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S T Chiu-Tsao

Publications and source records attributed to S T Chiu-Tsao.

At least 19 recordsLinked to original sources
PubMed2001

The use of cylindrical coordinates for treatment planning parameters of an elongated 192Ir source.

PURPOSE: The doses given to the intima, media, and adventitia are very crucial quantities in intravascular brachytherapy. To facilitate accurate computerized treatment planning calculations, we have determined dose distributions in away-and-along table format around an 192Ir wire source and developed pertinent dosimetric parameters in cylindrical coordinates. METHODS AND MATERIALS: The Monte Carlo method (MCNP4C code) was used to calculate the dose distributions for the AngioRad 192Ir wire source (model SL-77HS, Interventional Therapies). The calculations were carried out for photon, beta, and electron (conversion and Auger) contributions for radial distances from 0.03 to 2.0 cm with 0.01-cm increments, and up to 2.24 cm from the source center in the longitudinal direction with 0.04-cm resolution. Dose rate values are determined in away-and-along format (cylindrical coordinates) and then converted to spherical coordinate format. Dosimetric parameters, such as the geometry factor, G(r, theta), and anisotropy function, F(r, theta), are generated in both cylindrical (R, Z, phi) and spherical (r, theta, phi) coordinates. The use of a cylindrical coordinate system for treatment planning parameters is proposed as a more suitable approach for accurate calculations. RESULTS: The photon contribution to dose varies nearly inversely with radial distance (from the source center) along the perpendicular bisector with 0.199 x 10(-3) cGy U(-1) s(-1) (0.802 cGy Ci(-1) s(-1)) at 1 cm. The beta and electron contributions start at very high values of about 35.5 x 10(-3) cGy U(-1) s(-1) and 11.0 x 10(-3) cGy U(-1) s(-1), respectively, at 0.03 cm and fall off exponentially to negligible amount near 0.2 cm. The total dose rate at 0.2 cm is 1.428 x 10(-3) cGy U(-1) s(-1) (5.754 cGy Ci(-1) s(-1)). The radial dose function, g(R), is nearly unity between 0.2 cm and 2 cm. Due to the beta and electron dose contributions, g(R) increases steeply to 5.5 as radial distance decreases from 0.2 cm down to 0.03 cm. The F(R, Z) values are close to unity for the majority of the region of interest. In contrast, F(r, theta) experiences a steep rise as shallow angles are approached (closer to the source), related to the beta dose contributions. Accurate treatment planning calculations would be possible with linear interpolation of F(R, Z), but difficult with F(r, theta) in the spherical coordinate system and the original normalization point as recommended in the American Association of Physicists in Medicine Task Group 60 (AAPM TG-60) formalism. CONCLUSION: The AngioRad 192Ir wire source, model SL-77HS, was completely characterized dosimetrically using Monte Carlo methods. The use of cylindrical coordinates and a modified anisotropy function normalization point for dosimetric parameters of an elongated 192Ir source is more suitable for accurate computerized treatment planning calculations in intravascular brachytherapy.

Algorithms↗
PubMed2001

Thermoluminescent dosimetry of the Symmetra 125I model I25.S06 interstitial brachytherapy seed.

As the efficacy of brachytherapy prostate treatment is becoming realized, new models of 125I seeds are being introduced. In this article we present thermoluminescent dosimetry (TLD) in a solid water phantom for a new design of 125I seed (UroMed/Bebig Symmetra, Model I25.S06). TLD cubes, LiF TLD-100, from Bicron (Solon, OH) with dimension 1 x 1 x 1 mm3 were irradiated at various distances from the seed at angles ranging from 0 degrees to 90 degrees in 10 degrees increments. The TLD detectors were calibrated by irradiation in a 60Co teletherapy beam. Monte Carlo simulation was used to account for TLD energy dependence and the deviation of solid water composition (as determined by chemical analysis of a sample) from liquid water. Dose rates per unit air kerma strength were determined based on calibrations traceable to the 1999 NIST standard (corrected for NIST measurement errors made in 1999) for the Symmetra seed. Dose data is presented in TG-43 format as a function of distance and angle. Values for lambda, F(r, theta), g(r), and the anisotropy constant are obtained for use in radiation treatment planning (RTP) software. The dose rate constant was determined to be 1.033+/-6.4% cGy h(-1) U(-1), which is comparable to model 6702 and higher than model 6711. We find the relative dose distributions of the Symmetra seed are similar to model 6702, and less anisotropic than model 6711. After accounting for deviation of measured solid water composition from the manufacturer's specification, good agreement between TLD results and Monte-Carlo-aided values was found.

