Biomedical subjects

C H Sibata

Publications and source records attributed to C H Sibata.

31 records · Page 2Linked to original sources

PubMed1994

Comparison of Nucletron and ROCS brachytherapy treatment planning systems for LDR and HDR applications.

The Nucletron Plato system is being used in our Institute for both high dose rate (HDR) and low dose rate (LDR) treatment planning, while the radiation oncology computer system (ROCS) is used for external beam planning. This paper compares both systems, using a gynecological application for the LDR case and an esophageal application for the HDR. It is shown that ROCS may be used as a backup to the Nucletron treatment planning system for LDR and simple HDR cases. The Nucletron planning system is a better system than ROCS for both LDR and HDR applications.

Brachytherapy↗

PubMed1994

Electron energy constancy check using a five-chamber detector array.

Two methods are shown here to determine the 50% depth ionization (d50) using buildup materials of different thickness placed on top of a five-chamber detector array. In the first method, two sets of different thickness buildup material are required to perform the check, one set for checking 6 and 9 MeV, while another set is used for 12, 16, and 20 MeV electron beam from a Varian Clinac 2100C. The second method only requires two data points to determine the d50 depth for each energy. The d50 depths determined were compared with the d50 depth obtained using a water phantom with ionization chamber measurements. The method is simple to use especially for departments that use a similar detector to perform quality assurance tests such as output/symmetry/flatness check.

Electrons↗

PubMed1994

Uncertainty in delivered dose resulting from the distribution of source activities in a Selectron LDR afterloader.

The uncertainty in the delivered dose resulting from the distribution of 137Cs source activity in a clinical Selectron LDR unit has been studied. A comparison is made of the dose delivered to a point 'A' in an implant with sources of equal activity to the actual dose delivered in the same implant with source activities randomly chosen from the population in the afterloader.

Cesium Radioisotopes↗

PubMed1991

Influence of detector size in photon beam profile measurements.

Correction is necessary to account for the detector size in clinical dosimetry of photon and electron beams. This correction is due to the absorbed dose gradient present in a finite-size detector. Further corrections are necessary when the detector and phantom materials are not the same. These corrections are due to the perturbation in the charged-particle fluence. Generally these corrections are applied to measurements along the central axis of the beam. Cross beam profile measurements, however, are not usually corrected for detector size. The ionization profile is also usually assumed to be equivalent to the absorbed dose profile. We have corrected the ionization chamber size effect by two approaches: extrapolation of measurements to zero detector size and deconvolution of measurements using a simple model for the detector response function. We have measured absorbed dose profiles to water using a small water-equivalent plastic scintillation detector. Film profile measurements were also studied. The ionization profile corrected for detector size and absorbed dose profile were not equal, probably due to loss of charged-particle equilibrium in the beam edges. For ionization chamber measurements, knowledge of the charged-particle spectrum is needed to convert ionization to absorbed dose to water. This is not necessary for relative absorbed dose measurements under charged-particle equilibrium. Film has been shown to be a straightforward and reliable method for cross beam profile measurements.

Film Dosimetry↗

PubMed1990

Influence of hip prostheses on high energy photon dose distributions.

Radiotherapy treatment of patients having a hip prosthesis is a common problem facing dosimetrists and physicists when the treatment plan requires irradiation of the pelvic area. To quantify the perturbation of these devices, attenuation studies were done with 6 and 18 MV photon beams using various hip prostheses models with varying size and composition. These studies have shown that an attenuation of as much as 50% can be found in a single beam profile under the prosthesis. We have studied the capability of a dose planning system to predict the transmission of these devices as compared with measurements.

Alloys↗

PubMed1989

Application of thermal dilution measurements for thermal treatment planning.

The use of thermal dilution measurements is demonstrated in quantifying effective thermal conductivity distributions through tumours. For focused acoustic sources these data are applied in the solution of the heat diffusion equation to estimate steady-state temperature distributions. The advantages and disadvantages of this approach are presented. The effective conductivity measurements are found to be useful in predicting the circumstances under which this simple model can be applied and in aiding the selection of the most appropriate heating modality.

Animals↗

PubMed1988

Experimental derivation of beta for high-energy photons.

