Imaging and Dosimetry Study of Inter-fraction Setup Error in a Murine Xenograft Flank Tumor Radiation Model

Modern small animal irradiation platforms provide for accurate delivery of radiation under 3D image guidance. However, leveraging these improvements currently comes at the cost of lower-throughput experimentation. Herein, we characterized setup accuracy and dosimetric robustness for mock/sham irradiation of a murine xenograft flank tumor model using the X-RAD SmART+ with the vendor-supplied Monte Carlo (MC) treatment planning system (SmART ATP). The chosen beam arrangement was parallel-opposing using a 20 mm square collimator, aligned off-axis for ipsilateral lung sparing. Using a cohort of five mice imaged with cone beam computed tomography (CBCT) over five consecutive mock-irradiation fractions, we compared inter-fraction setup variability resulting from a vendor-supplied multi-purpose bed with anesthesia nose cone with a more complicated immobilization solution with an integrated bite block with nose cone and Styrofoam platform. A hypothetical "high-throughput" image-guidance scenario was investigated, wherein the day 1 stage coordinates (resulting from CBCT guidance) were applied on days 2-5. Daily inter-fraction setup errors were evaluated per specimen (days 2-5) using CBCT-derived offsets from day 1 stage coordinates. Using the CBCT images and Monte Carlo dose calculation, 3D dosimetric plan robustness was evaluated for the vendor-supplied immobilization solution, for both the high-throughput guidance scenario as well as for use of daily CBCT-based alignment. Inter-fraction setup offset magnitude was 3.6 (±1.5) mm for the vendor-supplied immobilization compared to 3.3 (±1.8) mm for the more complicated solution. For the vendor-supplied immobilization, we found that daily CBCT was needed to adequately cover the flank tumors dosimetrically, given our chosen treatment approach.