Biomedical subjects

K J Weeks

Publications and source records attributed to K J Weeks.

At least 19 recordsLinked to original sources

PubMed1999

Method for determining photonuclear production of radioisotopes using high-energy electron beams.

Linear accelerators can produce electrons at high power and energy. These electrons can be targeted at materials to produce radionuclides. Monte Carlo simulation is used to follow the path of the linac electrons (15-35 MeV) through materials. The production of photons and their passage through the material is modeled. The method of using this Monte Carlo information to calculate activation in the materials is presented. It is found that kilowatt power levels can produce mCi amounts of radioactivity in minutes. This work permits systematic evaluation of the potential for designing linear accelerators for in-house production of radionuclides.

Computer Simulation↗

PubMed1998

Computed axial tomography tandem and ovoids (CATTO) dosimetry: three-dimensional assessment of bladder and rectal doses.

The purpose of this work is to compare bladder and rectal dose rates in brachytherapy for carcinoma of the cervix using two different dosimetry systems: traditional orthogonal radiograph-based dosimetry vs. computed axial tomography tandem and ovoids (CATTO) dosimetry. Twenty-two patients with carcinoma of the uterine cervix received the brachytherapy component of their radiotherapy with a computed-tomography compatible Fletcher-Suit-Delclos device. A total of 27 implants were performed. The average maximum bladder dose (Bmax) for the implants was 85.8 cGy/hr using the CATTO system as compared to 42.6 cGy/hr using traditional dosimetry, (P < 0.005). The average maximum rectal dose (R.) using the CATTO system was 59.2 cGy/hr as compared with 46.3 cGy/hr using the traditional system (P < 0.05). The traditional methods for choosing points to determine bladder and rectal dose rates underestimated the true Bmax in all cases and the R. in most. Based on the complication rates published in the literature, it is likely that the maximum tolerance dose of both the rectum and bladder, but especially the bladder, is higher than previously thought.

Brachytherapy↗

PubMed1998

Production of radioisotopes by direct electron activation.

High-energy electrons bombarded on materials can induce radioactivity by either directly knocking out neutrons or by first converting a fraction of the electron kinetic energy into electromagnetic energy, with subsequent neutron emission induced by the photons produced. The purpose of this paper was to develop a calculation method for estimating neutron emission and radionuclide production by high-energy (15-25 MeV) electrons directly interacting with a nucleus. The reaction (e,n) is considered using the method of virtual photons. The cross section for electron bombardment of lead, tantalum, rhenium, and tungsten targets is calculated. The electron cross sections are roughly 100 times less than the corresponding photon cross sections. The cross section increases monotonically with incident energy. A traveling wave linear accelerator was used for a qualitative test of the magnitude and energy dependence of the calculated cross sections. Tantalum was bombarded with electrons and the resultant emission of neutrons was inferred from the induced activation of 180Ta. The energy dependence and magnitude of the calculated electron cross sections agree with experiment within experimental uncertainties. It is concluded that accurate estimates of electron activation via the direct process is possible.

Electrons↗

PubMed1998

Monte Carlo dose calculations for a new ovoid shield system for carcinoma of the uterine cervix.

The dose distribution for an ovoid with a new tungsten shielding design was determined using Monte Carlo simulation. Standard Cesium-137 tube sources, tungsten shielding, and aluminum ovoid applicator were each modeled as a collection of solid objects. Dose was calculated in planes above, below, in front of, and on the sides of the colpostat. The Monte Carlo results were compared with the results from a parametrized calculation algorithm and good agreement was obtained. The dose distribution matrix derived from the parametrized algorithm can be used for clinical treatment planning.

Algorithms↗

PubMed1997

Three-dimensional applicator system for carcinoma of the uterine cervix.

