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S L Meeks

Publications and source records attributed to S L Meeks.

32 records · Page 2Linked to original sources
PubMed1999

Analytic characterization of linear accelerator radiosurgery dose distributions for fast optimization.

Linear accelerator (linac) radiosurgery utilizes non-coplanar arc therapy delivered through circular collimators. Generally, spherically symmetric arc sets are used, resulting in nominally spherical dose distributions. Various treatment planning parameters may be manipulated to provide dose conformation to irregular lesions. Iterative manipulation of these variables can be a difficult and time-consuming task, because (a) understanding the effect of these parameters is complicated and (b) three-dimensional (3D) dose calculations are computationally expensive. This manipulation can be simplified, however, because the prescription isodose surface for all single isocentre distributions can be approximated by conic sections. In this study, the effects of treatment planning parameter manipulation on the dimensions of the treatment isodose surface were determined empirically. These dimensions were then fitted to analytic functions, assuming that the dose distributions were characterized as conic sections. These analytic functions allowed real-time approximation of the 3D isodose surface. Iterative plan optimization, either manual or automated, is achieved more efficiently using this real time approximation of the dose matrix. Subsequent to iterative plan optimization, the analytic function is related back to the appropriate plan parameters, and the dose distribution is determined using conventional dosimetry calculations. This provides a pseudo-inverse approach to radiosurgery optimization, based solely on geometric considerations.

Humans↗
PubMed1999

Dosimetric characteristics of a double-focused miniature multileaf collimator.

The dosimetric characteristics of a double-focused miniature multileaf collimator (mMLC) attached to a Philips SL75/5 linear accelerator (linac) have been investigated. Output factors, percentage depth-dose, penumbra, leaf transmission, and leakage between the leaves were measured for the 6 MV x-ray beam on this accelerator. Because leakage both through and between the leaves is minimal, the linac jaws can be kept fixed while the mMLC leaf configuration is modified for different aperture shapes. This allows for accurate output prediction using the equivalent square formalism. Percent depth-dose measured for fields defined by the mMLC show little deviation from the percent depth-dose measured for fields defined by the machine jaws or Lipowitz metal blocks. Because the mMLC matches beam divergence in both directions, allows minimal beam transmission, and has a large source-to-collimator distance, the penumbra is sharper for fields defined by the mMLC than for fields defined by the linac jaws or Lipowitz metal blocks. Based on these data, dose calculations for mMLC-defined fields can be applied with no change in procedures from those used for fields defined using conventional methods.

Radiotherapy Planning, Computer-Assisted↗
PubMed1998

Preliminary experience with frameless stereotactic radiotherapy.

PURPOSE: To report initial clinical experience with a novel high-precision stereotactic radiotherapy system. METHODS AND MATERIALS: Sixty patients ranging in age from 2 to 82 years received a total of 1426 treatments with the University of Florida frameless stereotactic radiotherapy system. Of the total, 39 (65%) were treated with stereotactic radiotherapy (SRT) alone, and 21 (35%) received SRT as a component of radiotherapy. Pathologic diagnoses included meningiomas (15 patients), low-grade astrocytomas (11 patients), germinomas (9 patients), and craniopharyngiomas (5 patients). The technique was used as means of dose escalation in 11 patients (18%) with aggressive tumors. Treatment reproducibility was measured by comparing bite plate positioning registered by infrared light-emitting diodes (IRLEDs) with the stereotactic radiosurgery reference system, and with measurements from each treatment arc for the 1426 daily treatments (5808 positions). We chose 0.3 mm vector translation error and 0.3 degrees rotation about each axis as the maximum tolerated misalignment before treating each arc. RESULTS: With a mean follow-up of 11 months, 3 patients had recurrence of malignant disease. Acute side effects were minimal. Of 11 patients with low grade astrocytomas, 4 (36%) had cerebral edema and increased enhancement on MR scans in the first year, and 2 required steroids. All had resolution and marked tumor involution on follow-up imaging. Bite plate reproducibility was as follows. Translational errors: anterior-posterior, 0.01 +/- 0.10; lateral, 0.02 +/- 0.07; axial, 0.01 +/- 0.10. Rotational errors (degrees): anterior-posterior, 0.00 +/- 0.03; lateral, 0.00 +/- 0.06; axial, 0.01 +/- 0.04. No patient treatment was delivered beyond the maximum tolerated misalignment. Daily treatment was delivered in approximately 15 min per patient. CONCLUSION: Our initial experience with stereotactic radiotherapy using the infrared camera guidance system was good. Patient selection and treatment strategies are evolving rapidly. Treatment accuracy was the best reported, and the treatment approach was practical.

