CONTEXT: MR and ultrasound can noninvasively assess the relative blood volume (rBV) within a tumor to possibly aid diagnosis and treatment planning. The relationships among rBV, blood vessel density, and tumor size had not been reported, however, until investigators at Toronto's Sunnybrook and Women's College Health Sciences Centre presented study results at the ISMRM meeting in May.
RESULTS: Contrast-enhanced MR, vascular ultrasound, and micro-CT were performed on New Zealand white rabbits harboring VX2 tumors with a median size of 9 cm3. Blood volume fraction (BVF) was calculated by comparing MR data collected from the tumor site before and after the injection of Clariscan, an intravascular superparamagnetic iron oxide contrast agent. Blood vessel density was also assessed by 3D MR. Ultrasound evaluation of tumor rBV involved injection of an intravascular microbubble contrast. Principal investigator Dr. Xiuling Qi compared contrast signals from the tumor, adjacent muscle, and, when possible, a major vessel. Micro-CT studies were used to assess microcirculation in the tumor tissue rim. The results indicate that regional blood volume in the tumor rim decreases as tumor size increases. RBV, measured with contrast-enhanced MR and ultrasound, was qualitatively consistent with blood vessel density. Micro-CT and 3D MR results suggest an inverse relationshi!p between tumor vessel density in the rim and tumor size, meaning that blood vessel density decreases as tumor size increases.
IMAGES: Enhanced 3D MR image of a 15-cm3 tumor (top) demonstrates less vessel density in its peripheral regions than a 9-cm3 tumor (bottom).
IMPLICATIONS: The observation that small tumors have increased blood vessel density might reflect rapid growth and the relative absence of necrosis. Clinically, the results suggest that these molecular imaging approaches have a role in optimizing the timing of cancer therapies designed to inhibit development of tumor vasculature.