TU‐D‐332‐06: Quantitative in Vivo Measurements of Treatment Response in Murine Tumors Using Diffusion‐Weighted Magnetic Resonance Imaging

Purpose: Post‐irradiation changes in tissue structure can lend insight into how a tumor is responding to therapy. Here, diffusion‐weighted magnetic resonance imaging (DW‐MRI), which observes Brownian motion within a tissue sample, was used to quantify changes in the diffusion properties of irradiated tumors in vivo. Method and Materials: MCa‐29 mammary carcinoma was grown in the hind legs of C 3 Hk/fam mice. Tumors were grown to a diameter of 8mm, treated with 20Gy of 137 Cs gamma‐rays (mean energy = 662keV), and imaged with DW‐MRI. After this initial scan, half the mice were sacrificed and the tumors excised for histological analysis. The remaining animals were re‐scanned three days later before being sacrificed for histological analysis. Diffusion data were collected on a 4.7T Bruker Biospin scanner (Billerica, MA) and analyzed using a two‐compartment model, representing MR signal as originating from a slow‐diffusing component, often associated with intracellular space, and a fast‐diffusing component, often associated with extracellular space. The analysis yielded rates of diffusion for the slow‐ and fast‐diffusing components and the volume fraction occupied by slow‐diffusing component, or cellular space (CVF). Results: Between imaging sessions the mean rates of diffusion in the slow‐ and fast‐diffusing components increased 96% (p=0.03) and 62% (p=0.30), respectively. The DW‐MRI‐based CVF remained approximately unchanged (CVF=0.40±0.04 versus CVF=0.43±0.08, p=0.88). This did not correlate with the histology‐based CVF measurements which, during the same time period, decreased significantly from 0.76±0.03 to 0.56±0.01 (p