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Biomedical subjects

Peter Winkler

Publications and source records attributed to Peter Winkler.

14 recordsLinked to original sources
PubMed2006

An intercomparison of 11 amorphous silicon EPIDs of the same type: implications for portal dosimetry.

The use of electronic portal imaging devices (EPIDs) for portal dosimetry requires knowledge of their dosimetric properties. The pixel value response of amorphous silicon EPIDs of type Elekta iViewGT is known to be nonlinear with dose. However, it is not clear whether these nonlinearities vary with time and from one detector to another, respectively. In the present study, the dose-response characteristics of 11 iViewGT EPIDs were investigated with respect to dose rate, total dose and field size. It was found that each detector needs to be individually calibrated, not only in terms of absolute sensitivity but also with respect to its relative response variations with exposure parameters. Doubling the dose rate typically increased the EPID signal between 1.4% and 2.8%. Changing the number of monitor units from 30 to 500 was accompanied by an increase in detector sensitivity between 1.7% and 2.8%. The EPID scatter factors were always within +/-1%. It was observed that the dose-response behaviour was not stable with respect to time. Particularly within the first weeks of operation, detector ageing caused variations in both absolute sensitivity and relative response curves. It is recommended to establish a quality assurance programme if the amorphous silicon EPIDs are intended to be used for clinical portal dosimetry.

Dose-Response Relationship, Radiation↗
PubMed2006

Dural ectasia in children with Marfan syndrome: a prospective, multicenter, patient-control study.

UNLABELLED: The clinical diagnosis of Marfan syndrome in childhood is difficult, because symptoms may not have developed to their full expression until adulthood. The Ghent nosology for the diagnosis of Marfan syndrome classifies dural ectasia as a major diagnostic criterion. More than two thirds of adult patients with Marfan syndrome show dural ectasia, while the frequency in childhood is unknown. This prospective multicenter observational patient-control study was performed to identify pathologic changes of the lumbosacral spine in young patients with Marfan syndrome. DESIGN: Prospective clinical trial, multicentric, cross-sectional. SETTING: MRI of the lumbosacral spine. PATIENTS: Twenty patients with proven Marfan syndrome, 20 patients suspicious for Marfan syndrome and 38 healthy controls. OUTCOME MEASURES: Vertebral body diameter (VBD) from L1 to S1, dural sac diameter (DSD) from L1 to S1, dural sac ratio (DSR), qualitative assessment of the lumbosacral spine. RESULTS: DSD and VBD in different age groups were higher in patients with proven or suspected Marfan syndrome than in healthy controls (DSD: L1, 6-8 years, P < 0.05). VBD related to body height showed a similar growth related increase in patients with proven or suspected Marfan syndrome and controls. DSD related to body height was elevated in patients with proven or suspected Marfan syndrome at different levels of the lumbar spine. DSD at levels L1, L5, and S1, and DSR at levels L5 and S1 of patients with proven Marfan syndrome were significantly higher (P < 0.05) than in controls. CONCLUSION: Even during childhood pathologic changes inside the lumbosacral spine of patients with Marfan syndrome can be observed. Dural ectasia, which occurs at different levels of the lumbar spine, can be detected at levels L5 and S1 in up to 40% of patients with Marfan syndrome.

Adolescent↗
PubMed2006

Inverse planning--a comparative intersystem and interpatient constraint study.

PURPOSE: To compare commercial treatment-planning systems (TPS) for inverse planning (IP) and to assess constraint variations for specific IMRT indications. MATERIAL AND METHODS: For IP, OTP, XiO and BrainSCAN were used and step-and-shoot intensity-modulated radiotherapy (IMRT) delivery was assumed. Based on identical constraints, IP was performed for a prostate, head and neck, brain, and gynecologic case. IMRT plans were compared in terms of conformity/homogeneity, dose-volume histograms (DVHs), and delivery efficiency. For ten patients each of a class of indications, constraint variations were evaluated. RESULTS: IMRT plans were comparable concerning minimum target dose, homogeneity, conformity, and maximum doses to organs at risk. Larger differences were seen in dose gradients outside the target, monitor units, and segment number. Using help structures proved efficient to shape isodoses and to reduce segmentation workload. For IMRT class solutions, IP constraint variations depended on anatomic site. CONCLUSION: IP systems requiring doses as input and having objective functions based on physical parameters had a very similar performance. Constraint templates can be established for a class of IMRT indications.

