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Biomedical subjects

D D Leavitt

Publications and source records attributed to D D Leavitt.

18 recordsLinked to original sources
PubMed1992

Computed tomography treatment planning in IR-192 brachytherapy in the head and neck.

Brachytherapy dose prescription and treatment planning lag behind the state-of-the-art for external beam therapy. As altered fractionation of external beam therapy improves patient outcome in head and neck cancer, there will be an increased need to compare the two radiotherapy techniques. Currently, implant techniques and dose prescription documentation are not uniform, dose prescription to a target volume is subjective, and implant quality is poorly understood and not routinely assessed. All contribute to a lack of scientifically rigorous brachytherapy clinical trials. Studies designed to combine tumor imaging and dosimetry data are important in the evolution of brachytherapy treatment planning. Head and neck implants, which often require nonparallel, arching, or looping source carriers for all but small tumors in order to encompass the target volume adequately, were used to evaluate the clinical utility and feasibility of computed tomography as a treatment planning tool in brachytherapy. Following placement of plastic afterloading tubes under general anesthesia, orthogonal radiographs with dummy sources in the afterloading tubes are obtained as customary for source localization. With the patient in the same position, axial CT scans are obtained with the dummy seeds still in place for treatment planning. The implant physician, using data from the pre-treatment diagnostic CT scan, outlines target areas on sequential images creating a 3-dimensional target volume. By superimposing anatomic data with isodose curves one can objectively define implant parameters important in clinical trials analysis. These include minimum target absorbed dose, implant uniformity, and treatment to target volume ratio. The results of the first 10 patients are presented and implications of these data regarding the analysis of implant technique, implant quality, and implant optimization are discussed. The technique as performed is laborious but practicable in the clinical research setting of head and neck implant. Further research efforts should improve, simplify, and objectify brachytherapy and hasten the time when rigorous multi-institutional brachytherapy trials will be reality.

Adult↗
PubMed1991

Dynamic field shaping to optimize stereotactic radiosurgery.

A dynamic field shaping collimation system is evaluated for use in stereotactic radiosurgery of non-spherical lesions. The concept is as follows: (a) use the existing circular collimators to define a cone which encompasses the maximum dimensions of the target volume; (b) position two sets of independent rectangular photon collimators immediately upstream from the circular aperture and allow each collimator to have independent translational and rotational motion in order to define, for each increment of arc, a polygonal field shape having up to four straight and four curved edges which enscribe the beam's eye projection of the target; (c) modify the translational and rotational position of each independent collimator with each change in arc angle to continuously shape the instantaneous field to the target shape. A prototype device has been constructed and uses vane control technology developed in a related research project in electron arc therapy. The efficacy of this device is illustrated by dose calculations and measurement based on actual clinical data. Dose volume histograms are used to compare the dose received by three techniques: single isocenter treatment using a single circular aperture, dual isocenter treatment, and single isocenter treatment using dynamically shaped fields. Doses were calculated throughout the brain using a volume grid of 3 mm spacing. Dose volume histograms comparing dose within the target volume and brain volume excluding target volume, as well as computed isodose distributions, demonstrate the possible reduction in normal tissue dose burden while simultaneously preserving dose uniformity throughout the prescribed target volume. This simple four-vane collimation system may provide a viable alternate treatment technique for non-spherical lesions.

Humans↗
PubMed1991

Dose volume histogram analysis of lung radiation from chest wall treatment: comparison of electron arc and tangential photon beam techniques.

The technique of electron arc irradiation of the post-mastectomy chest wall was developed to improve dose uniformity and to reduce lung irradiation in comparison to that seen with standard chest wall tangent photon beam methods. Because of the cephalocaudal variation in chest wall shape and thickness, electron arc treatment planning requires anatomical detail provided by multiple axial CT images of the thorax. To compare the fixed beam and rotational techniques, computer simulated beams covering the chest wall and internal mammary lymphatics were retrospectively applied to the CT-derived contours obtained during treatment planning for 12 consecutive patients receiving adjuvant chest wall treatment by electron arc. The lung dose distribution for each technique was calculated using heterogeneity corrections. The multiplanar 2-dimensional isodose distributions were summed to provide estimated 3-dimensional dose distributions of integral histograms. These reveal that for most of these patients a modest to large improvement in volume-dose relationship occurs with the electron arc technique.

Breast Neoplasms↗
PubMed1991

Graduated block technique for the treatment of paranasal sinus tumors.