Adolescent↗
PubMed1998

IRLED-based patient localization for linac radiosurgery.

PURPOSE: Currently, precise stereotactic radiosurgery delivery is possible with the Gamma Knife or floor-stand linear accelerator (linac) systems. Couch-mounted linac radiosurgery systems, while less expensive and more flexible than other radiosurgery delivery systems, have not demonstrated a comparable level of precision. This article reports on the development and testing of an optically guided positioning system designed to improve the precision of patient localization in couch-mounted linac radiosurgery systems. METHODS AND MATERIALS: The optically guided positioning system relies on detection of infrared light-emitting diodes (IRLEDs) attached to a standard target positioner. The IRLEDs are monitored by a commercially available camera system that is interfaced to a personal computer. An IRLED reference is established at the center of stereotactic space, and the computer reports the current position of the IRLEDs relative to this reference position. Using this readout from the computer, the correct stereotactic coordinate can be set directly. RESULTS: Bench testing was performed to compare the accuracy of the optically guided system with that of a floor-stand system, that can be considered an absolute reference. This testing showed that coordinate localization using the IRLED system to track translations agreed with the absolute to within 0.1 +/- 0.1 mm. As rotations for noncoplanar couch angles were included, the inaccuracy was increased to 0.2 +/- 0.1 mm. CONCLUSIONS: IRLED technology improves the accuracy of patient localization relative to the linac isocenter in comparison with conventional couch-mounted systems. Further, the patient's position can be monitored in real time as the couch is rotated for all treatment angles. Thus, any errors introduced by couch inaccuracies can be detected and corrected.

Humans↗
PubMed1998

Treatment planning optimization for linear accelerator radiosurgery.

PURPOSE: Linear accelerator radiosurgery uses multiple arcs delivered through circular collimators to produce a nominally spherical dose distribution. Production of dose distributions that conform to irregular lesions or conformally avoid critical neural structures requires a detailed understanding of the available treatment planning parameters. METHODS AND MATERIALS: Treatment planning parameters that may be manipulated within a single isocenter to provide conformal avoidance and dose conformation to ellipsoidal lesions include differential arc weighting and gantry start/stop angles. More irregular lesions require the use of multiple isocenters. Iterative manipulation of treatment planning variables can be difficult and computationally expensive, especially if the effects of these manipulations are not well defined. Effects of treatment parameter manipulation are explained and illustrated. This is followed by description of the University of Florida Stereotactic Radiosurgery Treatment Planning Algorithm. This algorithm organizes the manipulations into a practical approach for radiosurgery treatment planning. RESULTS: Iterative treatment planning parameters may be efficiently manipulated to achieve optimal treatment plans by following the University of Florida Treatment Planning Algorithm. The ability to produce conformal stereotactic treatment plans using the algorithm is demonstrated for a variety of clinical presentations. CONCLUSION: The standard dose distribution produced in linear accelerator radiosurgery is spherical, but manipulation of available treatment planning parameters may result in optimal dose conformation. The University of Florida Treatment Planning Algorithm organizes available treatment parameters to efficiently produce conformal radiosurgery treatment plans.

Algorithms↗
PubMed1998

Potential clinical efficacy of intensity-modulated conformal therapy.

