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Biomedical subjects

B J Gerbi

Publications and source records attributed to B J Gerbi.

At least 19 recordsLinked to original sources
PubMed1996

Stereotactic radiosurgery in pediatric patients.

Although stereotactic radiosurgery has been studied extensively in adults, the data demonstrating its efficacy in children is limited. Medical records were reviewed to identify the indications for and outcomes of patients treated with this modality. Linear accelerator-based radiosurgery was used to treat 11 recurrent brain tumors and one posterior fossa arteriovenous malformation over 3 years. The mean and median age of those treated was 10 and 8 years, respectively (range 1-20 years). Patients received 700 to 3,000 cGy delivered to the 50-90% isodose line in a single fraction. The mean and median follow-up was 15 and 17 months, respectively. Three of the four children with malignant disease died 6 to 9 months after treatment. One patient died of recurrence outside the treatment field. Another child died of complications related to radiation injury, and the third died of disease progression. All children with low-grade tumors remain alive without complications. Six of eight (75%) children exhibit substantial radiographic reductions in tumor size. The child with a vascular malformation has been followed for 26 months, without hemorrhage and with a radiographically proved decrease in size. Our series suggests that radiosurgery has limited usefulness in malignant disease. Therapeutic response is influenced by lesion size and/or location. Stereotactic radiosurgery appears to be effective in children with low-grade intracranial tumors or arteriovenous malformations. Further experience is required to establish the role and long term side effects of radiosurgery in pediatric patients.

Adolescent↗
PubMed1995

Evaluation of dose variation during total skin electron irradiation using thermoluminescent dosimeters.

PURPOSE: To determine acceptable dose variation using thermoluminescent dosimeters (TLD) in the treatment of Mycosis Fungoides with total skin electron beam (TSEB) irradiation. METHODS AND MATERIALS: From 1983 to 1993, 22 patients were treated with total skin electron beam therapy in the standing position. A six-field technique was used to deliver 2 Gy in two days, treating 4 days per week, to a total dose of 35 to 40 Gy using a degraded 9 MeV electron beam. Thermoluminescent dosimeters were placed on several locations of the body and the results recorded. The variations in these readings were analyzed to determine normal dose variation for various body locations during TSEB. RESULTS: The dose to flat surfaces of the body was essentially the same as the dose to the prescription point. The dose to tangential surfaces was within +/- 10% of the prescription dose, but the readings showed much more variation (up to 24%). Thin areas of the body showed large deviations from the prescription dose along with a large amount of variation in the readings (up to 22%). Special areas of the body, such as the perineum and eyelid, showed large deviations from the prescription dose with very large (up to 40%) variations in the readings. DISCUSSION: The TLD results of this study will be used as a quality assurance check for all new patients treated with TSEB. The results of the TLDs will be compared with this baseline study to determine if the delivered dose is within acceptable ranges. If the TLD results fall outside the acceptable limits established above, then the patient position can be modified or the technique itself evaluated.

Adolescent↗
PubMed1995

Maintaining accuracy in stereotactic radiosurgery.

PURPOSE: To provide the manufacture's specification for the base phantom of a commercially available stereotactic radiosurgery system so that its accuracy can be confirmed, and to describe a calibration device that allows the accuracy of the base phantom to be verified quickly and on a routine basis. Modifications to the target pointer system that make matching the pointer tips easier and less likely to damage the pointer tips are also described. METHODS AND MATERIALS: In stereotactic radiosurgery, spatial accuracy is the key factor for successful dose delivery. With some commercially available systems, this accuracy depends on the accuracy of the base phantom coordinate system, how closely the tip of the target pointer can be matched to the tip of the base phantom pointer, and how accurately the coordinates set on the isocentric subsystem match those set on the base phantom. Two major problems, usually overlooked when evaluating system accuracy are, first, the base phantom, which establishes the stereotactic coordinate system, is assumed to be completely accurate. This is a dangerous assumption because the base phantom is used frequently for routine patient treatments and for standard quality assurance tests. To exacerbate the problem, no independent device is provided with stereotactic systems to check the accuracy of the base phantom. Second, the accuracy of the isocenter coordinates set on the head support stand depends upon how closely the target pointer and the base phantom pointer can be aligned. The hardware provided with the system is difficult to use and easily leads to damage of the pointer tips. RESULTS: In this work, we provide the manufacturer's specifications for a popular stereotactic system, describe a device that can be used to check quickly and easily the accuracy of the base phantom, and describe a modification to the transfer pointer system that allows the pointer tips to be more easily aligned with reduced possibility of damage to the pointer tips. CONCLUSION: The methods and apparatus described in this paper should be useful to anyone using a base phantom for testing radiosurgery accuracy.

