Rectangular edge synchronization for intensity modulated radiation therapy with dynamic multileaf collimation

The implementation of intensity modulated radiotherapy by dynamic multileaf collimator control involves the use of interpreter software which creates leaf trajectory plans for each leaf pair. Interpreter software for use with an Elekta SL15 linear accelerator and dedicated multileaf collimator has been written and tested. In practice the ideal trajectory plans often predict contact between leaves from opposing leaf banks, but this is prohibited by control software on the Elekta system as it could lead to mechanical damage. If the modulation within the geometric limits of a shaped field is not to be compromised then strategies to avoid leaf contact result in additional unwanted doses outside the geometric edge. The magnitude of any such additional dose can be reduced to acceptable levels by a technique which we have called rectangular edge synchronization. The performance of interpreter software which incorporates rectangular edge synchronization has been compared with that of potentially more efficient software which does not. The option containing the rectangular edge synchronization algorithm was shown to work consistently well at high monitor unit rates, and without incurring leaf contacts, under a wide range of test conditions. It therefore provides a sound basis for using intensity modulation to replace mechanical wedges, to simulate customized patient shape compensators, or to implement the results of inverse treatment planning processes that require superimposed intensity modulated beams.     

Generation of discrete beam-intensity modulation by dynamic multileaf collimation under minimum leaf separation constraints

An algorithm to generate discrete beam-intensity modulation by dynamic multileaf collimation is presented which incorporates constraints on minimum allowed leaf separations. MLC positioning information is derived simultaneously for all leaf pairs and back-up diaphragms as they progress across the field. A feedback mechanism allows corrections to be applied to eliminate potential violations of minimum separation conditions and any underexposure in the interleaf tongue-and-groove region as they are encountered. The resulting motion correctly delivers the intended modulation and is physically realizable. Implementation of the algorithm is described. Results of the algorithm can also alternatively be interpreted as defining a series of static fields to deliver the same modulation.     

Physical and dosimetric aspects of a multileaf collimation system used in the dynamic mode for implementing intensity modulated radiotherapy

The use of a multileaf collimator in the dynamic mode to perform intensity modulated radiotherapy became a reality at our institution in 1995. Unlike treatment with static fields using a multileaf collimator, there are significant dosimetric issues which must be assessed before dynamic therapy can be implemented. We have performed a series of calculations and measurements to quantify head scatter for small fields, collimator transmission, and the transmission through rounded leaf ends. If not accounted for, these factors affect the delivered dose to the prostate by 5%-20% for a typical plan. Data obtained with ion chambers and radiographic film are presented for both 6 and 15 MV x-ray beams. The impact on the delivered dose of the mechanical accuracy of the multileaf collimator, achieved during leaf position calibration and maintained during dose delivery, is also discussed.     

Accurate portal dose measurement with a fluoroscopic electronic portal imaging device (EPID) for open and wedged beams and dynamic multileaf collimation

Measuring portal dose with an electronic portal imaging device (EPID) in external beam radiotherapy can be used to perform routine dosimetric quality control checks on linear accelerators and to verify treatments (in vivo dosimetry). An accurate method to measure portal dose images (PDIs) with a commercially available fluoroscopic EPID has been developed. The method accounts for (i) the optical 'cross talk' within the EPID structure, (ii) the spatially nonuniform EPID response and (iii) the nonlinearity of the EPID response. The method is based on a deconvolution algorithm. Measurement of the required input data is straightforward. The observed nonlinearity of the EPID response was largely due to the somewhat outdated EPID electronics. Nonlinearity corrections for more modern systems are expected to be smaller. The accuracy of the method was assessed by comparing PDIs measured with the EPID with PDIs measured with a scanning ionization chamber in a miniphantom, located at the same position as the fluorescent screen. For irradiations in open, wedged and intensity modulated 25 MV photon beams (produced with dynamic multileaf collimation) EPID and ionization chamber measurements agreed to within 1% (1 SD).     

Technical report: an alternative mechanical technique of pseudoaneurysm compression therapy

Graded manual compression therapy under ultrasound guidance has become the standard first line treatment of post-catheterization femoral pseudoaneurysms. Although effective, this treatment is often poorly tolerated by both patient and operator. We describe a new mechanical technique which has proven successful in our department, and is well tolerated by both patients and staff.     

Hemispherical deafferentation: an alternative to functional hemispherectomy

An alternative technique for so-called functional hemispherectomy has been developed to be used for the classical indications of hemispherectomy or the various modifications of functional hemispherectomy. The technique entails a smaller trepanation, less operation time, and less blood loss, and it leaves more brain tissue in place as compared with other functional hemispherectomy techniques. It starts with either hippocampectomy alone or with hippocampectomy and anterior temporal lobectomy. After this, deafferentation of the white matter of the temporal, occipital, parietal, and frontal lobe, using either a transcortical transventricular approach along the outline of the lateral ventricle or a sylvian key hole approach, is performed. The technique includes a transventricular callosotomy, and it leaves in place only a small portion of the suprainsular cortex and the insular cortex. However, as one modification, removal of the insular cortex can easily be performed, if necessary, and, as a second modification, the entire transventricular deafferentation can be performed through a sylvian key hole. In this report, the technique is described and the surgical experience for the first 13 patients is outlined. The immediate seizure relief with an average follow-up of 12 months was similar to that for patients with functional hemispherectomy, but the follow-up period for these 13 patients is not long enough to allow definite conclusions concerning long-term control of seizures and long-term complications.     

