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R Kortmann

Publications and source records attributed to R Kortmann.

7 recordsLinked to original sources
PubMed2001

Evolution of visual resolution constrained by a trade-off.

The evolution of visual systems is constrained by a trade-off between spatial and temporal resolution. In this article we aim at identifying the causes of the trade-off at the retinal level in both artificial and natural visual systems. We start by selecting two factors that limit the values of spatial and temporal resolution. Then we show in two experiments on the evolution of an artificial system that the two factors induce trade-off curves connecting the evolved values of spatial and temporal resolution. A comparison of the experimental results with the resolution evolved in natural visual systems leads us to the conclusion that in natural systems the same factors are responsible for the observed trade-off.

Animals↗
PubMed1996

[Gamma knife versus stereotactic linear accelerator. Utilization, clinical results and cost-benefit relations].

INTRODUCTION: The principles of radiosurgery were developed in 1951 by Leksell. Their technical realization led to the development of the gamma knife and stereotactically modified linear accelerator. METHODS: In addition to the gamma knife, we present the different principles of convergent beam irradiation (radiosurgery with linear accelerator), the further development to fractionated stereotactic conformal radiotherapy, and the necessary quality-assurance steps. RESULTS: The greatest uncertainties in the precision of radiosurgery result from medical imaging (CT 0.7 x 0.7 x 1 mm; DSA 1-5 mm; MR angiography < 2 mm). The focusing accuracy of the gamma knife (+/- 0.3 mm) can also be achieved today by linear accelerators using a stereotactic floorstand. For the same indication and the same dosage for the target volume, there are no clinical differences between the gamma knife and the linear accelerator (AVM: 80% complete obliteration; metastases: 85% local tumor control; AN: 90% tumor control). However, there are greater differences in costs. There is no constellation where the gamma knife is just as expensive or more cost-effective than the linear accelerator treatment. The most cost-effective solution is modification of an available linear accelerator, resulting in treatment costs per patient of 9,201.25 DM (50 patients/year). CONCLUSION: There seem to be no methodological, physical, clinical or cost reasons for using a gamma knife, especially because the trend is going towards fractionated conformation radiotherapy instead of the application of high single doses.

Brain Neoplasms↗
PubMedDate not supplied

[HIT-LGG: effectiveness of carboplatin-vincristine in progressive low-grade gliomas of childhood--an interim report].

BACKGROUND: The rationale of the HIT-LGG protocol is to delay standard radiotherapy through administration of chemotherapy for children with progressive low grade glioma at an age under 5 years and in older children upon individual decision. PATIENTS AND METHOD: Until October 10th., 1999, 402 patients from 69 hospitals were registered. 130 children with progressive tumors were treated after a median observation time of 5 months: 46 patients received primary radiotherapy and 84 primary chemotherapy. A ten week induction period with weekly Vincristine and pulses of Carboplatin at weeks 1, 4, 7 and 10 is followed by consolidation with simultaneous application of both drugs every 4 weeks until week 53. RESULTS: Of 84 patients in the chemotherapy arm of the study (49 male, 35 female, 23 NF I, median age 2.99 years) 36 received treatment at diagnosis and 43 after a median observation time of 19.7 months. 94.3% achieved a clinical and neuroradiological response according to protocol criteria (5 CR, 30 PR/OR, 31 SD) after a median of 5.1 months. 4 tumors showed primary progression (9 too early, 5 not known). Only 6 of 84 children received radiation therapy for progressive disease during (2) or after termination (4) of chemotherapy, after a median delay of 25.6 months at a median age of 6.0 years. At a median observation time of 21.0 months, 6 children are in CR, 11 in PR, 48 have SD, 4 tumors are progressive and 3 children died of their tumor. (9 too early, 3 not known). PFS is at 72% after 36 months. 24 of 27 children experiencing allergic reactions to Carboplatin had to interrupt chemotherapy prematurely. CONCLUSIONS: Combination therapy with Carboplatin and Vincristine can effectively delay the start of radiotherapy in children with progressive low-grade glioma. The high rate of hypersensitivity reactions has to prompt future modifications of treatment.

Adolescent↗
PubMedDate not supplied

Radiosurgery for the treatment of brain metastases in renal cell carcinoma.

BACKGROUND: In the treatment of brain metastases using a stereotactically modified linear accelerator it could be shown that a single dose between 15 and 25 Gy leads to partial or complete remission of so-called radioresistant metastases from melanoma and hypernephroma. Radiosurgery of brain metastases then started in centers all over the world, however, experiences with brain metastases of renal cell carcinoma are yet limited. The aim of this analysis is therefore to present the treatment results of radiosurgery of brain metastases. Furthermore, in this paper prognostic subgroups shall be defined, in order to establish guidelines for an optimal therapy strategy. MATERIALS AND METHODS: Radiosurgery means stereotactically guided high-precision irradiation methods by extremely focussing ionizing radiation within the target volume as a single dose application. The characteristic steep dose decrease allows the selective destruction of small intracranial lesions, while the surrounding brain tissue is optimally protected. Two methods, Gamma Knife and stereotactic modified linear accelerator are clinically available. RESULTS: In larger studies from different groups all over the world, local tumor control rates from 85% to 95%, recurrence rates from 6% to 15% and side effects between 3% and 15% have been attained, independent of the system used. Prognostic factors, like volume of metastases < 10 ml, applied dose > 18 Gy, one or two metastases, absence of extracranial metastases, good patient performance with a Karnofsky score > 70%, primary treatment and more than one year between primary diagnosis and brain metastases showed a trend toward improved survival. Depending on the prognostic factors the median survival after radiosurgery ranged from 6 to 12 months. Retrospective comparison of radiosurgery and surgical series suggest that both modalities attain similar results. The dose can be applied with an accuracy of 0.3 mm. DISCUSSION: Based on these experiences, brain metastases can be treated by radiosurgery, primarily in patients with one or two metastases or in combination with whole brain irradiation as a boost in patients with more than two metastases. Furthermore with radiosurgery a new treatment modality exists to re-irradiate patients who have been failed after surgery or whole brain irradiation.

Bone Neoplasms↗