Anisotropy↗
PubMed1998

The role of brachytherapy in head and neck cancer.

By precision dose delivery to a well-defined target, brachytherapy fulfills the complementary goals of delivering sufficient dose for tumor cell kill while sparing normal adjacent structures. The unique properties of this modality, either used alone or implemented in a combined modality setting with surgery or external-beam irradiation, offer an ideal means of enhancing the therapeutic ratio by exploiting the effect of local therapies while respecting normal tissue tolerance. Brachytherapy also provides an effective technique for the retreatment of patients with recurrent, persistent, or second primary head and neck malignant tumors in a previously irradiated region. The benefits of this treatment modality are reflected in excellent reported local control rates, shortened overall treatment times, reduced functional deficits, improved quality of life, and decreased costs. Successful brachytherapy relies, however, on close collaborative efforts between radiation oncologists and surgeons, careful assessment of the patient, precise planning, and technique that adheres to the rules of a system. Brachytherapy has been clearly identified as an effective, safe, and appealing modality in the treatment of head and neck malignancies.

Brachytherapy↗
PubMed1996

125I eye plaque dose distribution including penumbra characteristics.

The two main purposes of this work are (1) to determine the penumbra characteristics for 125I eye plaque and the relative influence of the plaque and eye-air interface on the dose distribution, and (2) to initiate development of a treatment planning algorithm for clinical dose calculations. Dose was measured in a newly designed solid water eye phantom for an 125I (6711) seed at the center of a 20 mm COMS eye plaque using thermoluminescent dosimeter (TLD) "cubes" and "minichips" inside and outside the eye, in the longitudinal and transverse central planes. TLD cubes were used in most locations, except for short distances from the seed and in the penumbra region. In the presence of both the plaque and the eye-air interface, the dose along the central axis was found to be reduced by 10% at 1 cm and up to 20% at 2.5 cm, relative to the bulk homogeneous phantom case. In addition, the overall dose reduction was greater for larger off-axis coordinates at a given depth. The penumbra characteristics due to the lip collimation were quantified, particularly the dependence of penumbra center and width on depth. Only small differences were observed between the profiles in the transverse and longitudinal planes. In the bulk geometry (without the eye-air interface), the dose reduction due to the presence of the plaque alone was found to be 7% at a depth of 2.5 cm. The additional reduction of 13% observed, with the presence of eye-air interface (20% combined), can be attributed to the lack of backscattering from the air in front of the eye. The dose-reduction effect due to the anterior air interface alone became unnoticeable at a depth of 1.1 cm (1.5 cm from the eye-air interface). An analytic fit to measured data was developed for clinical dose calculations for a centrally loaded seed. The central axis values of the dose rates multiplied by distance squared, Dr2, were fitted with a double exponential function of depth. The off-axis profile of Dr2, at a given depth, was parametrized by a modified Fermi-Dirac function to model both the penumbra characteristics due the plaque lip collimation and the effect of oblique filtration by silastic.

Algorithms↗
PubMed1994

High-sensitivity GafChromic film dosimetry for 125I seed.

The dose response of high-sensitivity GafChromic film to photons from 125I seeds for doses up to 200 Gy was established. The optical densities were measured using two types of densitometers: (a) a Macbeth spot densitometer with broadband light spectrum, and (b) an LKB He-Ne laser scanning microdensitometer with red light of wavelength 632.8 nm. The net optical density was found to be a power function of dose with exponents of 0.858 and 0.997, for the Macbeth and LKB densitometers, respectively. Film sensitivity with the LKB densitometer was about double of that with the Macbeth densitometer. The dose measurements were performed using the high-sensitivity GafChromic films for 125I model 6702 seed in solid water phantom. Each film was positioned parallel to the seed's long axis and centered at the seed's transverse axis. Films were exposed at various distances, ranging from contact to 3 cm from the seed center. The radiation dose delivered to the film center varied from 7 to 50 Gy, depending on the distance. The optical density at the film center was measured using both types of densitometers. Dose conversion was achieved with the established dose response curves for the respective densitometers. The dose values, along the seed's transverse axis obtained using both densitometers, were compared with each other, and also compared with published thermoluminescent dosimeter (TLD) data and Monte Carlo results. General agreement was found. It was concluded that the high-sensitivity GafChromic film measurement is a feasible method for 125I seed dosimetry in solid water phantom.(ABSTRACT TRUNCATED AT 250 WORDS)

Biophysical Phenomena↗
PubMed1994

Diode dosimetry of 103Pd model 200 seed in water phantom.