Brain Neoplasms↗
PubMed2005

Neuropathological spectrum of cortical dysplasia in children with severe focal epilepsies.

Cortical dysplasias comprise a variable spectrum of clinical, neuroradiological and histopathological findings. We report about a cohort of 25 pediatric patients (mean age 8.1+/-4.8 years) with severe drug-resistant early onset focal epilepsies (mean duration 2.1+/-0.4 years), mental/psychomotor retardation, and multilobar epileptogenesis. Compared to age-matched biopsy controls, microscopical inspection of neurosurgically resected specimens revealed dysplastic neurons with/without balloon cells in only 7 patients. According to Palmini's classification system, these lesions were categorized as focal cortical dysplasia (FCD) type II. All other patients presented with rather subtle but statistically significant neuroanatomical abnormalities. We identified increased numbers of ectopic neurons in white matter and cortical gliosis. However, most intriguing was our finding of a microcolumnar arrangement of cortical neurons in layer III. These microcolumns can be statistically defined as vertical lining of more than eight neurons (two times standard deviation of cell countings obtained from controls). In addition, neuronal perikarya were significantly smaller in epilepsy patients. Although histological abnormalities occurring during postnatal maturation of the brain challenge any neuropathological classification in this group of young patients, we propose that these findings are classified according to FCD type I. Our observations support a concept compatible with regional loss of high-order brain organization.

Adolescent↗
PubMed2005

Performance analysis of a film dosimetric quality assurance procedure for IMRT with regard to the employment of quantitative evaluation methods.

A system for dosimetric verification of intensity-modulated radiotherapy (IMRT) treatment plans using absolute calibrated radiographic films is presented. At our institution this verification procedure is performed for all IMRT treatment plans prior to patient irradiation. Therefore clinical treatment plans are transferred to a phantom and recalculated. Composite treatment plans are irradiated to a single film. Film density to absolute dose conversion is performed automatically based on a single calibration film. A software application encompassing film calibration, 2D registration of measurement and calculated distributions, image fusion, and a number of visual and quantitative evaluation utilities was developed. The main topic of this paper is a performance analysis for this quality assurance procedure, with regard to the specification of tolerance levels for quantitative evaluations. Spatial and dosimetric precision and accuracy were determined for the entire procedure, comprising all possible sources of error. The overall dosimetric and spatial measurement uncertainties obtained thereby were 1.9% and 0.8 mm respectively. Based on these results, we specified 5% dose difference and 3 mm distance-to-agreement as our tolerance levels for patient-specific quality assurance for IMRT treatments.

Benchmarking↗
PubMed2005

Dose-response characteristics of an amorphous silicon EPID.

Electronic portal imaging devices (EPIDs) were originally developed for the purpose of patient setup verification. Nowadays, they are increasingly used as dosimeters (e.g., for IMRT verification and linac-specific QA). A prerequisite for any clinical dosimetric application is a detailed understanding of the detector's dose-response behavior. The aim of this study is to investigate the dosimetric properties of an amorphous silicon EPID (Elekta IVIEWGT) with respect to three photon beam qualities: 6, 10, and 25 MV. The EPID showed an excellent temporal stability on short term as well as on long term scales. The stability throughout the day was strongly influenced by warming up, which took several hours and affected EPID response by 2.5%. Ghosting effects increased the sensitivity of the EPID. They became more pronounced with decreasing time intervals between two exposures as well as with increasing dose. Due to ghosting, changes in pixel sensitivity amounted up to 16% (locally) for the 25 MV photon beam. It was observed that the response characteristics of our EPID depended on dose as well as on dose rate. Doubling the dose rate increased the EPID sensitivity by 1.5%. This behavior was successfully attributed to a dose per frame effect, i.e., a nonlinear relationship between the EPID signal and the dose which was delivered to the panel between two successive readouts. The sensitivity was found to vary up to 10% in the range of 1 to 1000 monitor units. This variation was governed by two independent effects. For low doses, the EPID signal was reduced due to the linac's changing dose rate during startup. Furthermore, the detector reading was influenced by intrabeam variations of EPID sensitivity, namely, an increase of detector response during uniform exposure. For the beam qualities which were used, the response characteristics of the EPID did not depend on energy. Differences in relative dose-response curves resulted from energy dependent temporal output characteristics of the accelerator. If ghosting is prevented from affecting the results and all dose-response effects are properly corrected for, the EPID signal becomes independent of dose rate, dose, and exposure time.