PURPOSE: The purpose of this study was to examine the potential benefit of using intensity-modulated conformal therapy for a variety of lesions currently treated with stereotactic radiosurgery or conventional radiotherapy. METHODS AND MATERIALS: Intensity-modulated conformal treatment plans were generated for small intracranial lesions, as well as head and neck, lung, breast, and prostate cases, using the Peacock Plan treatment-planning system (Nomos Corporation). For small intracranial lesions, intensity-modulated conformal treatment plans were compared with stereotactic radiosurgery treatment plans generated for patient treatment at the University of Florida Shands Cancer Center. For other sites (head and neck, lung, breast, and prostate), plans generated using the Peacock Plan were compared with conventional treatment plans, as well as beam's-eye-view conformal treatment plans. Plan comparisons were accomplished through conventional qualitative review of two-dimensional (2D) dose distributions in conjunction with quantitative techniques, such as dose-volume histograms, dosimetric statistics, normal tissue complication probabilities, tumor control probabilities, and objective numerical scoring. RESULTS: For small intracranial lesions, there is little difference between intensity-modulated conformal treatment planning and radiosurgery treatment planning in the conformation of high isodose lines with the target volume. However, stereotactic treatment planning provides a steeper dose gradient outside the target volume and, hence, a lower normal tissue toxicity index. For extracranial sites, objective numerical scores for beam's-eye-view and intensity-modulated conformal planning techniques are superior to scores for conventional treatment plans. The beam's-eye-view planning technique prevents geographic target misses and better excludes healthy tissues from the treatment portal. Compared with scores for the beam's-eye-view planning technique, scores for intensity-modulated conformal plans using the Peacock Plan were significantly better for the lung and head and neck cases studied, equivalent for prostate cases, and inferior for breast cases. CONCLUSION: Using the entire 3D data set to construct radiotherapy plans through virtual simulation is always advantageous, whether done for stereotactic radiosurgery, beam's-eye-view conformal therapy, or intensity-modulated conformal treatment. Intensity modulation of the photon beam further enhances treatment planning under specific conditions. In general, the intensity-modulated technique is advantageous for large, irregular targets with critical structures in close proximity. Intensity-modulated treatment planning does not appear advantageous for stereotactic radiosurgery or treatment of the intact breast.

Aged↗
PubMed1998

The radiobiology of radiosurgery and stereotactic radiotherapy.

Radiation therapy has evolved into a complex amalgamation of treatment techniques that differ significantly according to the way the radiation is delivered to the patient and coincidentally according to the biologic effects that are observed with each technique. Although there are concepts within radiobiology that unify the field, it is not apparent that the biologic effects with one methodology of treatment resemble those of another. Radiosurgery, although initially developed in the 1950s, has become more commonly used in recent years. This treatment involves high-dose, single-fraction treatments with sharp dose gradients to small volumes of tissue. This contrasts with conventional external-beam radiotherapy which involves small-dose, multiple-fraction, broad-dose-gradient treatment to relatively large volumes of tissue. Stereotactic radiotherapy generally delivers small-dose, multiple-fraction treatments to small or intermediate volumes of tissue with a sharp dose gradient compared with conventional external-beam treatment. A discussion of these technique differences with reference to the radiobiologic implications may help elucidate the potential utility of the techniques in clinical radiotherapy.

Brain Neoplasms↗
PubMed1998

Linac scalpel radiosurgery at the University of Florida.

Optimal implementation of stereotactic radiosurgery requires multidisciplinary input from neurosurgeons, radiation oncologists, and physicists. Clinical processes of most importance to the physics staff include stereotactic imaging, treatment planning, and radiation delivery. Careful attention to each of these details helps to ensure the quality of the overall process. Here we provide a practical review of the clinical processes involved in linac scalpel radiosurgery. The linac scalpel system is a linear-accelerator-based radiosurgery system that was developed at the University of Florida. It has been used at the University of Florida to treat more than 1000 patients since 1988. The aim of the linac scalpel system is to minimize all possible uncertainties in imaging and treatment delivery. Once these errors are minimized, truly conformal treatment plans can be generated and delivered with confidence, allowing clinicians to focus solely on the patient's problem. By following practical examples of this well established system, many pitfalls in the clinical process can be avoided.

