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Biomedical subjects

J F Williamson

Publications and source records attributed to J F Williamson.

At least 91 records · Page 5Linked to original sources
PubMed1984

Serial thyroid function measurements in children with Hodgkin disease.

Thyroid function was measured serially in 28 children with Hodgkin disease diagnosed from 1971 to 1978. The patients' ages ranged from 4 to 16 years at diagnosis, and treatment consisted of chemotherapy only (four patients), radiation alone (15), or radiation plus chemotherapy (nine). None of the four children given chemotherapy only developed thyroid hypofunction, in contrast to 21 (88%) of the 24 children given high doses of radiation (P less than 0.001). Thyroid function in three patients with compensated hypothyroidism and in one child with primary hypothyroidism reverted to normal without thyroid replacement. One child given chemotherapy only and one child given radiation only became transiently hyperthyroid. These results indicate that patients given combined modality therapy for Hodgkin disease are at high risk for thyroid abnormalities. The results of long-term follow-up of thyroid function demonstrate, however, that all such thyroid abnormalities may not necessarily be permanent.

Adolescent↗
PubMed1984

The significance of electron binding corrections in Monte Carlo photon transport calculations.

Many Monte Carlo simulations ignore coherent scattering events and utilise the Klein-Nishina free electron distribution, rather than the incoherent differential cross-section, for choosing the trajectories of incoherently scattered photons. We assess the accuracy of this model by comparing its results with those of the complete bound electron model (form factor approach), which simulates coherent scattering events, and uses the appropriate bound electron angular scattering distributions. Both analytic and Monte Carlo calculations demonstrate that use of the free electron scattering distributions significantly underestimates the angular distribution of scattered photon energy resulting from low and medium energy photons incident upon carbon, iron, and platinum barriers. In using the free electron approximations to calculate barrier transmission, significant errors occur only for primary photon energies below 100 keV. Implementation of the complete bound electron model reduces the computational efficiency of our Monte Carlo code by only 10-25%.

Electrons↗
PubMed1983

Ovarian function after successful bone marrow transplantation in postmenarcheal females.

Ovarian function was followed serially in a group of six postmenarcheal females after successful bone marrow transplantation (BMT). The patients were between 13 9/12 and 22 6/12 (median 16 5/12) years of age at the time of BMT and were followed a median of 20 months (range 17-45 months) posttransplantation. Two subjects received short-term high-dose cyclophosphamide combined with single-dose total lymphoid irradiation (Group I), whereas the remaining four were treated with short-term, high-dose chemotherapy plus single-dose total body irradiation (Group II). Group II subjects also received combination chemotherapy prior to BMT. One subject from Group I continues to have regular menses and normal gonadotropin levels, 36 months post-BMT. The remaining five patients have demonstrated persistently elevated plasma concentrations of LH and FSH over a 17- to 45-month period of time. None of the four patients in Group II has menstruated since undergoing BMT. We conclude that single-dose radiation combined with short-term, high-dose chemotherapy results in profound ovarian damage in the majority of young women undergoing BMT.

Adolescent↗
PubMed1983

Monte Carlo evaluation of the Sievert integral for brachytherapy dosimetry.

The Sievert integral, widely used to compute dose distributions about filtered line sources, assumes that the emitted energy fluence is exponentially attenuated by the filter thickness traversed by the primary photons. To evaluate this approximation, a Monte Carlo simulation was performed realistically modelling the diffusion and energy degradation of primary photons due to coherent, incoherent and photoelectric interactions in the source filter. Estimates of the exposure rate at points near the source were obtained using analytical averaging. Comparison of the two models shows that for 226Ra and 192Ir sources, the Sievert algorithm consistently overestimates the exposure rate per unit activity. However, such errors may be significantly reduced if source intensity is expressed in terms of exposure rate. Computed exposure rate distributions based on exposure rate calibrations are also less sensitive to uncertainties in available spectroscopic data.

Brachytherapy↗
PubMed1983

Orthovoltage radiation of normal canine nasal passages: assessment of depth dose.

Frozen heads of 9 clinically normal dogs were irradiated with orthovoltage x-rays. Surface doses and nasal cavity depth doses were measured, and the percentage of surface dose (depth dose) was calculated at random depths from the dorsal cutaneous surface in transverse planes through the medial and lateral canthi. Depth dose of 2 orthovoltage x-ray beams having half-value layers of 1.5 mm of Cu (96 keV) and 2.6 mm of Cu (134 keV) were compared with and found to resemble that reported in depth dose tables based on soft tissue equivalent material. Any differences (identified graphically) in depth dose, compared with that described using a uniform (soft tissue equivalent) phantom, were explained by the variations in tissue composition and the presence of air within the normal nasal and paranasal cavities.

Animals↗
PubMed1982

Dosimetric analysis of stereo and orthogonal reconstruction of interstitial implants.