PURPOSE: Intracavitary dose prescription for cancer of the uterine cervix has been based on the use of plane orthogonal films. Computed tomography (CT) and magnetic resonance imaging can provide three-dimensional (3D) anatomic information with which more sophisticated treatment planning can be carried out. This work describes a new tandem and ovoids design that permits modern 3D dosimetry and has the same placement flexibility for the physician as the applicators currently being used. METHODS AND MATERIALS: The external shape of the Fletcher-Suit-Delclos (FSD) minicolpostat tandem and ovoids system has been used as a model to build a prototype of a new applicator. The prototype colpostats are constructed out of aluminum and steel. The tandems are made of aluminum. The Fletcher shields are eliminated. A new method of using tungsten for dose attenuation and shielding has been designed. Longitudinal alignment of the tungsten shields makes the new system possible. This applicator is CT-compatible. RESULTS: Dose calculations for the new design are compared to a commercial version of the FSD applicator. Both the aluminum prototype and a simple extension of the prototype to a plastic applicator system are considered. It is shown that the principal difference in dose is that the dose is reduced in the region inferior to the center of the ovoids. All configurations (plastic caps on or off) are equivalently shielded for the new device. In addition, an intermediate mini-ovoid configuration can be used clinically via the introduction of a D-shaped cap. The latter reduces the high dose to the vaginal mucosal surfaces. CONCLUSION: For a single ovoid, a comparison of dose with the FSD shows differences; however, the difference in dose is insignificant when the complete applicator, tandem, and ovoids are compared. With this new applicator, it is now possible to accumulate very accurate and detailed 3D dose-distribution data for the critical structures and other points of interest in the vicinity of the applicator. These data will permit future analysis of the correlation of dose and outcome for carcinoma of the cervix.

Brachytherapy↗

PubMed1997

The Compton backscattering process and radiotherapy.

Radiotherapy utilizes photons for treating cancer. Historically these photons have been produced by the bremsstrahlung process. In this paper we introduce Compton backscattering as an alternate method of photon production for cancer treatment. Compton backscattering is a well-established method to produce high-energy photons (gamma rays) for nuclear physics experiments. Compton backscattering involves the collision of a low-energy (eV) photon with a high-energy (hundreds of MeV) electron. It is shown that the photons scattered in the direction opposite to the direction of the initial photon (backscattered) will have the energy desired for photon beam therapy. The output of Compton backscattering is a high-energy photon beam (gamma-ray beam), which is well collimated and has minimal low-energy components. Such gamma beams may be used for conventional high-energy photon treatments, production of radionuclides, and generation of positrons and neutrons. The theoretical basis for this process is reviewed and Monte Carlo calculations of dose profiles for peak energies of 7, 15, and 30 MeV are presented. The potential advantages of the Compton process and its future role in radiotherapy will be discussed.

Biophysical Phenomena↗

PubMed1996

High-dose, hyperfractionated, accelerated radiotherapy using a concurrent boost for the treatment of nonsmall cell lung cancer: unusual toxicity and promising early results.

PURPOSE: The treatment of nonsmall cell lung cancer (NSCLC) with conventional radiotherapy (RT) results in inadequate local tumor control and survival. We report results of a Phase II trial designed to treat patients with a significantly increased total dose administered in a reduced overall treatment time using a hyperfractionated, accelerated treatment schedule with a concurrent boost technique. METHODS AND MATERIALS: A total of 49 patients with unresectable Stage IIIA/IIIB (38 patients) or medically inoperable Stage I/II (11 patients) NSCLC were prospectively enrolled in this protocol. Radiation therapy was administered twice daily, 5 days/week with > 6 h between each treatment. The primary tumor and adjacent enlarged lymph nodes were treated to a total dose of 73.6 Gy in 46 fractions of 1.6 Gy each. Using a concurrent boost technique, electively irradiated nodal regions were simultaneously treated with a dose of 1.25 Gy/fraction for the first 36 fractions to a total dose of 45 Gy. RESULTS: Median survival for the entire group of 49 patients is 15.3 months. Actuarial survival at 2 years is 46%: 60% for 11 Stage I/II patients, 55% for 21 Stage IIIA patients, and 26% for 17 Stage IIIB patients. The actuarial rate of freedom from local progression at 2 years is 64% for the entire group of 49 patients: 62% for Stage I/II patients, 70% for Stage IIIA patients, and 55% for Stage IIIB patients. Patients who underwent serial bronchoscopic reevaluation (4 Stage I/II, 8 Stage IIIA, and 6 Stage IIIB) have an actuarial rate of local control of 71% at 2 years. The median total treatment time was 32 days. Nine of 49 patients (18%) experienced Grade III acute esophageal toxicity. The 2-year actuarial risk of Grade III or greater late toxicity is 30%. The 2-year actuarial rate of severe-late pulmonary and skin-subcutaneous toxicity is 20% and 15%, respectively. CONCLUSION: This treatment regimen administers a substantially higher biologically effective dose compared with conventional and pure hyperfractionation treatment schedules. The overall rate of acute and late toxicity was acceptable. Preliminary rates of overall survival and local control and freedom from local progression compare favorably to results reported with pure hyperfractionated radiotherapy and chemoradiotherapy.