Cancers of the head and neck often present difficult dosimetric challenges; tumors of the paranasal sinuses, often advanced at diagnosis, pose several problems in treatment planning. Adequate coverage of involved areas often necessitates inclusion of the ipsilateral orbit due to direct extension of disease; sparing the uninvolved contralateral orbit may be difficult, especially if the superior nasal cavity and ethmiod sinus must be treated. We will report on a technique that allows delivery of a relatively homogeneous dose to a treatment volume that includes the paranasal sinus and ipsilateral orbit, with significant sparing of the anterior chamber of the contralateral eye. This technique uses a heavily weighted anterior field designed to deliver 100% to a plane posterior to the lens of each eye. From this plane posteriorly, lateral wedged fields are employed to increase the dose as the anterior contribution decreases. To achieve maximum homogeneity would require a wedge angle of greater than 60 degrees, the maximum wedge commonly available. To overcome this restraint, this technique uses multiple lateral wedged fields whose anterior field edges graduate in a posterior direction allowing for further compensation of the anterior field's fall-off. Film densitometry using a Rando phantom* is used to verify the technique.

Brachytherapy↗
PubMed1991

Documentation of ovoid cap size.

The need for precise documentation in radiation oncology is paramount; assurance of the quality of therapy delivered is the responsibility of everyone engaged in the treatment of cancer patients. Although all aspects of quality assurance require meticulous attention to detail, the documentation of brachytherapy procedures, both written dose prescription and film verification, is particularly important as no single method of dose reporting is universally accepted and used. The size of ovoid caps used in brachytherapy applications for gynecologic malignancies cannot be verified on treatment planning films, as the caps are not radio-opaque. If the cap size is improperly reported by the physician, resulting surface dose calculations used to determine source loading and implant duration could cause over- or underdosing. In an effort to improve and refine gynecologic brachytherapy record-keeping, we have devised a method to verify and document cap size. Narrow stainless steel bands have been embedded into grooves cut circumferentially on the surface of the ovoid caps. These bands are readily visible on localization films, producing documentation of the cap diameter, reducing the risk of error in dose reporting, and providing a permanent record of ovoid size. We will review the nature of possible dose errors affecting clinical decisions.

Brachytherapy↗
PubMed1991

Analysis of primate head irradiation with 55-MeV protons.

The distribution of the dose to the head of a primate phantom due to 55-MeV proton irradiation was calculated using a clinical radiotherapy treatment planning system, with anatomic definition through computerized tomography scans. Dose profiles, isodose distributions, and differential and integral dose-volume histograms are used to describe the probable proton dose to the brain of rhesus monkeys, irradiated over two decades ago, in which brain tumors have now developed. The dose analysis shows that 59% of the brain received a dose in excess of the reference surface dose, and that portions of the brain received doses greater than 300% of the reference surface dose. The regions of high dose are illustrated in isodose distributions. This information may be useful in evaluating potential tumor induction following radiation exposure.

Animals↗
PubMed1990

Improved dose homogeneity in electron arc therapy achieved by a multiple-energy technique.

Improved dose homogeneity throughout the treatment volume defined for electron arc therapy is achieved through superposition of multiple arcs of different electron energy to the same treatment surface. The relative weights for each arc segment and energy are determined by computer optimization which minimizes the variation in radial depth dose across the treatment volume. In addition to the standard electron beam energies of 6 MeV, 9 MeV, 12 MeV, 16 MeV and 20 MeV, a new electron arc field is created by adding bolus to the treatment surface during an additional pass using 6 MeV electrons. This modified field, having maximum dose on the patient surface and a reduced range, supplements the dose delivered by the standard electron arc fields in the buildup region. Through use of this multiple-energy technique, depending on clinical indications, electron arc therapy can be planned and delivered to allow either skin sparing or a uniform dose from the patient's surface to the desired treatment depth.

Electrons↗
PubMed1990

Dynamic beam shaping.

Computer control of independent collimator jaw positions and dose is combined with multiple-field summation techniques to design optimized radiation field profiles. Clinically relevant examples are shown for dynamic wedges and compensated mantle fields. Calculation, measurement, and verification techniques are discussed.

Humans↗
PubMed1990

Design and production of customized field shaping devices for electron arc therapy.

A key element in the implementation of electron arc therapy is the use of customized field shaping devices on or near the patient's surface to protect normal tissue surrounding the treatment surface. Techniques for design and production of field shaping devices have evolved to meet the requirements of improved efficiency, patient comfort and protection, and reproducibility of patient set-up. Techniques in current use at the University of Utah are described and illustrated.

Breast Neoplasms↗
PubMed1989

Electron arc therapy: design, implementation and evaluation of a dynamic multi-vane collimator system.

