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Biomedical subjects

I J Das

Publications and source records attributed to I J Das.

At least 55 records · Page 3Linked to original sources
PubMed1994

Evaluation of tabletop materials for autocontouring in CT treatment planning.

Computerized tomography (CT) treatment planning has been proven to be essential in precision radiotherapy. Typically CT planning requires a large number of CT scans (between 3 and 50 slices) to be entered into the treatment planning system. A significant amount of time in the planning process is devoted to outlining internal structures, organs, and the external patient contour in each CT slice. In principle, external contours could be generated easily using autocontouring routines; however, in reality it does not always provide a satisfactory contour due to the limitations of the CT tabletop having nearly the same CT number as that of the body structure. A solution to this problem is to create a large CT number gradient interface between the patient and CT tabletop by inserting a thin sheet of low CT number material. The optimum material for a tabletop was investigated from a range of low-density and low atomic number media. Various materials were studied by placing them underneath an unsliced humanoid phantom. A portion of the phantom abdomen was imaged and analyzed on a Picker Premier IQ CT scanner. Results indicate that the tabletop should be made of the material that has a CT number at least 10 times lower than the tissue in contact with the table. A simple and cost-effective method of avoiding failures in autocontouring is to place a thin sheet of low-density material such as cardboard or foam board on the tabletop. Such an insert creates a large CT gradient resulting in a significant improvement in the accuracy of the edge detection algorithm used for autocontouring. Detailed analysis is presented.

Algorithms↗
PubMed1994

Spatial distribution of bremsstrahlung in a dual electron beam used in total skin electron treatments: errors due to ionization chamber cable irradiation.

The spatial distribution of the bremsstrahlung dose in total skin electron therapy and large electron fields has been assumed to be symmetric along the match line; however, the measured data in the entire plane have not been available. Das et al. ["Spatial distribution of bremsstrahlung in total skin electron irradiation," Med. Phys. 19, 810 (1992)] reported the ion-chamber measured bremsstrahlung data in the Stanford six-field technique and found that the bremsstrahlung dose profile was 2.5 times greater in the caudal plane (beam facing toward floor) as compared to the cephalic plane. This observation was recently quoted by Klevenhagen [Physics and Dosimetry of Therapy Electron Beams (Medical Physics, Madison, WI, 1993)]. Further study using thermoluminescent dosimeters and careful measurements using ion chambers showed that the bremsstrahlung radiation was indeed symmetric at the match line as theoretically predicted and was not a function of the beam angle or the type of floor. The earlier observations of Das et al. were found to be due to cable effects. Ionization chamber cable response in a large electron field and the associated errors in the bremsstrahlung distribution are presented. It is suggested that a cable-free dosimeter should be used for measuring low levels of bremsstrahlung radiation in large electron fields and at extended distances.

Biophysical Phenomena↗
PubMed1994

Dosimetric problems at low monitor unit settings for scanned and scattering foil electron beams.

Electron beam dosimetry at low monitor unit (MU) settings is important for dosimetric applications. Dose linearity, beam flatness, and beam energies were studied at low MU settings with various dose rates for different types of linear accelerators. It is observed that for the scattering foil units, the dose/MU is a smooth function of MU for all beam energies. Discrepancies in dose/MU are highest at the lowest MU. Significant variation (5%-245%) in dose linearity is observed among various linear accelerators at low MU settings. Dose rate has no effect on the dose linearity for all energies for the scattering foil units tested. On the contrary, for the scanned beam, there is no predictable pattern as dose/MU is random in nature and varies with time and beam energy. The maximum dosimetric error is observed for the highest energy beam where the beam width is most narrow. Using film, the beam uniformity was noticed to be very poor at low MU and high energy for scanned beams. The beam uniformity and dose linearity are random at low MU due to the random nature of the scan cycle. Under the adverse conditions, the deviation in dosimetric parameters was observed up to 200 MU.

Electrons↗
PubMed1994

Backscattering and transmission through a high Z interface as a measure of electron beam energy.

The ratio of electron beam transmission (Tr) and backscattering (Bs) is used as a measure of beam energy. With a proper thickness of high Z medium placed at the surface of a low Z homogeneous medium, it is observed that the function log(Tr/Bs) is a linear function of electron energy. This method is applied to electrons produced with scattering foil and pulse-swept (scanned) beams for the nominal energies ranging from 6 to 20 MeV. This technique provides a unique alternative method which is easy, simple, and accurate within +/- 3% for estimation of electron beam energy.

Electrons↗
PubMed1994

Technical note: dosimetry in the moving gap region in craniospinal irradiation.