Cerebral Angiography↗
PubMed1998

Optic-guided stereotactic radiotherapy.

Stereotactic localization provides a powerful referencing system for radiation planning and treatment delivery. Although the standard approach of applying a rigid reference frame has been accepted for single-fraction therapy, a new, noninvasive approach is needed for fractionated therapy. The use of optical tracking allows a patient's position to be monitored in real time with a truly touch-free system. Our optical system provides high precision and accuracy for repeat fixation and can be applied to a wide spectrum of patients. With this system, a stereotactic radiation treatment can be delivered in the same length of time as a routine fractionated radiation treatment.

Brain Neoplasms↗
PubMed1997

Radiotherapy for pediatric brain tumors.

Brain tumors in children are a diverse group of diseases that require multidisciplinary and subspecialty expertise. Radiation therapy is an established treatment cornerstone for these pediatric tumors. Basic concepts of radiation biology and physics provide a framework for understanding the ongoing evolution in radiation delivery techniques and current treatment paradigms. Standard techniques of pediatric central nervous system radiotherapy are included in this review, as well as newer techniques including conformal therapy, stereotactic radiosurgery, and fractionated stereotactic radiotherapy. Examples are provided to illustrate differences in treatment approaches. The appropriate application of each technique is discussed, and then outcomes and treatment sequelae are compared.

Brain Neoplasms↗
PubMed1996

Clinical implications of collimator exchange effect, relative collimator and phantom scatter.

It is well established that the beam fluence on the central axis for rectangular or blocked fields is not always the same as that of its collimator equivalent square field. Many centers still determine monitor unit settings using a single output factor based on the collimator equivalent square field size because they feel the small errors introduced by this approach do not justify the extra data and effort required to correctly determine the monitor unit settings. Analyzing clinical monitor unit determinations for 6 and 15 MV photon irradiation. it is shown that failure to account for these discrepancies in central axis beam fluence may introduce 2.5-3% error in clinical situations. While the error is generally small, it must be eliminated if we intend to reach the commonly stated goal of 5% overall uncertainty in dose determination.

Humans↗
PubMed1984

Subarachnoid and intracerebral hemorrhage: natural history, prognosis, and precursive factors in the Framingham Study.

No uniformly accepted hypothesis explains the genesis and rupture of intracranial aneurysms. We followed 5,184 men and women prospectively for 26 years; 36 cases of aneurysmal subarachnoid hemorrhage (SAH) accounted for 62% of all intracranial hemorrhages. Blood pressure before SAH was higher in these patients than in controls. Definite hypertension (greater than or equal to 160 mm Hg and/or greater than or equal to 95 mm Hg) at entry to the study or at closest exam before SAH was more frequent than in controls. Cigarette smoking, particularly heavy smoking, was also more frequent among cases.

Adult↗
PubMedDate not supplied

Duration of atrial fibrillation and imminence of stroke: the Framingham study.

The role of Atrial Fibrillation (AF) as a precursor of stroke was examined in the Framingham cohort based on 30 years of follow-up during which time 501 strokes occurred. There were 59 persons who sustained stroke in association with AF excluding those with rheumatic heart disease. AF increased the risk of stroke five-fold and the excess risk was found to be independent of the frequently associated cardiac failure and coronary heart disease. The contribution of AF to stroke risk was also at least as powerful as that of the other cardiovascular precursors. Stroke associated with AF was not only independent and substantial but also imminent. There was a distinct clustering of stroke events at the time of onset of the AF. Thirty day case-fatality rates were no different in those with strokes accompanied by AF than not at 17% versus 19% respectively. Recurrences in those with AF were only slightly more frequent, 25% versus 20%, a difference that was not statistically significant. Stroke recurrence in the first 6 months following initial stroke was more than twice as common (47% versus 20%) in the AF group.

Adult↗