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D D Leavitt

Publications and source records attributed to D D Leavitt.

50 records · Page 3Linked to original sources
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↗
PubMed1988

Electron arc therapy: chest wall irradiation of breast cancer patients.

From 1980 to October 1985 we treated 45 breast cancer patients with electron arc therapy. This technique was used in situations where optimal treatment with fixed photon or electron beams was technically difficult: long scars, recurrent tumor extending across midline or to the posterior thorax, or marked variation in depth of target tissue. Forty-four patients were treated following mastectomy: 35 electively because of high risk of local failure, and 9 following local recurrence. One patient with advanced local regional disease was treated primarily. The target volume boundaries on the chest wall were defined by a foam lined cerrobend cast which rested on the patient during treatment, functioning as a tertiary collimator. A variable width secondary collimator was used to account for changes in the radius of the thorax from superior to inferior border. All patients had computerized tomography performed to determine Internal Mammary Chain depth and chest wall thickness. Electron energies were selected based on these thicknesses and often variable energies over different segments of the arc were used. The chest wall and regional node areas were irradiated to 45 Gy-50 Gy in 5-6 weeks by this technique. The supraclavicular and upper axillary nodes were treated by a direct anterior photon field abutted to the superior edge of the electron arc field. Follow-up is from 10-73 months with a median of 50 months. No major complications were observed. Acute and late effects and local control are comparable to standard chest wall irradiation. The disadvantages of this technique are that the preparation of the tertiary field defining cast and CT treatment planning are labor intensive and expensive. The advantage is that for specific clinical situations large areas of chest wall with marked topographical variation can be optimally, homogeneously irradiated while sparing normal uninvolved tissues.

Adult↗
PubMed1985

Electron arc therapy: physical measurement and treatment planning techniques.

An electron beam arc therapy technique has been developed for the treatment of the post-mastectomy chest wall using a clinical linear accelerator modified for arc therapy. The effects on the dose distribution of primary X ray collimators, secondary cerrobend blocks attached to the accelerator accessory tray, and tertiary cerrobend casting of the treatment area on the patient's thorax have been investigated. Multiple electron energies within the same arc, variable rad per degree, and variable shaped secondary and tertiary applicators have been employed to optimize dose uniformity across the treated surface. A computerized treatment planning program has been developed to aid in visualization and optimization of dose distributions. A simple technique to estimate the width variation in the secondary collimator necessary to compensate for radial patient thickness changes in the cephalocaudad direction is described. Electron beam energies of 6 MeV, 9 MeV, 12 MeV, 15 MeV, and 18 MeV have been studied. The physical measurements needed to implement this technique are described, and a comparison of electron arc therapy dose distributions with other standard treatment techniques is presented.

Breast Neoplasms↗
PubMed1984

Electron arc therapy: clinical experience with chest wall irradiation.

In order to overcome the technical problems commonly encountered in fixed field photon and electron beam chest wall irradiation, we have treated the chest wall in 22 breast cancer patients with a moving electron beam that rotates about the patient's thorax. This paper discusses the clinical results of chest wall treatment by electron arc therapy. Twenty-one patients were treated following mastectomy, 16 electively because of high risk for local failure, and 5 because of local recurrence. One patient with advanced local-regional disease was treated primarily. During a median follow-up period of 24 months there has been one chest failure in one of the patients treated for local recurrence. No major complications were observed in skin, lung, soft tissue or esophagus, even in the 7 patients recently or concomitantly treated with multiagent cytotoxic chemotherapy. In 3 patients, small areas of telangiectasia developed in the region of abutment of the electron arc field to a photon field used to irradiate the supraclavicular nodal area. We conclude that the technique as applied to these patients is safe and efficacious. In certain clinical settings it has advantage over standard, fixed field approaches to treat the chest wall.

Breast Neoplasms↗
PubMed1980

Direct sagittal computed tomographic scans in the radiographic evaluation of the pelvis.

The authors describe a method for obtaining direct sagittal and coronal computed tomographic (CT) scans of the pelvis using a special accessory table adapted for use in wide-aperture whole-body scanners. In these scans the spatial and density resolution are nearly equivalent to those in axial scans. A case is described in which sagittal scanning helped define a pelvic mass.

Adult↗
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↗