The relative dose distribution around the 103Pd model 200 implant seed was measured with a computerized data acquisition system employing a p-n junction silicon diode immersed in a water phantom. Data are acquired in polar coordinates by computer control of (1) the diode distance from the seed center and (2) the rotation angle of seed about a transverse axis. Transverse axis data are compared with thermoluminescent dosimeter (TLD) measurements and a Monte Carlo calculation by others.

Brachytherapy↗
PubMed1991

A new device to stabilize templates for transperineal I-125 implants.

Transperineal Iodine-125 implants of the prostate are currently being performed at Memorial Sloan-Kettering Cancer Center with CT-based treatment planning and transrectal ultrasound for verification of proper needle placement in the prostate at the time of implantation. An adjustable device, the WIPI, has been designed to stabilize the perineal template and rectal obturator during planning and implementation of the procedure. The device is simple to use and is compatible with CT scanning, transrectal ultrasound, and the Mick applicator. Its design and key functional features are described here.

Brachytherapy↗
PubMed1991

CT-based optimized planning for transperineal prostate implant with customized template.

A computerized planning procedure has been developed for CT-guided transperineal prostate implants. The segment for custom planning of perineal needle orientations allows placement of I-125 seeds in the entire prostate gland while avoiding the pubic bones. Least-squares optimization (LSO) is used to obtain the seed-loading pattern along the needles. The optimized seed distribution produces a better fit between treatment and target volumes than that obtained from our previous manual technique. Also, the present semi-automatic technique reduces planning time by about a factor of 10 compared to that of the manual approach.

Brachytherapy↗
PubMed1991

An improved method for computerized tomography-planned transperineal 125iodine prostate implants.

Transperineal 125iodine implants of the prostate can be performed with ultrasound guidance, a simple technique that has met with widespread acceptance. However, ultrasound does not allow good visualization of the pubic bones in relation to the pelvic outlet, and the pubic bones may interfere with needle placement in the anterior peripheral aspect of the prostate. Adequate irradiation of the entire periphery of the prostate is important to assure tumor control, since most tumors are multicentric and may involve the anterior aspect of the prostate. A computerized tomography-based treatment planning procedure that allows for angulation of transperineal needles to avoid the pubic bones and still reaches the most peripheral aspects of the gland is described. The technique also allows for the use of transrectal ultrasound and fluoroscopy to verify correct needle placement in the prostate at the procedure. Early treatment results, based on prostate specific antigen and regression of palpable tumors, are encouraging.

Antigens, Neoplasm↗
PubMedDate not supplied

Treatment planning dosimetric parameters for a (90)Y coil source used in intravascular brachytherapy.

BACKGROUND: (90)Y coil sources have been used in animal and clinical trials for treatment of restenosis in intravascular brachytherapy (IVBT). This study aims to determine the American Association of Physicists in Medicine (AAPM) Task Group-60 (TG-60) dosimetric quantities in regions surrounding the balloon wall for use in treatment planning computer systems. METHODS: The Monte Carlo method was used to determine the dose distribution, using MCNP4B2 code. The coil source was modeled by a hollow cylinder of 2.9 cm length centered in a balloon (2.5 mm diameter) filled with carbon dioxide (CO(2)) at 5 atm. Scoring voxels consisted of contiguous annular disk shells with 0.1 mm spacing in the radial direction and 0.2 mm spacing in the longitudinal direction. The scoring region ranges from the center of the source to 1.0 cm in the longitudinal direction, and from 0.13 to 1 cm in the radial direction. In the plane containing the source axis, the Monte Carlo-generated doses in rectilinear coordinates are converted to polar coordinates. RESULTS: The dose rate of the source is provided in both Cartesian and polar coordinates. The dose rate constant [D(r(0),theta(0))], anisotropy function [F(r,theta)], and radial dose function [g(r)] were generated from these values and listed in tabular format. At shallow angles and longer distances from the source center, large values of the anisotropy function resulted, deviating two orders of magnitude from unity. CONCLUSIONS The doses given to the intima, media, and adventitia are very crucial quantities in IVBT. The calculated TG-60 dosimetric quantities, used commonly in conventional brachytherapy applications, provide a means for the user to determine the three-dimensional dose surrounding the balloon catheter. These parameters can be used in future treatment planning system for IVBT. We also discuss the need to develop a new formalism specific to longer sources used in IVBT.

Angioplasty, Balloon, Coronary↗
PubMedDate not supplied

A new treatment planning formalism for catheter-based beta sources used in intravascular brachytherapy.