We have evaluated the performance of stereo and orthogonal computerized three-dimensional implant reconstruction algorithms. Sets of orthogonal and stereo radiographs of a simulated interstitial seed implant were taken. Computer reconstructed coordinates of each source were compared with their measured positions in order to assess the geometric accuracy of the two methods. We find that the effect of random measurement errors on reconstructed coordinates from stereo films is 3-5 times that of the orthogonal method. Although relative individual seed positions may be in error by as much as 1 cm, these errors do not significantly change the shape or area of isodose curves surrounding the implant periphery.

Brachytherapy↗
PubMed1982

Methods for routine calibration of brachytherapy sources.

Two types of routine geometry calibrators, the nuclear medicine dose calibrator and a Lucite jig holding a 30-ml external beam ion chamber with the source in a rigid geometry, were compared with open-air measurements for 137Cs, 192Ir, and 226Ra brachytherapy sources. The proximity of scattering surfaces in the second apparatus resulted in significant distortion of the buildup effect and deviation from the inverse square law (5% to 15%). For the dose calibrator, the response/Roentgen was found to be dependent not only on source energy, but on source capsule thickness as well. Approximately one half of the observed variation (27%) in calibration factors was accounted for by differences in filtration among sources. A mathematical logarithm that corrects for these geometric and filtration effects is presented. In addition, the activity assay procedure provided by the manufacturer of the dose calibrator is shown to be unsuitable for brachytherapy sources. Steps to overcome these problems are discussed.

Brachytherapy↗
PubMedDate not supplied

Radiation transport calculations in treatment planning.

This article reviews the role of radiation transport calculations in radiotherapy dose computation. The physical and mathematical principles underlying transport theory, as applied to electrons and photons, are discussed. Practical methods of solving the transport equation, with emphasis on Monte Carlo techniques, are reviewed. The contribution of analytic and Monte Carlo transport calculations to electron beam, photon beam, and brachytherapy treatment planning dosimetry is assessed. Currently, the most important roles of transport theory include using approximate solutions of the transport equation as theoretical foundations of dose computation algorithms, and using Monte Carlo simulation to calculate basic treatment planning data, characteristic of the treatment modality, which can serve as input to a dose computation algorithm.

Computer Simulation↗
PubMedDate not supplied

The American Brachytherapy Society perspective on intravascular brachytherapy.

BACKGROUND: Recent clinical studies indicate that intravascular brachytherapy (IVB) can reduce the rate of restenosis substantially after angioplasty procedures. However, no clinical guidelines exist for optimal therapy. METHODS: The members of the IVB Subcommittee of the American Brachytherapy Society (ABS) identified the areas of consensus and controversies in IVB to issue the ABS perspective on IVB, based on analysis of published reports and the clinical experience of the members in brachytherapy. RESULTS: IVB is still experimental. The long-term efficacy, toxicity, the target tissue, and dose required for IVB are not established. The ABS recommends that IVB procedures must be performed, with careful attention to radiation-related issues, in the context of controlled multidisciplinary clinical trials with the approval of the institutional review board, the Nuclear Regulatory Commission, the Food and Drug Administration, and under an Investigational Device Exemption. The therapeutic radiologist, with a qualified radiation physicist, is responsible for dose prescription and delivery and needs to be present during the IVB procedure as part of this multidisciplinary team. The long-term outcome from these studies should be reviewed critically and published in peer-reviewed journals. The ABS endorsed the dosimetric guidelines of the American Association of Physicists in Medicine Task Group 60 (AAPM TG-60) report. The ABS recommends that dose specification be defined clearly; to allow comparisons between studies, the dose should be prescribed at 2 mm from the source for intracoronary brachytherapy and at an average luminal radius of +2 mm for peripheral vascular brachytherapy. The prescription doses at the above point is generally in the 12-18 Gy range. Comprehensive procedures for quality assurance, radiation protection, and emergencies should be in place before initiating an IVB program. Higher energy beta sources, lower energy gamma sources, dose-volume histograms, and correlation of three-dimensional reconstructions of delivered dose with patterns of failure are areas for further research. CONCLUSION: The ABS perspective on IVB is presented to assist the interventional team in developing protocols for the use of IVB in the prevention of restenosis. Long-term outcome data with a standardized reporting system are needed to establish the role of brachytherapy in preventing vascular restenosis. Endovascular brachytherapy is a new and evolving modality, and these recommendations are subject to modifications as new data become available.

Brachytherapy↗
PubMedDate not supplied

Further characterization of MHC haplotypes demonstrates conservation telomeric of HLA-A: update of the 4AOH and 10IHW cell panels.

Cell panels have been used extensively in studies of polymorphism and disease associations within the major histocompatibility complex (MHC), but the results from these panels require continuous updates with the increasing availability of novel data. We present here an updated table of the typings of the 10IHW and 4AOH panels. Local data included are HFE, HERV-K(C4) and six microsatellites telomeric of HLA-A. Typings for class I, MICA (PERB11.1), MICB (PERB11.2), XA, XB, LMP2 and 10 microsatellites reported by others have also been consolidated in this table. The tabulation shows that the length of conservation in the human MHC is even more extensive than previously thought. Human MHC ancestral haplotypes are inherited as a conserved region of genomic sequence spanning some 6-8 megabases from the HLA class II region and beyond the HLA class I region up to and including the HFE gene. Numerous examples of historical recombinations were also observed.

