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D D Leavitt

Publications and source records attributed to D D Leavitt.

At least 37 records · Page 2Linked to original sources
PubMed1996

Multi-fractionated stereotactic radiotherapy.

Precision and accuracy of a patient's treatment are key advantages of single-fraction stereotactic radiosurgery (SRS) for arteriovenous malformations (AVMS) and some small brain metastases. These advantages are equally valuable in fractionated treatment of the pituitary, brain metastases and brain boost fields. The need to implement the preciseness from stereotactic radiosurgery to fractionated treatments was recognized. Using our experience with single-fraction stereotactic radiosurgery as a model, we developed a multi-fractionated stereotactic radiotherapy technique that allows us to immobilize a patient daily and implement important existing devices such as the Brown-Roberts-Wells (BRW) angiographic localizer, CT scan localizer, and non-coplanar shaped field treatment planning. Development of this technique also allows us to achieve reproducible patient positioning based on immobilization techniques using polyurethane foam immobilization and heat moldable plastic technology without the necessity of the invasive technique of skull fixation. The development, implementation and dosimetry of this technique will be discussed in this paper.

Humans↗
PubMed1996

Design and production of wax compensators for electron treatments of the chest wall.

The primary aim of electron treatment planning for the post mastectomy chest wall is to encompass the volume between the skin surface and the lung-rib interface while limiting dose to the lung. Electron energies for treatment of the chest wall are chosen based on the thickness of tissue between these two areas. Surgical defects or surface irregularities often result in differing thicknesses of tissue across the treatment volume, and patient-specific compensation is necessary to achieve the desired dose distribution. This is true whether the treatment plan is designed using fixed or rotational electrons to treat the chest wall. These clinical requirements are often met using custom shaped wax of varying thickness which conforms to the chest surface. This paper will discuss the treatment planning process used to design these compensators, creation and use of an exact duplicate of the patient's chest wall to aid in the production of these compensators, the production process itself, and verification of the completed compensator.

Breast Neoplasms↗
PubMed1995

Dosimetry and treatment planning of complex electron arc therapy.

Electron arc therapy has been primarily utilized for treatment of the post-mastectomy chest wall. Its clinical usefulness and benefits have been proven. Casting procedures and blocking techniques have been discussed. We have successfully applied electron arc therapy to the treatment of the nasal cavity and associated nodal regions. When compared to the chest wall, the smaller radii and pronounced irregularity of the surface anatomy of the facial region presents multiple new dosimetric and treatment planning challenges. We will discuss these challenges, along with those encountered in the casting procedures and reproduction of patient characteristics required for the execution of this treatment. Many aspects of dosimetry were utilized in unique ways to produce the treatment plans, cast and bolus needed. Due to the insertion of bolus into the nostrils, a unique hollow bite block was constructed to allow patient respiration during treatment. Film dosimetry was used to verify computer predictions and to compare this electron arc treatment technique with alternative fixed electron beam techniques. The potential benefits and difficulties of this technique will be discussed.

Carcinoma, Squamous Cell↗
PubMed1995

A method for total skin electron treatment for infants.

Diseases such as mycosis fungoides require the treatment of a patient's total skin surface with superficial radiation. In a unique clinical situation, a 14-month-old child presented with a need for total skin treatment. A typical total skin technique requires overlapping electron beams, using 6 body positions, each with the gantry rotated for 2 angulations, or '6 positions-12 fields'. Adaptation of this technique for infants is complicated by the small diameter of some body parts, and by the necessity to treat while the patient is anesthetized. Even degraded, low energy electrons can easily penetrate fingers and toes. Therefore, dose from 6 positions becomes additive, and the total dose to small circumferences can be 3 to 4 times more than skin dose on the torso, raising concerns about uneven bone growth in the developing child. Special phantoms were designed for extensive dosimetry needed to determine both dose rate and dose summation from the overlapping beams. Computerized electron pencil beam calculations were compared to TLD measurements. Unique compensating techniques were used to deliver uniform dose. A modification of the 6 position-12 field technique will be described; and accessories used to reduce high dose regions will be illustrated.

Bone Development↗
PubMed1994

Improved field edge definition in electron arc therapy with dynamic collimation techniques.