Aged↗

PubMed1994

Clinical use of a concomitant boost technique using a gypsum compensator.

PURPOSE: To develop a clinical procedure to treat field within a field (concomitant boost) portals with a single compensated field. METHODS AND MATERIALS: An ordinary manual cerrobend block former was used to produce styrofoam molds from simulator film data. A special gypsum compound was poured into the molds. The compensator block is independently mounted to the treatment machine via a custom-made compensator holder. RESULTS: Measurements confirm that the inhomogeneous dose distribution has been reliably delivered via this technique. The accuracy of placement of the high dose region is sufficient for clinical use. CONCLUSION: The procedure enables the concomitant boost effect to be easily implemented in the clinic without increasing clinical setup time.

Calcium Sulfate↗

PubMed1994

Characterization of gypsum attenuators for radiotherapy dose modification.

Gypsum has significant value as a compensator material for use with high energy x rays in radiation therapy. Compensator thickness may be calculated by using an effective attenuation coefficient (mu eff). Detailed measurements using narrow and broad beam geometry were collected to determine this effective attenuation coefficient as a function of energy (4-15 MV), field size, depth in tissue, and thickness of the compensator. An effective attenuation coefficient relation was defined using a least-square method. It was then determined that extrapolating the broad beam data to a 0 x 0 cm2 field size resulted in a good approximation to the measured narrow beam attenuation coefficient. The variations in surface dose produced by gypsum attenuators were compared to open beam results. For the energies studied, it was determined that the increase in surface dose was acceptable for clinical application.

Biophysical Phenomena↗

PubMed1993

3-dimensional optimization of multiple arcs for stereotactic radiosurgery.

PURPOSE: During linear accelerator-based radiosurgery, the physicians and physicists need to determine which combination of treatment arcs are "best" with regard to target coverage and incidental dose to adjacent structures. This is a complex problem, especially when targets are geometrically close to critical structures. The purpose of this article is to present a method to mathematically determine a set of arcs to produce desired target and normal structure dose distributions in linear accelerator radiosurgery. METHODS AND MATERIALS: Nonlinear least squares regression was used to determine the table angles and gantry angle arc ranges and their associated beam weights appropriate to linear accelerator radiosurgery. RESULTS: Three cases are presented: (a) critical structure close to target volume; (b) target volume too large for the largest collimator to cover the volume with one isocenter and a standard plan; (c) target volume located within one critical structure and close to another critical structure. The optimized treatment plans are all shown to be superior to a defined standard plan. CONCLUSION: The method successfully enables one to determine nonstandard arcs which achieve the desired results. In particular, the method enables one to find clinical treatment solutions, even when the desired results cannot be a priori defined.

Humans↗

PubMed1993

Enamel defects in 4- to 5-year-old children in fluoridated and non-fluoridated parts of Cheshire, UK.

The aim of this study was to compare the prevalence of developmental defects of enamel in the deciduous dentition of 4- to 5-year-old children residing in fluoridated (1 ppmF) and non-fluoridated (less than 0.2 ppmF) communities in Cheshire, UK. The significant difference in the prevalence of developmental defects of enamel between the two communities was accounted for by the higher prevalence of diffuse opacities in fluoridated Nantwich (29%), than in non-fluoridated Northwich (14%). The results also showed that when controlling for the age at which parents claimed toothbrushing commenced, the children in fluoridated Nantwich still had significantly more diffuse defects than the children in Northwich.

Child, Preschool↗

PubMed1991

Effect of backscatter on cell survival for a clinical electron beam.

Relative to homogeneous scattering conditions in tissue-like materials, a large increase in dose for clinical electron beams can occur upstream from high atomic number heterogeneities due to backscattered electrons. The degree of this dose increase is uncertain due to the unknown energy distribution of the backscattered electron fluence. Cell survival after irradiation was studied for Chinese hamster cells at the depth of maximum dose in a clinical 6 MeV electron beam under normal scattering conditions and with the addition of a lead backscatter. No significant difference in cell survival was found between the two geometries when the dose increase due to the backscattered electron fluence was approximated by the product of the measured ionization and the normal scattering stopping power ratio.

Animals↗

PubMed1991

Design of a plastic minicolpostat applicator with shields.

