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Biomedical subjects

C H Sibata

Publications and source records attributed to C H Sibata.

At least 19 recordsLinked to original sources
PubMed1996

A simple backup for a radiosurgery treatment planning system.

To calculate the dose distribution and the number of monitor unit (MU) per arc, all radiosurgery systems utilize some sort of computer. These computers are, of course, subject to equipment malfunction such as problems with the magnetic tape drive, keyboard, mouse, etc. Since most radiosurgery procedures are quite invasive and time consuming, it is important to have a reliable and reasonably accurate backup system for planning the treatment. This paper will show that a simple PC based system, along with a digitizer, may be used as a backup for a commercial, VAX based radiosurgery system. A complete radiosurgery planning procedure was carried out on a head phantom with a target imbedded inside. The treatment planning and verification using the PC based system is also compared with that using the VAX based system.

Computer Systems↗
PubMed1996

A well-type ionization chamber geometric correction factor.

To correct for the influence of source configuration on the measured activity of spherical and cylindrical brachytherapy sources, a geometric correction factor was calculated for the Standard Imaging HDR-1000 well-type ionization chamber. A Fortran program modelled each source as a lattice of point sources. Because of the cylindrical symmetry of the well chamber, it could be uniquely modelled by point detectors along the perimeter of the radial plane of the detection volume. Path lengths were calculated and attenuation factors were applied to each source-detector point combination individually. The total dose rate at each detection point was found through a Sievert summation of the point source contributions. For 137Cs sources with identical activities, a correction factor of 0.965 +/- 0.005 was calculated, equal to the ratio of the dose rate of the cylindrical source to that of the sphere. Experimental verification using a Nuclear Associates 67-809 series cylindrical sources and an Amersham spherical 137Cs source yielded a correction factor of 0.958 +/- 0.016.

Brachytherapy↗
PubMed1995

Evaluation of a well-type ionization chamber for calibration of HDL and LDR brachytherapy sources.

The Atomlab 44D well-type ionization chamber is being evaluated for calibration of high dose rate (HDR) Ir-192 and low dose rate (LDR) Cs-137 sources. The chamber has a flat response (sweet spot) of +/- 0.5% along approximately 3.5 cm for an Ir-192 HDR linear source and 3 cm for a Cs-137 LDR spherical source. The short-term stability of the chamber was determined using a cylindrical Cs-137 source positioned in the sweet spot region. The chamber response over a range of 17.4 mCi (644 MBq, Cs-137) and 8.82 Ci (326 GBq, Ir-192) is evaluated. The chamber may be used for calibrating the activities of both HDR Ir-192 and LDR brachytherapy sources.

Brachytherapy↗
PubMed1995

A simple device for prostate and rectal localization in radiation therapy.

A simple device for prostate and rectum localization in radiation therapy may be constructed as follows: first, dental wax is used to make up a shape of the rectal probe. Second, dental stone is used to make a two-piece mold by molding the dental wax probe. Third, using the mold, the rectal probe is made from a mixture of silicone and barium. The final step involves the attachment of the "plug," which is made of aluminum. From the simulation films for the prostate setup, the location of the anterior wall of the rectum, anal sphincter, and anal verge can easily be identified, such that the appropriate regions are shielded accordingly. A CT scan with the probe in the rectum did not give too much artifacts from the barium/silicone mixture. The rectal probe with the plug is useful for localizing the prostate more accurately, compared with using the other rectal localization techniques such as barium injection or some rectal markers.

Aluminum↗
PubMed1995

Application of the "bioeffects" algorithm of a treatment planning system.

In this study, both a four-field box and two-field AP/PA treatment plan are combined with two insertions of Cs-137 in a tandem and ovoids setup, to evaluate the bioeffects program of a treatment planning system. External beam energies studied are 18 and 6 MV. It is shown that there is a slight difference in the 50-70 time dose fractionation (TDF) isolines when comparing 6 MV and 18 MV, for the AP/PA setup. There is practically no difference for TDF isoline values larger than 80 for both energies with either the four-field or the two-field setup. This is because the brachytherapy contributed the majority of the dose to the regions near the applicator and the TDF values reflect the higher dose delivered by the brachytherapy relative to the external beams in that region. For this simple evaluation of the bioeffects program, the combination of the external beam plan and the brachytherapy plan does not give us enhanced information on the effectiveness of the plan.

Algorithms↗
PubMed1995

Accuracy of the point source approximation to high dose-rate Ir-192 sources.

