A NEW METHOD FOR DETERMINING DOSE-RATE DISTRIBUTION FROM RADIOIMMUNO-THERAPY USING RADIOCHROMIC MEDIA

Purpose: To describe and evaluate a new, simple, inexpensive method fo r directly measuring the radiation dose and its spatial distribution g enerated from explanted tissues of animals previously injected with ra diolabeled immunoconjugates or other agents. Methods and Materials: Th is technique uses the newly developed radiochromic dye medium (Gafchro mic(TM)) which responds reproducibly for therapeutic dose exposures, h as high spatial resolution, does not require film processing, and is r elatively insensitive to ambient light. We have evaluated the dose dis tribution from LS174T tumors and selected normal tissues in nude mice previously injected with Y-90 labeled anti-carcinoembrionic antigen an tibodies. Individual tissues from sacrificed animals are halved and th e flat section of the tissue is placed onto the dosimetry media and th en frozen. The dosimetry medium is exposed to beta and Bremsstrahlung radiation originating from the frozen tissues. The relative darkening of the dosimetry medium depends on the dose deposited in the film. The dosimetry medium is scanned with a commercial flatbed scanner and the image intensity is digitally stored and quantitatively analyzed. Isod ose curves are generated and compared to the actual tissue outline. Re sults: The absorbed dose distribution due to Y-90 exposure show only s light gradients in the interior of the tissue, with a markedly decreas ing dose near the edges of the tissue. In addition, the isodose curves follow the tissue outline except in regions having radii of curvature smaller than the range of the beta-particle (R90 = 5 mm). These resul ts suggest that the shape of the tumor, and its curvature, are importa nt in determining the minimum dose delivered to the tumor by radiation from Y-90 monoclonal antibodies, and hence in evaluating the tumor re sponse to the radiation. The dose and spatial dose distribution were c alculated assuming that the total Y-90 activity is distributed uniform ly throughout a half ellipsoid. The calculated spatial dose distributi ons for the half ellipsoids were similar to those observed from the do simetry media that had been exposed to radioactivity contained in the tumors. Conclusion: This method provides direct dose evaluation withou t elaborate summary calculations based on activity measurements from s erial slices. The measured radiation dose actually indicates the dose rate at the time of animal sacrifice. Quantitative analysis of radiati on emitted from the tissues is relatively fast, making it feasible to examine a number of tissues under a variety of conditions.