Artifacts↗
PubMed2004

Orbitofrontal epilepsy: electroclinical analysis of surgical cases and literature review.

Clinical and electrographic data were reviewed on 2 of our patients with orbitofrontal epilepsy who were seizure free at 5-year follow-up, and on 2 similar patients from the literature. One of our patients was lesional, and the other was nonlesional. Interictal EEG discharges were lateralized to the side of invasively recorded orbitofrontal seizures in the nonlesional case. In this case, no clinical manifestations occurred until the orbitofrontal discharge had spread to the opposite orbitofrontal and both mesial temporal areas. Unresponsiveness or arrest of activity were the initial manifestations of complex partial seizures in both cases. The 2 cases from the literature with long-term seizure-free follow-up had little impairment of awareness and displayed vigorous motor automatisms. Interictal epileptiform activity was bifrontally synchronous in 1 case. Ipsilateral frontotemporal discharges were seen in both. Invasive ictal epileptiform activity appeared maximal in the ipsilateral orbitofrontal region in both patients. No consistent electrographic or clinical pattern characterized these 4 cases. Seizures of orbitofrontal origin may be characterized by either unresponsiveness associated with oroalimentary automatisms or limited alteration of awareness and associated with vigorous motor automatisms. Invasive monitoring of the orbitofrontal cortex should be considered in nonlesional cases with complex partial seizures that show nonlocalizing ictal patterns and interictal frontal or frontotemporal epileptiform discharges.

Adult↗
PubMed2003

Introducing a system for automated control of rotation axes, collimator and laser adjustment for a medical linear accelerator.

Mechanical stability and precise adjustment of rotation axes, collimator and room lasers are essential for the success of radiotherapy and particularly stereotactic radiosurgery with a linear accelerator. Quality assurance procedures, at present mainly based on visual tests and radiographic film evaluations, should desirably be little time consuming and highly accurate. We present a method based on segmentation and analysis of digital images acquired with an electronic portal imaging device (EPID) that meets these objectives. The method can be employed for routine quality assurance with a square field formed by the built-in collimator jaws as well as with a circular field using an external drill hole collimator. A number of tests, performed to evaluate accuracy and reproducibility of the algorithm, yielded very satisfying results. Studies performed over a period of 18 months prove the applicability of the inspected accelerator for stereotactic radiosurgery.

Algorithms↗
PubMed2003

Normalized sensitometric curves for the verification of hybrid IMRT treatment plans with multiple energies.