Intravascular brachytherapy (IVBT) is an emerging modality for the treatment of atherosclerotic lesions in the artery. As part of the refinement in this rapidly evolving modality of treatment, the current simplistic dosimetry approach based on a fixed-point prescription must be challenged by future rigorous dosimetry method employing image-based three-dimensional (3D) treatment planning. The goals of 3D IVBT treatment planning calculations include (1) achieving high accuracy in a slim cylindrical region of interest, (2) accounting for the edge effect around the source ends, and (3) supporting multiple dwell positions. The formalism recommended by Task Group 60 (TG-60) of the American Association of Physicists in Medicine (AAPM) is applicable for gamma sources, as well as short beta sources with lengths less than twice the beta particle range. However, for the elongated beta sources and/or seed trains with lengths greater than twice the beta range, a new formalism is required to handle their distinctly different dose characteristics. Specifically, these characteristics consist of (a) flat isodose curves in the central region, (b) steep dose gradient at the source ends, and (c) exponential dose fall-off in the radial direction. In this paper, we present a novel formalism that evolved from TG-60 in maintaining the dose rate as a product of four key quantities. We propose to employ cylindrical coordinates (R, Z, phi), which are more natural and suitable to the slim cylindrical shape of the volume of interest, as opposed to the spherical coordinate system (r, theta, phi) used in the TG-60 formalism. The four quantities used in this formalism include (1) the distribution factor, H(R, Z), (2) the modulation function, M(R, Z), (3) the transverse dose function, h(R), and (4) the reference dose rate at 2 mm along the perpendicular bisector, D(R0=2 mm, Z0=0). The first three are counterparts of the geometry factor, the anisotropy function and the radial dose function in the TG-60 formalism, respectively. The reference dose rate is identical to that recommended by TG-60. The distribution factor is intended to resemble the dose profile due to the spatial distribution of activity in the elongated beta source, and it is a modified Fermi-Dirac function in mathematical form. The utility of this formalism also includes the slow-varying nature of the modulation function, allowing for more accurate treatment planning calculations based on interpolation. The transverse dose function describes the exponential fall-off of the dose in the radial direction, and an exponential or a polynomial can fit it. Simultaneously, the decoupling nature of these dose-related quantities facilitates image-based 3D treatment planning calculations for long beta sources used in IVBT. The new formalism also supports the dosimetry involving multiple dwell positions required for lesions longer than the source length. An example of the utilization of this formalism is illustrated for a 90Y coil source in a carbon dioxide-filled balloon. The pertinent dosimetric parameters were generated and tabulated for future use.

Beta Particles↗
PubMedDate not supplied

Ytterbium-169: a promising new radionuclide for intravascular brachytherapy.

PURPOSE: To explore the feasibility of 169Yb (gamma, 93 keV) as a new radionuclide for intravascular brachytherapy (IVBT) in terms of dose distribution, penetration power, and radiation safety features as compared with 125I and 192Ir. METHODS: The dose distributions for catheter-based sources, 169Yb, 125I, and 192Ir, in homogeneous water and in the presence of calcium and a steel stent have been determined and compared using the Monte Carlo method (MCNP4B2 code). The dose rates of the sources were evaluated from 0.02 to 100 cm. RESULTS: In the short distance range (0.02<r<1.0 cm), the dose distributions in homogeneous water are very similar for the three radionuclides when the dose rates are normalized at 2 mm. Between 1 and 20 cm, the relative dose rates fall off similarly for 169Yb and 192Ir, whereas for 125I, it decreases much more rapidly. At a distance further away (r approximately 100 cm), the dose rate of 169Yb is about 10 times lower than that of 192Ir, indicating the cathlab radiation shielding requirement for 169Yb is substantially reduced as compared with 192Ir. Calcified plaques and stents cause a drastic dose reduction in the arterial wall for 125I, but have no effect for 192Ir gamma-rays. Only slight dose reductions were detected for 169Yb beyond a layer of 1.0-mm calcium (2-3%), and behind a steel stent strut (5%). CONCLUSION: 169Yb is a promising new radionuclide for IVBT. It has a much better penetrating power through calcified plaques and stents compared with the low-energy source 125I. It also provides easier radiation protection measures for cardiac cathlab personnel than the high-energy source 192Ir, while preserving a favorable dose distribution in tissues surrounding an arterial vessel.

Brachytherapy↗
PubMedDate not supplied

Extended expression for tissue-maximum ratio fitted to the Varian Clinac-4 and Clinac-6 accelerators.

