Multi-Institutional Audit of FLASH and Conventional Dosimetry with a 3D-Printed Anatomically Realistic Mouse Phantom.

We conducted a multi-institutional dosimetric audit between FLASH and conventional dose rate (CONV) electron irradiations by using an anatomically realistic 3D-printed mouse phantom. A CT scan of a live mouse was used to create a 3D model of bony anatomy, lungs, and soft tissue. A dual-nozzle 3D printer was used to print the mouse phantom using acrylonitrile butadiene styrene (∼1.02 g/cm <sup>3</sup> ) and polylactic acid (∼1.24 g/cm <sup>3</sup> ) simultaneously to simulate soft tissue and bone densities, respectively. The lungs were printed separately using lightweight polylactic acid (∼0.64 g/cm <sup>3</sup> ). Hounsfield units (HU), densities and print-to-print stability of the phantoms were assessed. Three institutions were each provided a phantom, and each institution performed two replicates of irradiations at selected anatomic regions. The average dose difference between FLASH and CONV dose distributions and deviation from the prescribed dose were measured with radiochromic film. Compared to the reference CT scan, CT scans of the phantom demonstrated mass density differences of 0.10 g/cm <sup>3</sup> for bone, 0.12 g/cm <sup>3</sup> for lung, and 0.03 g/cm <sup>3</sup> for soft tissue regions. Differences in HU between phantoms were <10 HU for soft tissue and bone, with lung showing the most variation (54 HU), but with minimal impact on dose distribution (<0.5%). Mean differences between FLASH and CONV decreased from the first to the second replicate (4.3% to 1.2%), while differences from the prescribed dose decreased for both CONV (3.6% to 2.5%) and FLASH (6.4% to 2.7%). Total dose accuracy suggests consistent pulse dose and pulse number, though these were not specifically assessed. Positioning variability was observed, likely due to the absence of robust positioning aids or image guidance. This study marks the first dosimetric audit for FLASH using a non-homogeneous phantom, challenging conventional calibration practices reliant on homogeneous phantoms. The comparison protocol offers a framework for credentialing multi-institutional studies in FLASH preclinical research to enhance reproducibility of biological findings