Date of Award

5-2009

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Program

Biomedical Engineering and Imaging

Research Advisor

Gary S. Keyes, Ph.D.

Committee

M. Waleed Gaber, Ph.D. Mohammad F. Kiani, Ph.D. Erno Lindner, Ph.D. Thomas E. Merchant, D.O., Ph.D.

Keywords

Blood-Brain Barrier, Brain Tumor, Peritumoral, Radiation, Thalidomide, Vasculature

Abstract

In the USA, 200,000 brain tumors are diagnosed each year with glioma representing 8.4% of the 200,000. The standard treatment for glioma consists of surgical resection, when possible, followed by radiation therapy (RT) and/or chemotherapy. Radiation therapy is one of the most effective treatments of brain tumors; however, the therapeutic ratio of RT is limited by damage to the normal tissue. We hypothesize that tumor growth has an adverse effect on the peritumoral tissue through the angiogenic/inflammatory environment it creates rendering it susceptible to further damage by RT which may be prevented by using anti-angiogenic/anti-inflammatory agents. We have developed a rat C6 glioma brain tumor model to study the combination of tumor presence and radiation treatment on the peritumoral region both at early and late time points. We have also used this model to test the effect of thalidomide on limiting radiation toxicity to the normal tissue while not interfering with radiation efficacy.

Intravital microscopy was used in combination with a cranial window brain tumor model to assess the effect of glioma presence on neighboring tissue with and without RT (40Gy total, 8Gy/day starting on day 5 post-implant/surgery) and when RT was administered in combination with thalidomide (100mg/kg/day). Permeability of the blood-brain barrier (BBB) was determined by measuring the rate of extravasation of 3kDa Texas-Red dextran from the vasculature into the tissue. Leukocytes were stained using an intravenous injection of Rhodamine 6G and leukocyte interactions, an indicator of inflammation, were counted in venules ranging in size from 45 to 90μm. Staining for vascular endothelial growth factor (VEGF) and glial fibrillary acidic protein (GFAP), a marker of astrocytes, was also performed.

Our studies show that the presence of the tumor alone caused quantifiable changes in BBB permeability, and caused an increased in vascular endothelial growth factor (VEGF) protein expression in the peritumoral region. Astrogliosis, an increase in reactive astrocytes associated with inflammation, was detected in the peritumoral region and contralateral to the tumor.

RT of the implanted tumors caused a significant increase in BBB permeability and in adhered leukocytes in the peritumoral region, compared to the sham implant group. In addition following RT, VEGF increased both in the peritumoral region and in the middle of the tumor. Astrogliosis was also significantly higher in the tumor implant + RT animals compared to sham and tumor implanted animals. At 66 days post tumor-implantation the RT the BBB permeability and astrogliosis were still significantly higher compared to sham implanted animals.

We have also evaluated thalidomide as a potential anti-angiogenic/anti-inflammatory agent with the prospective to protect normal tissue and have shown that it had limited effects in a rat C6 brain tumor model and it interfered with RT tumor treatment efficacy. In addition, at 66 post tumor implant there was a significantly higher incidence of astrogliosis, BBB permeability, and adhered leukocyte counts in the animals treated with thalidomide compared to sham implanted animals.

In this work, we have developed and characterized a new rat radiation brain tumor model to study the effect of a brain tumor and RT on the normal brain tissue at acute and late time points. We have quantified the effect of tumor presence on the peritumoral microvasculature and observed a significant increase in vascular permeability but no significant effect on leukocyte interactions. The lack of leukocyte interactions might indicate that the increase in permeability is associated with the angiogenic signaling induced by tumor presence. In support of this conclusion, we observed an increased VEGF expression in the peritumoral region. The combination of RT and tumor presence had a greater damaging effect on peritumoral BBB integrity measured by an increase in leukocyte interactions and permeability which could not be inhibited by using thalidomide. Furthermore, the regression of tumor after RT and the achievement of 100% survival at 65 days post implant have allowed us to investigate late radiation damage.

DOI

10.21007/etd.cghs.2009.0370

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