Introduction: Recent developments in image-guided radiation therapy (IGRT) of small animals aim to adapt the radiation planning and delivery protocols employed in the clinical setting to preclinical investigations. Accurate and targeted delivery of radiation to the disease volume of interest is necessary for evaluation of new treatment strategies and their timely translation to the clinic. This abstract reports the feasibility of employing a 1T small animal MR scanner and a cone-beam CT (CBCT)-equipped small animal irradiator to effectively visualize and delineate orthotopically implanted tumors on the treatment bed prior to targeted radiotherapy. Methods: Male nude mice bearing orthotopically implanted prostate tumor (DU145 and 22Rv1) and female SCID mice bearing orthotopically implanted cervical tumor (ME-180) were positioned on a transportable multimodality imaging and treatment bed (Minerve System, Bioscan) and scanned using a 1T MR scanner (M2, Aspect Imaging) and a small animal irradiator equipped with CBCT (X-rad 255Cx, Precision X-ray). The MR images were acquired using a fast spin echo sequence (TE/TR = 80/4800, NEX of 4 or 6, voxel size of 0.2 x 0.2 x 1.0 mm, 22 to 24 slices and imaging time of 6 to 8 minutes). The CBCT images were acquired at 80 and 100 kVp (for reduced streaking at the bone/soft tissue interface) and 0.5 mA (60 seconds acquisition) and reconstructed at 0.1 x 0.1 x 0.1 mm voxel size. Image analysis was conducted using Microview (GE Healthcare) by manually contouring of the tumor in selected slices and then linearly extrapolating across non-contoured slices. Results: The overall soft tissue contrast in the lower abdominal cavity was poor in the CBCT data set. There are cases where there is sufficient contrast between the bladder and the tumor, and between the tumor and selected segments of the intestinal tract. Nonetheless, it was challenging in most cases to confidently delineate the tumor margins using the CBCT images alone. Side-by-side consultation with the MR images during contouring allowed for the tumors to be more confidently delineated on the CBCT images (without co-registration with the MR data set) with a volumetric accuracy of 92.4 ± 12.3% (7 distinct tumors with volumes ranging from 181 to 1399 mm3) relative to the volumes contoured using the MR data set alone. Conclusion: Precise and accurate delivery of radiation to preclinical animal models must be supplemented by imaging techniques that allow for visualization and delineation of the disease volume in the treatment position. MR images acquired with the animal on a transportable multimodality imaging and treatment bed facilitated the delineation of volumetrically accurate treatment regions using a CBCT-equipped small animal irradiator. Acknowledgements: The authors would like to thank Coackley C, Zafarana G and Bristow RG for providing the prostate mouse models, and Chaudary N and McKee T for providing the cervical mouse model. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4060. doi:1538-7445.AM2012-4060
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Zheng et al. (2012) studied this question.