Innovative techniques in motion control technology have been applied to the design and implementation of a portable computer-controlled multi-vane collimator for use in electron arc therapy. The collimator, consisting of 18 independently controlled vanes, is inserted into the standard accessory mount assembly of a linear accelerator, in the same fashion as standard field shaping blocks. Power is supplied to the collimator vane motors via a self-contained battery system. The range of motion of the vanes, symmetrically mounted nine on each side, provides a variable aperture width projected to isocenter of 2 cm minimum to 8 cm maximum. The projected length of the aperture at isocenter is 38 cm. The transition time between vane positions is less than 1 second, corresponding to gantry movement of less than 1 degree. The movement of each of the 18 vanes is monitored and controlled by six individually addressed three axis processors that are shielded from the electron beam. A table of collimator vane positions versus gantry angle, as determined by dose optimization calculations, is stored in a data file. The desired collimator vane position corresponding to the current arc segment is conveyed from the control console to each vane controller via packets within a token passing network. Communication between the computer in the console area and the vane controllers is accomplished through encoded infra-red pulse transmission, eliminating the need for additional communication lines between the console and the accelerator. This dynamic collimator offers improved dose uniformity while simplifying the delivery of electron arc therapy.

Electronics, Medical↗
PubMed1989

Optimization of electron arc therapy doses by multi-vane collimator control.

Retrospective computer simulations, based on clinical treatment planning data available from over 50 patients treated by electron arc radiotherapy to the chestwall following mastectomy, show that a dramatic improvement in dose uniformity can, in many clinical situations, be achieved by dynamic shaping of the electron arc collimator, under computer control, as a function of gantry angle and distance superior or inferior to the central plane. The greatest improvement in dose uniformity is seen in calculational planes in which the patient contour has the greatest departure from a circular shape. Dosimetric studies demonstrate this improvement. Indicators for use of variable-width multi-vane electron arc collimators include the following: (1) Mechanical constraints of the therapy equipment may limit the placement of isocenter to an inadequate depth which causes large variation in the SSD around the arc; (2) Out of the central plane, the shape of the chest wall may change dramatically across the limits of the arc, creating large variations in the dose distribution; (3) Clinical definition of the treatment surface to include surgical scars or other at-risk volume may create an irregularly shaped treatment surface, thereby changing the fraction of the arc included in the treatment surface from one plane to the next. Electron arc collimator shape determines both the dose rate and the electron arc beam profile. Both the dose rate and the beam profile must be included in the integration of dose to a point within the arc. The dose to a point within the arc can be modified by as much as a factor of 1.5 to 2.0 by increasing the collimator width from 3 cm to 7 cm. A multi-vane collimator allows these changes to be made in each specific plane to compensate for changes in patient contour.

Breast Neoplasms↗
PubMed1975

Optimization of dose distributions in moving-strip therapy using a minicomputer.

Optimized 60-Co dose distributions for the moving-strip technique were calculated for 3 patients using a PC-12 minicomputer program which corrects for field obliquity and changes in patient thickness. Beam profiles were measured using an ionization chamber in a water phantom. A Masonite phantom was constructed to simulate a patient and used to measure optimized and unoptimized midplane dose distributions by thermoluminescent dosimetry. Measured midplane doses agreed with computer-calculated doses within experimental error. The computer optimization technique improved dose uniformity, reducing the midplane dose variation from plus or minus 12-13% to plus or minus 3-4%.

Cobalt Radioisotopes↗
PubMedDate not supplied

Dosimetry measurements and analyses of 10-MV x rays for Clinac-18.

Radiation parameters for two Varian Clinac-18 10-MV x-ray units were measured and compared with the same parameters for the Toshiba LMR-13 and the Arco Mevatron XII. Comparison of the percentage depth dose, surface dose, and depth of maximum ionization as a function of field size are presented.

Radiotherapy↗
PubMedDate not supplied

Comparison of moving-strip therapy using a Cobalt-60 teletherapy unit, a Varian 4-MV linear accelerator, and a Varian 10-MV linear accelerator.

Cobalt-60 gamma rays and 4- and 10-MV x rays are compared for moving-strip therapy in terms of the dose uniformity and the given dose needed to deliver a prescribed tumor dose. Dose distributions in phantoms of 14--32-cm thickness were calculated for each therapy unit. Individual given doses which would deliver the most uniform dose along the midline of the treatment volume were determined by computer and were verified experimentally by thermoluminescent dosimetry. Using computer optimization techniques, the midplane dose uniformity is improved significantly for the three therapy units considered.

Cobalt Radioisotopes↗
PubMedDate not supplied

Dynamic wedge field techniques through computer-controlled collimator motion and dose delivery.

Clinical treatment planning situations arise which require different wedge angles within segments of a single therapeutic x-ray field. Idealized wedge-shaped dose distributions, including combination of several wedge segments of different angle within a single field, are generated and delivered through computer control of asymmetric collimator motion and dose per field segment. A dual-pass technique is introduced to provide improved adherence to the prescribed isodose distribution. Dynamic wedge distributions are verified by film densitometry and ionization chamber measurement. These results suggest the potential importance of this technique as an added clinical radiotherapy tool.

Humans↗