Equipment Design↗
PubMed1995

Design specifications for a treatment stand used for total body photon irradiation with patients in a standing position.

One total body photon irradiation technique used to treat patients employs a standing treatment position and a horizontally directed high-energy photon field. This standing technique presents special problems, including keeping the patient immobile during treatment and offering protection from injury if the patient develops weakness or loss of consciousness due to either medication (anxiolytics, narcotics, or antiemetics) or other causes. In this article we describe a treatment stand designed to manage these problems and use effectively total body photon irradiation. This stand has been used successfully in our clinic at the University of Minnesota for several years and has met or exceeded the original design expectations.

Equipment Design↗
PubMed1995

Evaluation of electronic portal imaging device for missing tissue compensator design and verification.

The purpose of this investigation is to determine if electronic portal imaging devices (EPIDs) can be used for the design and verification of compensating filters. In order to do this, we investigated the operating characteristics of a commercially available EPID and the variation in transmitted dose for various measurement situations. We performed four initial tests to determine the EPID response specific to compensator situations. The tests determined EPID response to variable patient SSDs, different gantry angles, positions of an inhomogeneity within a phantom, and the sensitivity variation of different parts of the imager. After these tests, we determined the attenuation functions relating EPID response to phantom thickness for various phantom materials. With these functions, we tested simple compensation situations to demonstrate that missing tissue compensators can both be designed and verified using EPIDs.

Biophysical Phenomena↗
PubMed1995

Stereotactic radiosurgery for recurrent malignant gliomas.

PURPOSE: To evaluate the role of stereotactic radiosurgery in the management of recurrent malignant gliomas. PATIENTS AND METHODS: We treated 35 patients with large (median treatment volume, 28 cm3) recurrent tumors that had failed to respond to conventional treatment. Twenty-six patients (74%) had glioblastomas multiforme (GBM) and nine (26%) had anaplastic astrocytomas (AA). RESULTS: The mean time from diagnosis to radiosurgery was 10 months (range, 1 to 36), from radiosurgery to death, 8.0 months (range, 1 to 23). Twenty-one GBM (81%) and six AA (67%) patients have died. The actuarial survival time for all patients was 21 months from diagnosis and 8 months from radiosurgery. Twenty-two of 26 patients (85%) died of local or marginal failure, three (12%) of noncontiguous failure, and one (4%) of CSF dissemination. Age (P = .0405) was associated with improved survival on multivariate analysis, and age (P = .0110) and Karnofsky performance status (KPS) (P = .0285) on univariate analysis. Histology, treatment volume, and treatment dose were not significant variables by univariate analysis. Seven patients required surgical resection for increasing mass effect a mean of 4.0 months after radiosurgery, for an actuarial reoperation rate of 31%. Surgery did not significantly influence survival. At surgery, four patients had recurrent tumor, two had radiation necrosis, and one had both tumor and necrosis. The actuarial necrosis rate was 14% and the pathologic findings could have been predicted by the integrated logistic formula for developing symptomatic brain injury. CONCLUSION: Stereotactic radiosurgery appears to prolong survival for recurrent malignant gliomas and has a lower reoperative rate for symptomatic necrosis than does brachytherapy. Patterns of failure are similar for both of these techniques.

Actuarial Analysis↗
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↗