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Biomedical subjects

B J Gerbi

Publications and source records attributed to B J Gerbi.

33 records · Page 2Linked to original sources
PubMed1989

An afterloading brachytherapy device utilizing thermoplastic material.

An afterloading brachytherapy device for treatment of residual cancer in an enucleated orbit with two cesium-137 sources was designed using a thermoplastic material, Aquaplast. The device consists of a face-mask support held in place with elastic bands around the head and an acrylic afterloading applicator. The device is very easy to make, holds the sources firmly in place, allows full mobility of the patient, and gives excellent dose distribution to the target area. It was easily tolerated by a 7-year-old child during the 50 h of treatment.

Brachytherapy↗
PubMed1989

Total skin electron arc irradiation using a reclined patient position.

Several techniques for the treatment of the total skin of the patient using electron beams have been described in the literature. However, most techniques presuppose that the patient is capable of maintaining a standing position for the duration of the treatment. For patients either weakened by disease or those suffering from a loss of limbs, this is often an unrealistic expectation. We will describe a total skin electron irradiation technique that allows the patient to remain in a reclined position without sacrificing dose uniformity. This technique uses two symmetric +/- 48 degrees arc electron beams to provide a field uniformity of +/- 5% over a range of approximately 250 cm X 45 cm. Six patient positions are used to provide a uniform dose around the periphery of the patient. A description of the treatment technique along with details of the dosimetry are given.

Electrons↗
PubMed1989

Quality control of custom block making in radiation therapy.

Custom blocks are the most commonly used field shaping device in radiotherapy. They offer many advantages in daily clinical use. However, as with all accessories used for patient treatments, care must be taken with their use. As such, a quality control program to check block cutting accuracy is recommended as a routine part of their clinical use. We have developed a three step method for checking custom blocks before they are used for patient treatment. These steps include: a static light check, a parallel opposed film check, and a block check involving the patient. By completing these three steps, we feel that we have improved the overall accuracy of our custom block making system which has resulted in more accurate treatments for our patients.

Humans↗
PubMed1987

Cervical cancer: intracavitary dose specification and prescription.

Dose and volume specifications for reporting intracavitary therapy were analyzed according to criteria recommended by the International Commission on Radiation Units and Measurements (ICRU). Ninety Fletcher-Suit radium applications were studied to examine the validity of the assumptions of the ICRU and the merit of their routine reporting. It was demonstrated that the reporting recommendations were inconsistent with clinical prescription systems and added little to dose specification. The distinction between dose specification and dose prescription was stressed.

Brachytherapy↗
PubMed1986

Role of point A in the era of computerized dosimetry.

Treatment prescriptions based on milligram-hours and point A doses were examined in light of the recommended dose and volume specification for intracavitary therapy proposed by the International Commission on Radiation Units and Measurements (ICRU). Because neither point A doses nor milligram-hours are to be included in dose reporting, it is necessary to translate the vast empirical experience with prescriptions based on these parameters into the ICRU schema. A total of 90 Fletcher-Suit radium applications were analyzed to explore relationships between point A doses, milligram-hours, and the ICRU guidelines. It was demonstrated that some definitions of point A can have essentially no utility within any dosimetric system, while others can lead to treatment prescriptions with some degree of correspondence to ICRU recommendations. Older dosimetric concepts must be retained while newer ones are being developed.

Brachytherapy↗
PubMedDate not supplied

Compensating filter design using radiographic stereo shift information.

This paper describes a method of designing 3-dimensional compensating filters for radiation therapy using photon beams. A radiopaque grid is placed on the patient surface and stereo shift radiographs are taken of the treatment area. With the aid of a computer, tissue deficit information is calculated. Isothickness lines are plotted for the different missing tissue thicknesses and lead sheets with proper magnification are cut from these plots and assembled into the final tissue compensator.

Humans↗
PubMedDate not supplied

Dosimetry of asymmetric x-ray collimators.

We have studied the dosimetry of an independent jaw system (provided with the Varian Clinac 2,500) using ionometric measurements performed both in air and in a water phantom. Our study shows that the effect of the independent jaw on the dose distribution is similar to that of secondary blocking except for changes produced in the collimator scatter. A system of dose calculation was developed which takes into account the changes in the collimator scatter as well as in the isodose distribution. A method is described to correctly generate isodose curves for fields shaped by an independent jaw using a modified AECL TP11 treatment planning system. The primary modification in the program consists of correcting the zero-area tissue-maximum ratios for the off-axis variation in beam quality.

Humans↗
PubMedDate not supplied

Validity of lung correction algorithms.

Our studies have compared the "effective tissue-air ratio (TAR) method" (ICRU Report No. 24), "equivalent TAR method," and the "generalized Batho method" (currently used by the TP-11 computer treatment planning system) with measured results for different energy photon beams using two lung inhomogeneities to simulate a lateral chest field. Significant differences on the order of 3%-15% were found when comparing these various methods with measured values.

Algorithms↗
PubMedDate not supplied

Dose buildup for obliquely incident photon beams.