The accuracy of the point source approximation used in dose calculations for an implant comprised of multiple high dose rate (HDR) Ir-192 source dwell positions is investigated. First, a single dwell position implant is modeled. The exposure rate about the source is calculated using both the point source approximation and the more rigorous line source formalism. A comparison of these calculated exposure rates is made. It is found that for each HDR Ir-192 source dwell position, the point source approximation results in a dose overestimation of 1% at a distance of 1 cm on the source transverse axis, while dose underestimations of more than 2% can be found at a distance of 1 cm on the source longitudinal axis. Even larger errors occur closer to the source. The results of this academic study are then extended to two clinical cases--an endobronchial treatment and a tandem and ovoids setup, both involving multiple source dwell positions. Since clinical HDR Ir-192 implants are comprised of many individual source dwell positions, there will be inaccuracy in the calculated overall dose distribution leading to dose delivery errors. For example, the dose delivered to a prescription point located 0.5 cm from an endobronchial applicator will be 3% lower than prescribed. Similar errors are produced in gynecologic implants. To decrease below 0.5% the dose delivery error resulting from the point source approximation, prescription points should be at a distance of at least 1 cm from any applicator. Since the dosimetry error is a direct result of the choice of model used to describe the source, the use of anisotropy factors accounting for the variation of photon fluence around the HDR Ir-192 source will not completely correct the calculation.

Brachytherapy↗
PubMed1995

Evaluation of dose delivery based on a comparison of dosimetry calculations using open beam and wedged beam depth dose data.

The purpose of this study is to evaluate the magnitude of the error in dose delivery caused by the use of open beam depth dose data in dosimetry calculations for wedged photon beams. Isodose plans were calculated for treatments given in a 3-field isocentric prostate or rectal setup using an open AP beam with two lateral wedged beams. The dose distributions were first calculated using open beam depth dose data for all three fields. Next, the open beam data was used only for the AP field and true wedged beam depth dose data was substituted for the two lateral wedged fields. The magnitude of the depth dose variations for wedged vs open beams depends on the nominal beam energy, the wedge angle, and the depth of measurement. Consequently, isodose distributions calculated for wedged fields were found to be different when true wedged beam depth dose data was used instead of open beam data as is commonly done. Monitor unit calculations using a field size specific wedge factor show that dose delivery errors up to 4% can result from the use of open beam depth dose data in wedged beam dose distribution calculations for a 6-MV photon beam. Accurate treatment planning for wedged fields requires the use of wedged beam depth dose data specific to each wedge. Simply using open beam depth dose data in dose calculations for wedged beams will result in dose delivery errors, the magnitude of which depends on the combination of wedge angle, field size, and nominal beam energy.

Evaluation Studies as Topic↗
PubMed1995

The use of customized spreadsheets in radiation therapy.

A number of radiation-therapy-related uses based on a commercially available spreadsheet program have been developed at our facility. The graphics and display capabilities inherent in these spreadsheet programs allow for concise visual results. The spreadsheets are used as an independent check for several types of radiation therapy dose calculations. External beam--a spreadsheet will verify the monitor units (MU) or time required to deliver a prescribed dose to a point on an isodose line as calculated by a commercial treatment planning system. Calibration--spreadsheet programs have been developed to perform the calculations necessary for the output calibration of cobalt and high-energy photon and electron beams according to the TG-21 protocol. The user must indicate which beam, electrometer, chamber, phantom material, temperature, pressure and depth of measurement that apply. Radiosurgery--the MU per arc is calculated based on the following: the average depth per arc as obtained from a commercial radiosurgery program, the collimator size, and the prescription dose. TBI--The patient's width is entered into the spreadsheet program, which then calculates the MU needed to deliver a prescribed dose to the midline.

Humans↗
PubMed1995

Deconvolution of detector size effect for small field measurement.

Parametrization of the small fields employed in stereotactic applications is a painstaking process involving extensive film dosimetry to achieve acceptable beam edge definition. Use of cylindrical or spherical detectors for profile measurements would simplify data acquisition but add a volume averaging artifact to beam edge definition. We demonstrate a simple approach to unfolding the chamber size artifact from measured small beam profiles using typical cylindrical chambers. In comparison with film measurements we have found good agreement when the detector response function is deconvoluted from the measured profiles, although the amount of correction needed is fairly minimal for the detectors studied.

Film Dosimetry↗
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↗
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↗