To minimize the risk of overlap of adjoining orthogonal fields in craniospinal irradiation, a skin gap is usually maintained between the caudal margin of the brain fields and the cephalic margin of the spine field. The moving gap technique (feathering) is commonly used to improve the dose in the gap region. Using Kodak XV ready-pack films and a polystyrene phantom, the dosimetry in the moving gap region at the depth of spinal cord for different gap sizes and feathering step sizes was studied. The dose profiles in the moving gap region with half-beam block set-up were also measured. Our results show that the combination of a small skin gap and a larger feathering step size results in a higher dose in the moving gap compared with that with identical gap and feathering step sizes. For example, with a 0.5 cm gap and 1 cm feathering step size, the dose in the moving gap region at depth of 5 cm ranges from 82 to 88% of the dose to the brain field. The corresponding dose range with 1 cm gap and 1 cm feathering step size is about 68-73%. With no gap and 1 cm feathering step, a dose range of 92-98% is achieved in the moving gap. The slightly steeper penumbra of a half-beam block used in the brain fields does not result in any dosimetric advantage in the moving gap compared with the conventional set-up when feathering is employed.

Brain↗
PubMed1993

Expression of p210 bcr/abl increases hematopoietic progenitor cell radiosensitivity.

PURPOSE: The cytogenetic finding of the Ph1+ chromosome and its molecular biologic marker bcr/abl gene rearrangement in cells from patients with chronic myeloid leukemia are associated with a proliferative advantage of the Ph1+ clone in vivo. Although the transition to the acute terminal phase or blastic crisis is often associated with additional cytogenetic abnormalities, the molecular events which correlate the initial cytogenetic lesion with the terminal phase are poorly understood. Defective cellular DNA repair capacity is often associated with chromosomal instability, increased mutation frequency, and biologic alterations. METHODS AND MATERIALS: We, therefore, tested whether the protein product of the bcr/abl translocation (p210) could alter DNA repair after gamma-irradiation of murine cell lines expressing the bcr/abl cDNA. RESULTS: The 32D cl 3 parent, 32D cl 3 pYN (containing the control vector plasmid) and each of two sources of 32D cl 3 cells expressing p210 bcr/abl cDNA (32D-PC1 cell line and 32D-LG7 subclone) showed a D0 of 1.62, 1.57, 1.16, and 1.27 Gy, respectively. Thus, expression of the p210 bcr/abl product induced a significant (p < 0.05) increase in radiosensitivity at the clinically relevant radiation therapy dose-rate (1.16 Gy/min). The increased radiosensitivity of p210 bcr/abl expressing cells persisted if cells were held before plating in a density-inhibited state for 8 hr after gamma-irradiation, indicating little effect on the repair of potentially lethal gamma-irradiation damage. The IL-3 dependent parent 32D cl 3 cells demonstrated programmed cell death in the absence of growth factor or following gamma-irradiation to 200 cGy. Expression of bcr/abl cDNA in the 32D-PC1 and 32D-LG7 sub clones abrogated IL-3 requirement of these cell lines and inhibited gamma-irradiation induced programmed cell death. CONCLUSION: These data suggest a role for bcr/abl p210 in amplifying gamma-irradiation DNA damage or broadly inhibiting DNA repair, conditions that may stimulate further cytogenetic alterations in hematopoietic cells.

Biomarkers↗
PubMed1993

Tolerances in setup and dosimetric errors in the radiation treatment of breast cancer.

PURPOSE: Treatment failure in radiation therapy, as well as unexpected complications, can be associated with set up changes or variations that can cause deviations from the prescribed radiation dose distribution both inside and outside the target volume. The effect of various deviations from the planned setup on the delivery of the prescribed radiation dose to the desired treatment volume was studied. METHODS AND MATERIALS: Adding a second simulation was investigated as means of minimizing setup changes on treatment. The first simulation was used for planning the treatment and the second simulation was essentially a mock treatment. Dosimetric evaluations based on dose volume histograms were analyzed for each deviation in the setup. RESULTS: In 95% of the patients, the frequency of the changes in the setup parameters between the second simulation and the treatment setup were reduced significantly from the changes that occurred between the first simulation and the second simulation. The changes in isocenter coordinates up to +/- 1.0 cm have minimal effects (+/- 2%) on the dose distributions. Gantry angle variations up to +/- 4 degrees produce a change of less than +/- 5% in the dose distribution within the target volume. However, this angular variation resulted in additional tissue irradiation outside of the desired treatment field (about 10 cm3 for a large patient). A gantry angle variation of +/- 6 degrees can change the volume of tissue that receives the prescribed dose by at least +/- 10%. In addition, such a change can increase the volume of tissue outside the desired treatment field that is irradiated. CONCLUSION: It is concluded that individually, deviations in one of the parameters from the planned setup of +/- 1.0 cm in isocenter position and +/- 4 degrees in gantry angle do not produce significant deviations from the planned dose distribution. However, a significant change in dose distribution is observed if the setup parameters are concurrently changed. A second simulation may minimize the deviations of the treatment setup from the planned setup and maximize the precision in dose delivery to the target volume.

Breast Neoplasms↗
PubMed1992

Photon beam dosimetry at a blocked beam edge using diffusion approximation.

A simple analytical model is presented for the transport of secondary electrons at a photon beam edge using the energy averaged solution of the Boltzmann equation, originally developed for beta-ray dosimetry at a plane interface. Dose at a point under a block is assumed to be due to secondary electrons and the scattered photons generated from the primary photon beam. The diffusion approximation is used for the secondary electron transport at a virtual plane interface created by the block. The dose from the scattered photon component is treated as decaying exponentially with distance from the beam edge. Comparisons made with the model and measurements are in general agreement for high energy accelerator beams.