The absorbed dose in a medium for a given beam of megavoltage photons is related to the collision kerma by the energy dependent parameter beta. Some theoretical methods of estimating and calculating beta have been proposed in the past. The majority of the methods take into account only Compton interactions, with just one method taking into account the beam spectrum, coherent and incoherent scattering and pair production effects. Experimentally measured data, on the other hand, implicitly include appropriate contributions of all these processes. Experimentally derived beta values are tabulated and the rationale for their measurement is discussed. The beta value for a low-Z ion chamber calibration in free space with a 60Co gamma-ray beam is 1.002. The dependence of beta on several factors such as energy, field size, phantom material and depth has been studied.

Dose-Response Relationship, Radiation↗

PubMed1985

Determination of contamination-free build-up for 60Co.

Experimental verification of the difference between absorbed dose in tissue and the collision fraction of kerma requires precise knowledge of the absorbed dose curve, particularly in the build-up and build-down regions. A simple method for direct measurement of contamination-free build-up for 60Co, which should also be applicable for most of the photon energies commonly employed for treatment, is presented. It is shown that the contribution from air-scattered electrons to the surface dose may be removed by extrapolating measurements of build-up to zero field size. The remaining contribution to contamination from the collimators and other source-related hardware may be minimised by measuring these build-up curves sufficiently far from the source. These results were tested by measuring the build-up using a magnet to sweep scattered electrons from the primary photon beam and by measuring the surface dose in the limit of an evacuated beam path. The relative dose at zero depth in polystyrene was found to be approximately 8.9 +/- 0.3% of the dose at the depth of maximum build-up.

Cobalt Radioisotopes↗

PubMed1979

[Isodose curves for the tangential application to the breast].

The auxiliary collimating device furnished by Atomic Energy of Canada Ltd is constituted of a metalic base with a acrilic plate at the end and is fixed at the trimmer. This device is generally used in the irradiation of the breast in order to minimize the dose to the lung. Since only one isodose curve was furnished, the construction of others field sizes became necessary. Two systems of measurements were used. Isodose curves were obtained by decrement lines and scanning semi-conductor detector techniques. The problem associated to the measurements are also discussed.

Breast Neoplasms↗

PubMedDate not supplied

Build-up curves of scanned high energy electron beams from the Sagittaire linear accelerator.

A study of the build-up curves using an extrapolation chamber for 7, 10, 13, 16, and 19 MeV electron beams, from a Sagittaire linear accelerator, is presented. The effect of the ionization chamber bias polarity, field size, collimation and surface obliquity on the shape of the relative ionization curve was investigated. No clinically significant change is observed except the displacement of the maximum ionization point was observed for the oblique incidence of the beam.

Electrons↗

PubMedDate not supplied

Calculational methods for estimating skin dose from electrons in Co-60 gamma-ray beams.

Several methods have been employed to calculate the relative contribution to skin dose due to scattered electrons in Co-60 gamma-ray beams. Either the Klein-Nishina differential scattering probability is employed to determine the number and initial energy of electrons scattered into the direction of a detector, or a Gaussian approximation is used to specify the surface distribution of initial pencil electron beams created by parallel or diverging photon fields. Results of these calculations are compared with experimental data. In addition, that fraction of relative surface dose resulting from photon interactions in air alone is estimated and compared with data extrapolated from measurements at large source-surface distance (SSD). The contribution to surface dose from electrons generated in air is 50% or more of the total skin dose for SSDs greater than 80 cm.

Cobalt Radioisotopes↗

PubMedDate not supplied

A graphite transmission ionization chamber.

A pancake-type transmission chamber made of high-purity graphite and open to the atmosphere has been designed and constructed at the Secondary Standard Dosimetry Laboratory (SSDL-Rio de Janeiro). Tests performed on the chamber following the International Electrotechnical Commission recommendations indicate that its performance characteristics are comparable to those expected from a secondary standard ionization chamber.

Cobalt Radioisotopes↗

PubMedDate not supplied

Scatter factor corrections for elongated fields.

Measurements have been made to determine scatter factor corrections for elongated fields of Cobalt-60 and for nominal linear accelerator energies of 6 MV (Siemens Mevatron 67) and 18 MV (AECL Therac 20). It was found that for every energy the collimator scatter factor varies by 2% or more as the field length-to-width ratio increases beyond 3:1. The phantom scatter factor is independent of which collimator pair is elongated at these energies. For 18 MV photons it was found that the collimator scatter factor is complicated by field-size-dependent backscatter into the beam monitor.

Health Physics↗