For obliquely incident photon beams, the buildup of dose with depth is markedly different from normally incident beams. However, relatively little data on this topic exists for high-energy photon beams of energy greater than 6 MV. Measurements of dose in the buildup region were made using a plane-parallel ionization chamber in a polystyrene phantom with obliquely incident 6-, 10-, 18-, and 24-MV x-ray beams angled 0 degrees to 84 degrees. Buildup curves at these angles were plotted and from these an obliquity factor, defined as the ratio of ionization charge collected at a point for a particular angle of incidence to that collected at the same point at normal incidence, was determined. For each energy, the obliquity factor as a function of depth, field size, and source-chamber distance was studied. Results indicate that the obliquity factor is highly dependent on the beam energy, angle of incidence, the collimator opening, and the source-skin distance. A mathematical expression has been developed to predict the dose in the buildup region of high-energy photon beams for various angles of beam incidence, field size, and chamber distance.

Humans↗
PubMedDate not supplied

The polarity effect for commercially available plane-parallel ionization chambers.

The polarity effect was investigated for three different commercially available plane-parallel ionization chambers: the Memorial Pipe chamber, the Victoreen/Nuclear Associates model 30-329 chamber manufactured by PTW, Frieburg, and the Capintec PS-033 thin-window ionization chamber. The primary study was the polarity effect versus depth below the phantom surface for 6-, 10-, 18-, and 24-MV x-ray beams, and 9- and 22-MeV electron beams. The polarity effect in the region of nonelectronic equilibrium that exists at the interface of two dissimilar materials, polystyrene and aluminum, was investigated as well as the effects of field size. For the group of plane-parallel ionization chambers that we studied, we found a polarity effect of only 1%-2% for electron beams at the depth of dmax. At depths greater than dmax, the polarity effect for electrons increased and was as high as 4.5% for some chambers. When used in the buildup region of high-energy photon beams, these same chambers exhibited up to a 30% difference in collected charge between one polarity and the other. This effect and its relationship to physical chamber characteristics is discussed.

Radiometry↗
PubMedDate not supplied

Interface dose perturbation as a measure of megavoltage photon beam energy.

The description of the quality of a photon beam has usually been characterized by a single value such as the half-value layer, the effective attenuation coefficient, the percent depth dose, and most recently by the ionization ratio (IR). Although the IR is simple and easy to measure, it lacks sensitivity at photon energies above 10 MV. This paper describes a method based on dose perturbation at an interface and defines the forward dose perturbation factor (FDPF) as a measure of beam quality. Comparisons between the two methods are given for photon energies ranging from 60Co to 24 MV. The results show that the FDPF method is more sensitive to spectral changes at photon energies above 10 MV than the IR.

Cobalt Radioisotopes↗
PubMedDate not supplied

Measurement of dose in the buildup region using fixed-separation plane-parallel ionization chambers.

Accurate measurement of dose at the surface of a phantom and in the buildup region is a difficult task but one that is important for the proper treatment of patients. The instruments of choice for these measurements are extrapolation chambers but few institutions have these instruments at their disposal. As a result, fixed-separation plane-parallel ionization chambers are most commonly used for this purpose. Recent papers have re-emphasized the inaccuracies in the measurement of dose in the buildup region of normally incident photon beams when using fixed-separation plane-parallel ionization chambers. Data for Co-60, 6-, 10-, 18-, and 24-MV photon beams are presented that show the magnitude of this over response in the buildup region for several commercially available plane-parallel ionization chambers versus results obtained using both an extrapolation chamber and LiF thermoluminescent detectors. Differences in the percent depth dose at the surface of a phantom of greater than 19% were found for one of the chambers. All chambers over responded in the buildup region to some degree based upon their internal dimensions. The appropriateness of published corrections for these chambers is evaluated and guidelines for the accurate measurement of dose in the buildup region are presented.

Humans↗
PubMedDate not supplied

A mathematical expression for %DD accurate from Co-60 to 24 MV.

Obtaining accurate %DD values for routine treatment calculations is essential in radiation therapy. Many papers have presented expressions to calculate this or related parameters but these expressions often required that the parameters needed by the equation be determined for each individual treatment unit. This paper presents an expression that calculates %DD values with a mean-square accuracy of approximately 1.0% versus measured values. The expression is applicable to beam energies ranging from Co-60 to 24 MV, field sizes from 4 X 4 to 40 X 40 cm2, and depths from 1 cm deeper than dmax to 30 cm. The only information required by this expression that is machine specific is the ionization ratio.

Cobalt Radioisotopes↗
PubMedDate not supplied

The response characteristics of a newly designed plane-parallel ionization chamber in high-energy photon and electron beams.

A new plane-parallel ionization chamber has been designed by Attix to overcome the shortcoming of previous commercially available parallel-plate ionization chambers for dosimetry in high-energy photon and electron beams in radiation oncology. This investigation details the performance characteristics of this new, commercially available plane-parallel chamber. The magnitude of the polarity effect in high-energy electron beams is shown to be less than 1% while the polarity effect in high-energy photon beams is lower than several other plane-parallel ionization chambers. The over response of the chamber in the buildup region of normally incident high-energy photon beams is less than 1% for 6- and 24-MV x rays while the response of the new chamber to obliquely incident x-ray beams was affected much less by the angle of beam incidence than the other chambers tested. These superior response characteristics are primarily due to the construction characteristics of the collecting electrode arrangement. The Attix chamber, with a wall diameter (w) of 40 mm and a plate separation (s) of 1 mm, has an aspect ratio, (w/s), of 40. This exceeds the previously reported design criterion of w/s > or = 25 required to properly measure surface and buildup dose in either conventional therapy beams or in beams that are highly contaminated.

Biophysical Phenomena↗