PURPOSE: The diffuse shape of the electron beam used for arc therapy requires collimation on the patient's surface to sharply define treatment field edges. The electron beam arcs 10-15 degrees past field edges defined by a custom fitted and manufactured cast which acts as a tertiary collimator. This allows the entire beam profile to be integrated at the field edge. The tertiary collimator is heavy and bulky, requiring two therapists to lift and position the cast. An alternative technique for field edge definition would require the electron arc collimators to dynamically close to zero, while maintaining the projection of the leading edge of the field coincident with the geometric edge of the treatment field. This would allow integration of the entire electron arc profile and maintain a sharply defined treatment edge at the medial and lateral margins of the arc. The present customized cast could be replaced by generic lead strips at only the superior and inferior treatment field borders. This study investigates the dosimetry of dynamically collimated electron arc treatment volumes at field margins and its potential for eliminating the need for tertiary collimation at the arc field margins. METHODS AND MATERIALS: Electron arc isodose distributions were calculated using a pencil beam algorithm for treatment volumes defined by tertiary collimation at the surface of a cylindrical phantom and compared to distributions generated by simulating dynamic collimation to define the same field edges. Phantom measurements were performed using film densitometry to verify computer predictions. RESULTS: Penumbra width is one measure of the sharpness of dose fall off at a treatment field edge. We define it as the distance between the 90% and 20% isodose lines at the field edge measured orthogonal to the incident electron beam. Calculations and phantom film densitometry measurements were performed for electron energies from 6-20 MeV. Dynamic and tertiary collimation both reduce penumbra width by approximately 50% compared to no collimation. There is a small advantage in minimizing penumbra width at low electron energy with tertiary collimation. This shifts to a small advantage with dynamic collimation at high electron energy. CONCLUSION: Dynamic collimation produces a field edge isodose distribution equivalent to tertiary collimation for clinical purposes. These results suggest that tertiary collimation at medial and lateral electron are treatment field margins can be eliminated with dynamic collimation. This should result in greater clinical acceptance of breast electron arc therapy as the capacity for dynamic collimation is added to the next generation of linear accelerators.

Breast Neoplasms↗
PubMed1994

Electron arc therapy of the postmastectomy prosthetic breast.

PURPOSE: Reconstructive surgery of the postmastectomy breast presents new challenges to postoperative radiation therapy. This paper evaluates the dosimetric significance of oblique incidence of the electron field on the reconstructed mound during arc therapy. METHODS AND MATERIALS: Using film densitometry, the relative dose distributions resulting from electron arc therapy incident on phantoms simulating the breast and chest wall dimensions of an actual patient are evaluated. RESULTS: Irradiation of the breast phantom in normal supine position results in constriction of the dose distribution at the junction of the reconstructed mound with the chest wall. Angulation of the phantom to provide a more normal incidence of the electron beam during arc reduces the constriction by minimizing obliquity of the incident electrons. CONCLUSION: These measurements suggest that, with proper positioning of the patient relative to the incident electron beam, electron arc therapy may be used as an alternative treatment technique for treatment of the postmastectomy reconstructed breast and chest wall.

Breast Neoplasms↗
PubMed1994

Experimental simulation of proton space radiation environments: a dosimetric perspective.

Three-dimensional dose calculation techniques developed for radiotherapy treatment planning were used to calculate dose distributions from unidirectional, planar rotational and omnidirectional incident radiation (experimental proton beams and solar flares). The calculations predicted regions of high dose within primate heads exposed to 55-MeV protons, supporting the postulate of radiation-induced brain tumors within this population. Comparisons among predicted doses to the human head from solar flares of three different energies demonstrated differences between unidirectional and omnidirectional irradiation in the space environment. The results can be used to estimate dose distributions based on a) limited phantom measurements, or b) nonuniformly incident radiation in orbit; both situations are difficult to replicate under laboratory exposure conditions.

Animals↗
PubMed1993

Measurement of mechanical accuracy of isocenter in conventional linear-accelerator-based radiosurgery.

PURPOSE: Five Varian linear accelerators were studied to determine whether their mechanical isocentric accuracies were sufficient for radiosurgery and, if not, if the observed errors were sufficiently consistent and predictable to be correctable by some form of secondary collimator steering device to maintain isocentric alignment. METHODS AND MATERIALS: A 0.3 mW 670 nm diode laser was mounted in the secondary collimator insert of a radiosurgery extended collimator assembly. A cylindrical lens was used to create a laser fan beam that passed through isocenter and could be oriented parallel or perpendicular to the plane of rotation. A position sensitive photo-diode having an electrical output that varied with the portion of its surface illuminated was mounted at isocenter in a rotational mount. This mount tracked the accelerator gantry such that the surface of the photo-diode remained perpendicular to the laser beam during gantry rotation. An X/Y recorder was connected to the gantry-angle potentiometer of the accelerator and to the photo-diode and plotted the positional variation from isocenter with gantry rotation. RESULTS: The root-mean-square error for the five machines was +/- 0.06 to +/- 0.08 mm in the plane of rotation and +/- 0.17 to +/- 0.35 mm out of (perpendicular to) the plane of rotation. The in-plane-of-rotation errors tended to be maximal near the diagonal gantry angles and the out-of-plane-of-rotation errors were maximal in the over and under vertical positions. CONCLUSIONS: Both types of errors were predictable but only the out-of-plane-of-rotation errors were considered large enough to warrant consideration of correction (although the need is debatable). On all the tested machines, the out-of-plane-of-rotation error curve was a relatively smooth bell-shaped function that would be readily amenable to correction. The diode laser/photo-detector system used should prove useful in accurately defining isocenter and facilitating the precise adjustment of the laser isocenter lights.

Humans↗
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↗