An expression which generates tissue maximum ratio (TMR) data for the Varian Clinac-4 and Clinac-6 accelerators is presented. This semiempirical expression can be programmed easily for radiotherapy computer systems or the new generation of scientific calculators. Our technique leads to good agreement with the published data, as demonstrated by a total average percent deviation of the measured TMR values of -0.115% for the Clinac-4 and -0.002% for the Clinac-6.

Particle Accelerators↗
PubMedDate not supplied

Monte Carlo dosimetry for 125I and 60Co in eye plaque therapy.

Monte Carlo calculations of radiation dosimetry using MORSE code are performed for 125I and 60Co point sources in a cylindrical head phantom that simulates the geometry of eye plaque therapy for choroidal melanoma. We obtain the dose variation in the eye at submillimeter intervals over distances as close as 1 mm and up to 2.5 cm from the source. The calculations for 125I are performed for the phantom media of water, protein, and a homogenized protein-water mixture simulating the composition of the eye. Relative dose functions for 125I for these phantom media are fitted to second-degree polynomials. Agreement is found with published results. The relative dose function for 60Co at eye position in the water head phantom is fitted to a third-degree polynomial and compared with that for 60Co at the center of a large water sphere. A boundary effect due to the head phantom-air interface on the dose distribution for 60Co is demonstrated. The dose falloff with distance is faster for the eye geometry compared with the bulk geometry. We also show that the relative dose distributions within the tumor are comparable for 125I and 60Co by comparing their relative dose functions. This result is consistent with the success of clinical trials of large melanoma treatments with 125I plaques.

Biophysical Phenomena↗
PubMedDate not supplied

Dose rate determination for 125I seeds.

Dose rates in water have been determined for the two types of 125I seed currently used in brachytherapy. The need for such determinations became evident when water/air ratios measured with a silicon diode were found to be lower than expected. Extensive measurements using lithium fluoride thermoluminescent dosimeters (TLD's) have been performed in a solid water phantom, at distances from 0.1 to 10 cm from the seed center and at angular increments of 10 degrees, 15 degrees, or 30 degrees within a plane through the seed axis. Dose calibration of the TLD's was accomplished by irradiation in air with 125I seeds of the same type and of strengths traceable to a calibration at the National Institute of Standards and Technology (NIST). Relative calibration of TLD's was monitored by irradiation, in an oven-type x-ray machine, of control dosimeters simultaneously and all dosimeters intercurrently with the 125I irradiations. Values obtained for the dose rate constant, i.e., dose rate per unit air-kerma strength at 1 cm on the transverse axis, were 0.853 and 0.932 cGy h-1 U-1 (1.08 and 1.18 cGy h-1 mCi-1) for the 6711 and 6702 seeds, respectively. Measured data were supplemented with Monte Carlo-calculated relative dose rate data generated using the MORSE code. These calculations used 100 energy groups from 10 to 35.4 keV and involved energy collection bins ranging from 0.025 to 1.2 cm on an edge. Normalized at 1 cm, transverse axis calculated data are not significantly different from measured data (ours or cited literature) at distances either less than 2.5 or greater than 8 cm. Normalized at different distances along the transverse axis, our off-axis calculated and measured distributions agree closely at all angles but differ from literature measured distributions at small (less than or equal to 1 cm) distances and, for small angles, increasingly at larger distances (greater than or equal to 5 cm).

Brachytherapy↗
PubMedDate not supplied

Thermoluminescent dosimetry for 103Pd seeds (model 200) in solid water phantom.

Dose measurements using LiF thermoluminescent dosimeters (TLD) have been performed for single 103Pd seeds (model 200) at the center of a solid water phantom. TLD cubes 1 mm on an edge were used for measurements from 1 mm to 1 cm at 1-mm intervals. The cubes were centered along transverse and longitudinal axes and along radial lines from seed center at 10 degrees increments. TLD chips of dimension 3.1 X 3.1 X 0.89 mm were used at distances of 2, 2.5, 3, and 4 cm at 15 degrees angular intervals. Data are presented as the product of distance squared and dose rate per unit source strength, plotted versus distance and angle. At 1 cm from seed center along the transverse axis this product was found to be 0.88 cGy cm2 mCi-1h-1. A dose-rate table in polar coordinates has been formulated for use with multiseed dose distribution calculations. Comparison with data of Meigooni et al. [Endocuriether./Hyperthermia Oncol. 6, 107-117 (1990)] shows general agreement for distances of 2 cm or greater. A comparison of our transverse axis data with Russell's calculated values (Theragenics Internal Report, 4 November 1984) for an ideal point source of 103Pd shows very good agreement except at distances less than 0.5 cm, where differences are attributable to the extended source effect in the actual seed.

Brachytherapy↗