results : The physical dose based plans provide slightly better tumor coverage and dose uniformity than that from a commercial planning system. Compared with physical dose optimization, the biological-model-based plans provide better protection for vagina and the same tumor control probability. It is found that clear specification of regions of interest is important in biological-model-based system as the system will deliver high doses to regions not considered for potential complications. Conclusion : HDR treatment plan optimization using ASA in conjunction with physical dose or biological-model-based objectives is feasible. Further studies in various disease sites, especially for cases with higher prescription dose, should be explored.

Cone-beam computed tomography integrated with a linear accelerator is one of the more promising ways of bringing image guidance into radiotherapy. We will describe the clinical introduction of such a system in our hospital and a number of image guided protocols we have developed over the past three years.

We present a Monte Carlo based QA process for IMRT that uses both the planning MLC control files and the post-delivery MLC diagnostic files, thereby opening the possibility of performing highly accurate, automatic, patient- specific IMRT QA, without any measuring devices. This method also allows the accumulation of the actually delivered dose to the irradiated volume of the patient over the entire course of treatment.

: This study investigates feasibility of CBCT-based treatment planning.

: Hounsfield unit (HU) values and profiles of phantoms and patients in CBCT images were compared to those in CT images. CBCT-based treatment plans for the phantoms and patients were compared to CT-based treatment plans dosimetrically.

: Mean HU values of different materials in CBCT images were very close to those in CT images for Catphan. CBCT images included scatters and artifacts. As a consequent, the HU profiles of the homogeneous phantoms in CBCT images showed inhomogeneous HU distribution and the peripheral areas near the edge of the field-of-view showed reduced HU values. The HU profiles of the inhomogeneous phantom showed reduced HU values throughout the phantom and more significantly around the peripheral areas and lungs than other areas. The HU values in patients were also reduced in most tissue regions in CBCT images. Most plans based on CBCT with a bowtie filter showed good agreement with those based on CT with less than 1 % of MU/cGy difference or 1 – 2 mm of isodose discrepancy. Large dosimetrical discrepancy occurred when CBCT was scanned without a bowtie filter, when a treatment beam passed through a significant amount of lung, and when a treatment beam was largely off-centered so as to pass through a body part imaged near the edge of the field-of-view. All IMRT plans based on CBCT and CT showed isodose distribution with very good agreement for patient cases except for one lung cancer patient.

: This study proves feasibility of CBCT-based treatment planning by comparing HU values, MU/cGy and isodose distributions to the gold standard CT-based treatment planning. CBCT could be used for treatment planning purposes for most cases except for the cases with large discrepancy shown in this study.

: To evaluate the impact of static dosimetric leaf gap on MLC-based small beam dose distribution for intensity modulated radiosurgery. : We determined the optimal dosimetric static leaf gap by comparing the profiles of MLC based small beam with those of the collimated fields. The measurement detectors were Stereotactic Field Detector (SFD, Scanditronix-Wellhofer, Germany), 0.01 cc cylindrical ion-chamber (CC01, Scanditronix-Wellhofer, Germany), and extended dose ranged radiographic film (EDR2, Kodak, USA). The doughnut shaped PTV (6.1 cm3) and inner OAR (0.3 cm3) were delineated for delicate intensity modulated radiosurgery test plan. In this study, Millennium 120 leaf-MLC was used. For the measurement of dose, we used radiosurgery head phantom (model 605, CIRS, Norfolk, Virginia). : We found that 2 mm gap was optimal for the MLC based small beam. The maximum dose differences at the inside PTV, outside PTV, and inner OAR were 22.3%, 20.2%, and 35.2% for the 0 mm leaf gap, 17.8%, 22.8%, and 30.8% for the 1 mm leaf gap, and 5.5%, 8.5%, and 6.3% for the 2 mm leaf gap, respectively. In a humanoid head phantom study, the final dose distribution from the Eclipse planning system was significantly different from the measured values. The planned results were similar, while the measured showed large differences in dose according to the leaf gaps (range: 1.3–12.7%). : An inadequate determination of the dosimetric static leaf gap during the RTP configuration can make errors from the final dose calculation, which can sometimes be confused with unwanted QA results of IMRS. An appropriate dosimetric leaf gap setting is critical during the commissioning of an inverse planning system and an incorrect setting can produce large dose delivery errors particularly in the delicate IMRS treatment.