With the clinical implementation of time-variable dose patterns and intensity modulated radiotherapy (IMRT) film dosimetry has regained popularity. Films are currently the most frequently used dosimetric means for patient specific quality assurance in IMRT. A common method is to verify a so-called hybrid IMRT plan, which is the patient specific treatment plan with unmodified fluence patterns recalculated in a dedicated phantom. For such applications the sensitometric curve, i.e., the relation between optical density (OD) and absorbed dose, should not depend critically on beam energy, field size and depth, or film orientation. In order to minimize the influence of all these variables a normalization of sensitometric curves is performed at various photon beam energies (6 MV, 10 MV, 25 MV). By doing so one unique sensitometric curve can be used for these three beam qualities. This holds for both film types investigated: Kodak X-Omat V films and EDR-2 films. Additionally, the influence of field size, depth and film orientation on a normalized sensitometric curve is determined for both film types. For doses smaller than 0.8 Gy for X-Omat V and doses smaller than 3 Gy for EDR-2 films the field size variation of normalized sensitometric curves is much smaller than 3% for fields up to 20 x 20 cm2. For X-Omat V films all differences between sensitometric curves determined at depths of 5, 10, and 15 cm are smaller than 3%. For EDR-2 films deviations larger than 3% are only observed at low net OD smaller than 0.25. The dependence of film orientation (parallel versus perpendicular) on a normalized sensitometric curve is found to be not critical. However, processing conditions have the largest influence and can result in differences up to 20% for sensitometric curves derived from films of the same batch but using different film processors. When normalizing sensitometric curves to the dose value necessary to obtain a net OD=1 for that respective geometry and energy the large energy dependence of sensitometric curves can be almost eliminated. This becomes especially important for the verification of hybrid IMRT plans with multiple energies. Additionally, such a normalization minimizes other influences such as field size, depth, and film orientation. This method is generally applicable to both Kodak X-Omat V and EDR-2 films. In order to achieve the highest accuracy level an upper dose limit of 0.8 Gy for X-Omat V films and 3 Gy for EDR-2 films should be taken into account. However, these dose limits may vary with film reading instrument and film processor.

Calibration↗
PubMed2002

Naming tools and animals: asymmetries observed during direct electrical cortical stimulation.

Semantically bounded disorders of verbal processing that result in selective dysnomias for items belonging to specific semantic categories have been well documented in lesion studies. Most commonly dissociations between the categories of living versus non-living things have been reported. Processing of living things such as animals seems to be impaired after bilateral lesions, whereas lesions resulting in impairment of processing of non-living things such as tools seem to be restricted to the left hemisphere. In this study, we tested the naming capabilities of epilepsy patients with subdural electrodes implanted for localization of the epileptogenic zone and preoperative mapping of cognitive functions. Tool and animal items were used, and the results show that during stimulation of the left hemisphere dysnomias for tool items were more pronounced than for animal items. This asymmetry is discussed within a model of more widely bilaterally distributed processing of living category members as compared to more restricted left-sided representation of non-living category members.

Adolescent↗
PubMed2002

Effects of lithium carbonate on hematopoietic cells in patients with persistent neutropenia following chemotherapy or radiotherapy.

Neoplasm therapy is restricted by the haematological side effects of tumour-destructive therapy, requiring expensive supportive care to some extent to overcome and treat leucopenia and its consequences. An effective and very cost-effective alternative for treating neutropenia is to administer lithium carbonate. Lithium leads to a release of hematopoietic growth factors (CSF) and therefore to proliferation of neutrophil granulocytes. Normally, recombinant CSF is only administered when there are indications of severe neutropenia because of the high costs involved, all the more evident in the long-term treatment of persistent leucocytopenia. On the other hand, CSF and leucocytes play an essential role in tumour immunology and with regard to response rates to cytostatic drugs. Lithium salts have shown that they can increase the number of neutrophil granulocytes quite significantly and, to a lesser extent, the number of eosinophil granulocytes and lymphocytes as well. The average number of erythrocytes does not change significantly. Patient tolerability to lithium carbonate therapy is very good. It can be used to treat patients with chronic leucopenia following chemotherapy or radiotherapy extremely cost-effectively. Unfortunately this treatment has not won acceptance in clinical oncology in the face of highly cost-intensive treatment with recombinant CSF.

Adult↗
PubMed2002

Aneurysm of the inferior vena cava in a 5-year-old boy.

The authors present a case of an aneurysm of the inferior vena cava in a 5-year-old boy. The boy suffered from acute pulmonary and paradoxical cerebral embolism after a minor abdominal trauma. Magnetic resonance angiography was the key for correct diagnosis of the retroperitoneal process and helped develop a strategy for surgery. The boy underwent resection of the aneurysm and a prosthetic replacement of the distal inferior vena cava. Aneurysms of the inferior vena cava are very rare conditions. The diagnosis is difficult because they may mimic a retroperitoneal tumor. Thrombosis and embolism are common complications of aneurysms of the vena cava.

Abdominal Pain↗