Best evidence synthesis: an intelligent alternative to meta-analysis

Research review has long been one of the most important scholarly activities in all branches of science. While there is sometimes a single study so well-designed, well carried out, and difficult to replicate that its findings are accepted as conclusive, more often there are many studies on a given topic, no one of which clearly supersedes the others. These studies may be done by different investigators using different methods or different populations. They may arrive at different conclusions. When this is the case, there is a need for reviewers to carefully consider the evidence and to put forth conclusions or hypotheses about where the weight of the evidence lies.     

The dystrophinopathies: an alternative to the structural hypothesis

Two studies with college students were conducted to further explore the reliability and construct validity of fear of emotion, as assessed by the Affective Control Scale (ACS), a measure of fear of anxiety, depressed mood, anger, and strong positive emotion. The ACS fared well in tests of internal consistency and of convergent and divergent validity. In an analogue study of panic onset with college students with no history of panic, the predictive and incremental validity of the ACS was demonstrated: the ACS predicted students' fear of induced panic-like bodily sensations even once trait and state anxiety were statistically controlled. A second aspect of the investigation was an analogue test of the hypothesis that stress from interpersonal conflict (induced via remembrance of conflictual interactions with an important person) would moderate the ACS's effects on anxiety and fear of induced bodily sensations. This hypothesis was not confirmed.     

Embolization therapy as an alternative to thoracotomy in vascular injuries of the chest wall

Hemothorax and persistent thoracic bleeding is frequently an indication for thoracotomy after trauma. Unfortunately, the source of the hemorrhage is often not identified. Presently, selective arteriography and transcatheter embolization (SATE) offers a good and safe alternative to localize and control hemorrhage from arterial injuries in selected patients. The records of eight patients who underwent SATE were reviewed. There were six blunt and two penetrating chest injuries. Four patients had significant preexisting medical comorbidities. Three patients with blunt injuries had undergone exploratory thoracotomy, but continued to bleed postoperatively. In three patients, angiography was indicated for associated thoracic and pelvic injuries, and five patients had SATE specifically due to thoracic hemorrhage. In all patients, SATE was effective to diagnose and control the hemorrhage. There were no complications related to the SATE procedure. Two patients died secondary to severe cerebral injuries. Given hemodynamic stability, SATE can be considered in patients who have already had a thoracotomy, have significant associated medical conditions, or those in need of other angiographic studies. Careful technique and a readiness to abandon SATE in unstable patients or when a suitable catheter position cannot be achieved are important technical points.     

Colposacropexy: an alternative technique

Colposacropexy procedures restore anatomically correct apical vaginal support on the levator plate at the ischial spine level. Venous hemorrhage resulting from laceration of presacral veins during suture fixation is the major hazard of this procedure. Titanium orthopedic bone anchor fixation minimizes this risk through precision placement of the bone anchor-suture unit.     

Pegaspargase: an alternative?

OBJECTIVE: To review the chemistry, pharmacology, pharmacokinetics, clinical activity, adverse effects, dosage, and administration guidelines for pegaspargase. DATA SOURCES: A MEDLINE search (1980-1996), a CANCERLIT search (1983-1996), and a CURRENT CONTENTS search (1980-1996) using the terms pegaspargase, PEG-asparaginase, PEG-L-asparaginase, polyethylene glycol L-asparaginase, polyethylene glycol conjugated L-asparaginase, and Oncaspar were conducted. STUDY SELECTION AND DATA EXTRACTION: All articles were considered for possible inclusion in this review. Abstracts were included only when they were judged to add critical information not otherwise available in the medical literature. DATA SYNTHESIS: L-Asparaginase has been a main component of treatment regimens for acute lymphocytic leukemia. A key limiting factor of L-asparaginase use has been the development of hypersensitivity to the drug. Recently, a polyethylene glycol (PEG) conjugated form of L-asparaginase, pegaspargase, has been made available. PEG modification of L-asparaginase has been shown to alter the tendency of the enzyme to induce an immune response and to extend the half-life of the drug. The majority of patients with hypersensitivity to the native enzyme preparations tolerate pegaspargase without further clinical hypersensitivity. The adverse effect profile of pegaspargase is similar to that of the native forms of L-asparaginase. The recommended dosage of pegaspargase is 2500 IU/m2 administered by intramuscular or intravenous injection every 2 weeks in combination with other chemotherapeutic agents. CONCLUSIONS: Pegaspargase is a safe, effective alternative to L-asparaginase in patients who have had clinical hypersensitivity reactions to both Escherichia coli- and Erwinia carotovora-derived L-asparaginase. However, pegaspargase should not be routinely substituted for L-asparaginase.