Biomedical subjects

S G Scheib

Publications and source records attributed to S G Scheib.

4 recordsLinked to original sources

PubMed2006

Development of a 3-D convolution/superposition algorithm for precise dose calculation in the skull.

In this paper an algorithm for calculating 3-D dose distributions within the brain is introduced and adapted to the demands of modem radiosurgery. The dose calculation with this model is based on a 3-D distribution of the primary photon intensity which is calculated with a ray casting algorithm. A prelocated matrix takes into account field sizes as well as modifying elements as collimator positions (MLC), blocks, wedges and compensators. Monte Carlo precalculated monoenergetic kernels from 0.1 MeV to 50 MeV were at our disposal. The components of the spectrum were either determined by deconvoluting depth dose curves measured in water or analyzed with a Ge-Li detector system in the case of 60Co. The calculated fluence distribution has to be superposed to the complete kernel containing the spatial energy deposition. Inhomogeneities and tissue interface phenomena (rhoe, Z) have been investigated. The divergence of the rays and the curved surface of the patient are taken into account. Assuming homogenous media, it is possible to shorten the computation time by using the Fast Fourier Transformation (FFT) delivering a first overview within seconds. The algorithm was evaluated and verified under specific conditions of small fields as used in radiosurgery and compared to dose measurements and Monte Carlo calculations. In using both the fast algorithm (FFT) for mainly homogenous conditions on one hand and the very precise superposition for inhomogeneous cases on the other, this algorithm can be a very helpful instrument especially for critical locations in the skull.

Algorithms↗

PubMed2005

Development of an interactive graphical user interface for therapy simulation.

Because of the ongoing development to more complex non-coplanar techniques in radiotherapy, the use of modem computer graphics while designing a dose plan becomes increasingly essential and more important. In this paper, we describe a concept to simulate 3D conformal treatment techniques on the computer. All important components of the treatment device and relevant patient structures are mapped to an internal model, which allows simulation of motion sequences as well as the interactive adjustment of treatment parameters. The intention of this user interface is to save time by using mainly graphical modules in the optimization process instead of running through the dose calculation every time.

Algorithms↗

PubMed2004

High precision radiosurgery and technical standards.

BACKGROUND: A high degree of precision and accuracy in radiosurgery is a fundamental requirement for therapeutic success. Small radiation fields and steep dose gradients are clinically applied thus necessitating a dedicated quality assurance program in order to guarantee dosimetric and geometric accuracy. MATERIAL AND METHODS: A detailed analysis of the course of treatment independent of the irradiation technique used results in the so-called chain of uncertainties in radiosurgery (immobilisation, imaging, treatment planning system, definition of regions of interest, mechanical accuracy, dose planning, dose verification). Each link in this chain is analysed for accuracy and the established quality assurance procedures are discussed. A "System Test" was used to check the whole chain of uncertainties simultaneously. RESULTS: The tests described are compatible with published reports on quality assurance in radiosurgery. In terms of accuracy the weakest link in the chain of uncertainties is stereotactic MR imaging. Geometric overall accuracy measured in the "System Test" is less than 0.7 mm. CONCLUSION: The established quality assurance routines have clinically been validated. MR imaging dominates geometric overall accuracy in radiosurgery, which can be limited to less than 1 mm by an adequate quality assurance protocol.

Adenoma↗

PubMed2000

Gamma knife radiosurgery as a primary treatment for prolactinomas.

OBJECT: The purpose of this study was to estimate the efficacy of gamma knife radiosurgery (GKS) in controlling tumor growth and endocrinopathy associated with prolactinomas. METHODS: Between 1993 and 1997, 164 of 469 patients with pituitary adenomas treated by GKS harbored prolactinomas. The dose to the tumor margin ranged from 9 to 35 Gy (mean 31.2 Gy), and the visual pathways were exposed to a dose of less than 10 Gy. The mean tumor diameter was 13.4 mm. The mean follow-up time for 128 cases was 33.2 months (range 6-72 months). Tumor control was observed in all but two patients who underwent surgery 18 and 36 months, respectively, after GKS. Clinical cure was achieved in 67 cases. Clinical improvement was noted with a decrease in the hyperprolactinemia after GKS. Nonetheless, in 31 (29%) of 108 patients who were followed for more than 2 years no improvement in serum prolactin levels was demonstrated, although this could be normalized by bromocriptine administration after treatment. Nine infertile women became pregnant 2 to 13 months after GKS and all gave birth to normal children. There was no visual deterioration related to GKS. Five women experienced premature menopause. In these patients there was subtotal disappearance of the tumor and an empty sella developed. CONCLUSIONS: Gamma knife radiosurgery as a primary treatment for prolactinomas can be safe and effective both for controlling tumor growth and for normalization of prolactin hypersecretion. A higher margin dose (> or = 30 Gy) seemed to be associated with a better clinical outcome. Gamma knife radiosurgery may make prolactinomas more sensitive to the bromocriptine.

Humans↗