There is a need for stable gel materials for phantoms used to validate optical CT scanners for the evaluation of radiotherapy gel dosimeters. Phantoms have previously been proposed based on addition of food coloring dyes to gelatine to simulate polymer gels. However, because optical extinction in these dyes operates via light absorption, they would be more suitable for simulating radiation sensitive radiochromic dosimetry gels in which the exposed region of the gel becomes more optically absorbing. To correctly simulate polymer type gels which operate via light scattering, it would be more realistic to add light scattering centers such as colloidal suspensions (sols) to the gels used in such phantoms. In this paper we present some of the results of the evaluation of a Vista optical CT scanner (Modus Medical Devices Inc.) using novel gel phantoms in which radiation exposed polymer gels are simulated by the addition of largely colorless sols of varying turbidity.

: To estimate the dose to the embryo/fetus of a pregnant patient with brain tumors, and to design an shielding device to keep the embryo/fetus dose under acceptable levels

: A shielding wall with the dimension of 3meter height, 2 meter width, and 30mm thickness is fabricated with 4 trolleys under the wall. It is placed between a patient and the treatment head of a linear accelerator to attenuate the leakage radiation effectively from the treatment head, and is placed 1cm below the lower margin of the treatment field in order to minimize the dose to a patient from the treatment head.

A anti-patient scattering neck supporters with 2cm thick Lipowitz metal is designed to minimize the scattered radiation from the treatment fields, and it is divided into 2 section. They are installed around the patient neck by attach from right and left sides.

A shielding bridge for anti-room scattered radiation is utilized to place 2 sheets of 2mm lead plates above the abdomen to setup three detectors under the lead sheets.

Humanoid phantom is irradiated with the same treatment parameters, and with and without shielding devices using TLD, and ionization chambers with and without a build-up cap.

: The dose to the embryo/fetus without shielding was 3.20, 3.21, 1.44, 0.90 cGy at off-field distances of 30, 40, 50, and 60 cm. With shielding, the dose to embryo/fetus was reduced to 0.88, 0.60, 0.35, 0.25 cGy, and the ratio of the shielding effect varied from 70% to 80 %. TLD results were 1.8, 1.2, 0.8, 1.2, and 0.8 cGy. The dose measured by the survey meter was 10.9 mR/h at the patient’s surface of abdomen. The dose to the embryo/fetus was estimated to be about 1 cGy during the entire treatment.

: According to the AAPM report 50 regarding the dose limit of the embryo/fetus during the pregnancy, the dose to the embryo/fetus with little risk is less than 5 cGy. Our measurements satisfy the recommended values. Our shielding technique was proven to be acceptable

The results of investigation of the effect of water, activated by nonionizing Molecular Resonance Effect Technology (MRET), on prophylaxis and treatment of two kinds of oncology diseases of mice are presented. It was discovered in detailed experiments that under the action of this activator there are very essential modifications of the basic physical-molecular properties of distilled water (decrease of viscosities, electrical conductivity and permittivity by several times, sharp increasing or oscillation of pH exponent etc.). The effectiveness of action on water depends on time of irradiation. Potent effect of activated water on total tumor cell content has been observed. In particular, the total tumor cell content in mice of “prophylactic treatment”group which received water activated in the most optimal regime (t_act = 30 min) was 4.2-fold decreased in comparison with control mice! It has been shown that average survival time of mice which received optimal activated water (t_act = 30 min) in “prophylactic treatment” regime increased to 61.7% compare to control groups. Very marked increase in life span (about 45%) was observed when mice were treated with activated water (t_act = 15 min and 45 min) in “prophylactic treatment” regime. Eeffectiveness of action of such activated distilled water approximately equal action of chemotherapy! The positive influence (increase by 20%) of optimal fractions of activated water on Cytotoxic activity of mouse lymphocytes in vitro at prophylaxis action is also presented.