Cell Line↗
PubMedDate not supplied

High dose ratio (HDR) cervical ring applicator to control bleeding from cervical carcinoma.

The objective of this study was to evaluate the use of the high dose rate (HDR) cervical ring applicator to control acute cervical bleeding from carcinoma of the uterine cervix. This study consists of 15 patients presenting with invasive carcinoma of the uterine cervix with acute vaginal bleeding requiring transfusion. Initial irradiation, delivered emergently because of vaginal bleeding, consisted of two fractions (5 Gy each fraction to the surface of the cervix) utilizing the HDR intracavitary vaginal ring applicator. Two fractions were administered at one week intervals for a total of 10 Gy to the surface of the cervix. Irradiation doses from the HDR ring applications were not considered into the composite total dose to point A. Diagnostic imaging evaluation and initiation of external irradiation were commenced during this initial weekly interval between fractions. Vaginal bleeding requiring no additional transfusions was achieved in 93% (14/15) of patients. No acute or long-term Grades 3, 4, or 5 bowel or bladder sequelae were noted. In conclusion, HDR cervical ring brachytherapy is effective in controlling acute vaginal bleeding and can be delivered without undue acute or long-term toxicity.

Brachytherapy↗
PubMedDate not supplied

Revision of tissue-maximum ratio and scatter-maximum ratio concepts for cobalt 60 and higher energy x-ray beams.

The concept of tissue-maximum ratios (TMR) as a basis of dose computations has posed problems when applied to a whole range of clinically used megavoltage beams. Another problem is the definition of scatter-maximum ratio (SMR) which assumes a value of zero at the depth of maximum dose. This paper describes new methods of measuring collimator and phantom scatter correction factors. The definitions of TMR and SMR are modified in order to compute phantom scatter at any depth, including depth of maximum dose. The revised concept is basic and general enough that it can be applied to x-ray beams of any energy fields of any shape and isocentric as well as non-isocentric modes of treatment.

Cobalt Radioisotopes↗
PubMedDate not supplied

Film dosimetry of megavoltage photon beams: a practical method of isodensity-to-isodose curve conversion.

The central problems of photon beam film dosimetry are the dependence of film response upon photon energy, processing conditions, and film plane orientation. We have overcome these problems by accurately fitting the depth-dependent sensitometric curve of Kodak XV-2 film (exposed parallel to beam axis) to the equation OD(D,d) = ODs(1 - exp [- alpha 0[1 + beta(d-dm)]D]) where OD(D,d) is the optical density for dose D at depth d. ODs, alpha 0, and beta are constants characteristic of the film and beam energy but are independent of field size. Only central axis depth dose data for a single field are required to determine their values. A computer program based upon this equation has been written which successfully generates single field isodose curves from film data for a variety of field sizes (including wedged fields) with an accuracy of +/- 3%. Data are presented for 60Co, and 4 and 10 MV x rays.

Film Dosimetry↗
PubMedDate not supplied

Dose calibrator response to brachytherapy sources: A Monte Carlo and analytic evaluation.

It is well known that the dose calibrator response/unit exposure rate depends significantly upon source energy. However, investigation of 137Cs, 192Ir, and 226Ra brachytherapy sources by empirical, analytical, and Monte Carlo techniques shows that source filtration significantly affects the calibrator reading to exposure rate conversion factor. The results demonstrate that for each clinically used filtration thickness an exposure calibrated standard source is required to establish the response of the well chamber. An interesting consequence of this analysis is that the Sievert point dose algorithm for clinical sources overestimates the dose on the order of 3% at distances of approximately 3.5 cm from the source.

Brachytherapy↗
PubMedDate not supplied

Monte Carlo evaluation of kerma at a point for photon transport problems.

Estimation of collision kerma at a geometric point arising from scattered photons is a potentially important application of Monte Carlo simulation, especially in the presence of steep flux gradients. We examine the usual method of extracting point-kerma estimates from randomly generated photon trajectories which consists of tallying the energy lost by photon collisions occurring in the vicinity of the point of interest. Several other methods derived from the equivalence of track length per unit volume and flux are evaluated as to accuracy and efficiency. Finally, a next-flight estimator is discussed in which the expected contribution of each simulated photon collision to kerma at the point of interest is calculated regardless of proximity of the collision to the point. All of these techniques are shown to involve a trade-off between statistical precision and spatial resolution: increasing the number of contributing collisions requires averaging kerma over a larger volume. Based upon both analytic models and realistic Monte Carlo simulations, use of next-flight and track-length estimators is shown to improve simulation efficiencies by factors of 2 to 20 compared to analog scoring. Practical guidelines as to choice of estimator and successful implementation are presented.

Models, Theoretical↗