A plastic intracavitary applicator system for the treatment of cancer of the uterine cervix is described. This applicator has a minicolpostat and a mechanism for affixing the tandem to the colpostats. Traditional afterloading refers only to the radioactive source. Both the source and the ovoid shield are afterloaded together in this applicator in contrast to traditional afterloading systems which afterload the source alone. A potential advantage of our applicator system is that it allows high quality CT localization because the sources and shields can be removed and the applicator is made of plastic. The advantages and disadvantages of this variation to the Fletcher system as well as other aspects of applicator design are discussed. An experimentally verified dose calculation method for shielded sources is applied to the design problems associated with this applicator. The dose distribution calculated for a source-shield configuration of the plastic applicator is compared to that obtained with a commercial Fletcher-Suit-Delclos (FSD) applicator. Significant shielding improvements can be achieved for the smallest diameter ovoid, that is, in the minicolpostat. The plastic minicolpostat dose distributions are similar to those produced by the conventional larger diameter colpostats. In particular, the colpostat shielding for rectum and bladder, which is reduced in the metal applicator's minicolpostat configuration, is maintained for the plastic minicolpostat. Further, it is shown that, if desired, relative to the FSD minicolpostat, the mucosa dose can be reduced by a suitable change of the minicolpostat source position.

Brachytherapy↗

PubMed1991

A head immobilization system for radiation simulation, CT, MRI, and PET imaging.

An aquaplast mask/marker immobilization system for the routine radiation therapy treatment of head and neck disease is described. The system utilizes a commercially available thermoplastic mesh indexed and mounted to a rigid frame attached to the therapy couch. The apparatus is designed to permit CT, MRI, and PET diagnostic scans of the patient to be performed in the simulation and treatment position utilizing the same mask, thereby facilitating image correlation. Studies employing weekly simulation indicate that patient treatment position movement can be restricted to 3 mm over the course of treatment. This easily constructed system permits rapid mask formation to be performed on the treatment simulator, resulting in an immobilization device comparable to masks produced with vacuum-forming techniques. Details of construction, verification, and central axis CT, MRI, PET markers are offered.

Carboxymethylcellulose Sodium↗

PubMed1990

Desmond Greer Walker Award. Enamel mottling in a non-fluoride community since the advent of fluoride toothpastes.

In 1987 an assessment of mottling in incisor teeth was carried out on a random sample of 12-year-old children in Liverpool, England. The number of children examined was 471. All of these children were born and had lived in Liverpool when the fluoride content of the water supply had averaged 0.1 parts/10(6). During this time the market share of fluoride toothpastes had increased from 5% in the early 1970s to 91.8% in 1987. The mouth prevalence of mottling was found to be 31.1%. The tooth prevalence of mottling was found to be 7.83%. There was no sex difference. The data obtained was compared with findings in 1974-75 in Liverpool before the advent of fluoride toothpastes. The data suggests that there has been a decrease in the prevalence of mottling in Liverpool since 1974-75.

Chi-Square Distribution↗

PubMed1990

Dose calculation and measurements for a CT compatible version of the Fletcher applicator.

Experimental measurements in a water phantom were made for a new shielded plastic Fletcher applicator. A dose calculation algorithm which allows the description of asymmetrically shielded source containers is presented. The traditional primary plus build-up factor description of the dose in a homogeneous volume is corrected by using effective attenuation coefficients measured in a water phantom for all the source-shield-container materials. Comparison of the theoretical results to the experimental data shows excellent agreement.

Brachytherapy↗

PubMed1989

Comparison of automated and manual shielding block fabrication.

This work reports the results of a study comparing computer controlled and manual shielding block cutting. The general problems inherent in automated block cutting have been identified and minimized. A system whose accuracy is sufficient for clinical applications has been developed. The relative accuracy of our automated system versus experienced technician controlled cutting was investigated. In general, it is found that automated cutting is somewhat faster and more accurate than manual cutting for very large fields, but that the reverse is true for most smaller fields. The relative cost effectiveness of automated cutting is dependent on the percentage of computer designed blocks which are generated in the clinical setting. At the present time, the traditional manual method is still favored.

Humans↗

PubMedDate not supplied

Selenium-75: a potential source for use in high-activity brachytherapy irradiators.

A study was made to find a high-specific-activity radioisotope having a longer half-life and lower gamma-ray energies than 192Ir in order to replace 192Ir in high-activity afterloading brachytherapy devices. Based upon a number of considerations, 75Se was selected. The present paper describes the physical properties of 75Se, its method of production, and the properties of an encapsulated source that was fabricated to prove the accuracy of our initial calculations.

Biophysical Phenomena↗