Biomedical subjects

T D Solberg

Publications and source records attributed to T D Solberg.

23 records · Page 2Linked to original sources

PubMed1996

Arteriovenous malformation animal model for radiosurgery: the rete mirabile.

PURPOSE: To study the effects of single-dose radiation on the porcine rete mirabile, a tangle of microvessels that mimics human arteriovenous malformations of the brain. METHODS: Eight retia mirabilia received a single dose of radiation under stereotactic location with digital angiography and CT. The following doses were applied: 20, 30, 40, 50, 60, 70, 80, and 90 Gy. The animals were followed up for a period of 7 months. Findings at neurologic examination, serial angiography, and histopathologic examination were analyzed. RESULTS: Progressive occlusion as observed by angiography corresponded to the histopathologic finding of intimal hyperplasia; that is, marked thickening of the vessel wall, progressing to occlusion of the vascular lumen, and associated thrombosis. A direct dose response was noted for these changes. Neurologic findings were related to the dose distribution and to histologic findings in structures adjacent to the rete mirabile. CONCLUSION: The rete mirabile is an excellent model by which to study the radiologic and histologic effects of single-dose radiation to the microvasculature of the central nervous system.

Angiography, Digital Subtraction↗

PubMed1995

Implications of tissue heterogeneity for radiosurgery in head and neck tumors.

PURPOSE: This study was undertaken to investigate the perturbation of small radiation beams by low density heterogeneities and to evaluate the ability of a Monte Carlo code to account for such perturbation. Performance of an inexpensive film scanning system was also evaluated. METHODS AND MATERIALS: Film and diode measurements were made in an acrylic phantom in which the size and position of an air gap were varied. Monte Carlo analysis was used to obtain additional verification of the measurements, to provide insight into photon and electron transport phenomena not directly measurable, and as a benchmark for the code. RESULTS: With 10 MV photons and a 1 cm circular field, a small 3-mm air cavity placed 2.6 cm deep in acrylic (full buildup) results in a reduction in central axis dose of 21% immediately following the cavity. Equilibrium is then reestablished over the next centimeter, after which the dose exceeds that of the homogeneous case by 3-4%. The loss in central axis equilibrium is highly field-size dependent, with some loss occurring even for the largest (32 mm) collimator. In addition, the presence of the air cavity produces a significant increase in dose up to 2 cm lateral and outside the primary field. CONCLUSIONS: Tissue heterogeneities are not presently accounted for in radiosurgery calculations, yet have the ability to perturb dose significantly. Targets may potentially be underdosed, and adjacent critical structures overdosed. Inability to account for tissue heterogeneities may limit the use of the radiosurgery approach in some areas. A Monte Carlo approach may be the method of choice for small field dose calculation when tissue heterogeneities are encountered.

Air↗

PubMed1995

A verification of the Monte Carlo code MCNP for thick target bremsstrahlung calculations.

The bremsstrahlung spectra from thick targets of Be, Al, and Pb are calculated using the Monte Carlo code MCNP (Monte Carlo N-particle). The current version of MCNP (v.4A) incorporates a coupled electron-photon transport scheme that allows the user to estimate the photon fluence produced from primary electron interactions. The simulation parameters are based on bremsstrahlung measurements of 15 MeV electrons incident on thick targets of Be, Al, and Pb at various angles between 0 degree and 90 degrees. The integrated yield and mean energy of each bremsstrahlung spectrum is calculated for the three targets at these angles. For angles less than 60 degrees the integrated yield calculated by MCNP4A is within 6% of measured values for the three targets. Furthermore, predicted mean energy is within 7% of the values derived from measurement for all angles tested. Also compared are the performances of two MCNP4A fluence tallies; a next-event estimator (detector tally) and a track length estimator (cell tally). Timing studies indicate the detector tally will perform the integrated yield calculations to a precision of 1% approximately 10 to 50 times faster than the conventional cell tally for an emission angle of 0 degree.

Aluminum↗

PubMed1994

A universal, multi-modality localization system for animal radiosurgery.

We have developed a stereotactic localization system allowing a radiosurgical approach in a number of animal models. The system utilizes fixation adapters specially designed for a particular animal, which in turn are attached to a common Brown-Roberts-Wells (BRW) compatible Delrin head ring. Each fixation adapter is constructed using materials compatible with CT, MRI, PET, and angiographic imaging studies. With such a system, radiographic localization, computerized treatment planning, and stereotactic radiation delivery can subsequently be performed in a manner identical to the procedures used for humans.

Animals↗

PubMed1992

Calculation of radiation dose enhancement factors for dose enhancement therapy of brain tumours.

When brain tumours are loaded with iodinated contrast media (CM) and exposed to x-rays, the photoelectrons, Auger electrons and fluorescent x-rays from the iodine enhance the radiation dose absorbed by the tumour. A modified CT scanner, the CTX, can be used to localize the tumour and to deliver the dose enhancement therapy. Monte Carlo calculations are presented here of the central-axis radiation depth dose in a brain containing a tumour loaded with an iodine concentration of 5 mg ml-1 and irradiated with the CTX operated at various kV settings. The dose enhancement factor (DEF) is also calculated for various field sizes and for 5 mg ml-1 of gadolinium in the tumour when the CTX is operated at 140 kV. The calculated values of the DEF are close to published experimental results.

Animals↗