Diffusion↗
PubMed1992

The v-src oncogene may not be responsible for the increased radioresistance of hematopoietic progenitor cells expressing v-src.

Infection of the IL-3-dependent, myeloid progenitor cell line 32D cl 3 with murine retroviruses that contain either the wild-type or a temperature-sensitive mutant v-src can render these cells growth-factor independent. These cells also became resistant to gamma irradiation administered at the low-dose rate of 0.05 Gy/min, which is used clinically. The v-src-dependent nature of resistance to gamma irradiation was examined by studying four clones of 32D cl 3 cells that had been infected with a retrovirus carrying the tsLA31A mutant of v-src. The tyrosine-specific kinase activity of this mutant is dramatically reduced at the nonpermissive temperature of 39 degrees C. Cells transformed by v-src and grown at either 34 or 39 degrees C, in the presence or absence of IL-3, demonstrated a significantly higher D0 compared to parental cells examined under identical conditions. In addition, expression of v-src abrogated the synergistic killing effect of heat and gamma irradiation. The D0 of parental 32D cl 3 cells kept at 39 degrees C after gamma irradiation was reduced significantly compared to the D0 of these cells kept at 34 degrees C. This contrasts with data from 32D cl 3 cells infected with either the wild-type v-src or the temperature-sensitive mutant, neither exhibited a synergistic effect in the D0 at either 34 or 39 degrees C. Therefore, while continuous expression of a v-src gene product is required for maintenance of the growth-factor-independent state, v-src does not appear to be responsible for the increased gamma-radiation resistance of these cells at low dose rate.

Animals↗
PubMed1991

Electron beam modifications for the treatment of superficial malignancies.

For the treatment of superficial tumors, the surface dose should be high; unfortunately, because of pronounced dose buildup in low energy electron beams, their efficacy for such treatment is reduced. Electron beams can be modified by placing a low atomic number material called a beam spoiler in the beam. In general, the surface dose is a function of electron energy, source to surface distance, field size, thickness of beam spoiler, distance of beam spoiler from surface, atomic number of beam spoiler, and angle of the beam. The effects of these parameters are evaluated with respect to surface dose, bremsstrahlung dose, and field size changes for small fields at standard SSD and electron energies from 6 to 17 MeV. It was found that the use of a beam spoiler can generally increase the surface dose to values exceeding 90% of the maximum buildup value while maintaining a bremsstrahlung dose less than 3%. Changes in field size related to the placement of the beam spoiler were considerable in some cases.

Electrons↗
PubMed1991

Dosimetric accuracy at low monitor unit settings.

Dosimetric accuracies at low monitor units are evaluated for linear accelerators from various manufacturers. A large error is observed in the majority of the accelerators. The error can be positive or negative. Although the error can exceed 20% for the first few monitor units, it is usually less than 5% when more than 10 monitor units are delivered. When low doses are required proper precautions should be taken for dosimetric accuracy including the beam energy, beam flatness and dose per monitor unit.

Humans↗
PubMed1990

Study of dose perturbation parameters for eye shielding in megavoltage photon beam therapy.

Shielding blocks are frequently used to minimize dose and shield sensitive organs in radiation therapy. The blocks, which are made of high atomic number materials, produce significant dose perturbations in megavoltage photon beams. The effects of these perturbations are studied with special interest in the eye shielding in the treatment of head and neck malignancies. Optimum parameters for the treatment are suggested.

Eye Injuries↗
PubMed1988

Evaluation of several film/screen combinations for megavoltage radiation therapy treatment field verification.

The introduction of a lower cost alternative to a film commonly used for megavoltage radiation therapy port films led to a comparison of two films (Kodak X-TL and X-OMAT G) in four metal screen cassette configurations. In addition to determination of H-D curves and point spread functions, images of humanoid phantom sections were obtained and evaluated for clinical "usefulness." The X-OMAT G film-screen combinations compared favorably with the X-TL film combinations with respect to the H-D Curves and point spread functions but there was a slight preference for X-TL film. There were no differences noted whether the film was used in the standard "ready pack" or naked in the cassettes.

Evaluation Studies as Topic↗
PubMed1987

An empirical method for beta-ray dosimetry at a homogeneous plane interface.

A simple equation is presented for the dose calculation at a plane interface from monoenergetic beta sources distributed uniformly in a half space of an infinite water medium. The calculated dose distribution is accurate to within 1.2% of Monte Carlo data for energies ranging from 0.5 keV to 10 MeV.

Beta Particles↗
PubMedDate not supplied

The production of secondary electrons in an electron beam.

Convenient methods for calculating the ratio of restricted to unrestricted collision stopping power in water over a wide range of initial and cutoff energies, the rate of production of secondary electrons, and the primary electron dose distribution in an electron beam, are presented.

Electrons↗