  We conducted a multi-institutional audit of dosimetric variability 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 ($~1.02 g/cm^3$) and polylactic acid ($~1.24 g/cm^3$)
simultaneously to simulate soft tissue and bone densities, respectively. The
lungs were printed separately using lightweight polylactic acid ($~0.64
g/cm^3$). Hounsfield units (HU) and densities were compared with the reference
CT scan of the live mouse. Print-to-print reproducibility of the phantom was
assessed. Three institutions were each provided a phantom, and each institution
performed two replicates of irradiations at selected mouse 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 $0.10 g/cm^3$ for bone, $0.12 g/cm^3$ for lung, and
$0.03 g/cm^3$ for soft tissue regions. Between phantoms, the difference in HU
for soft tissue and bone was <10 HU from print to print. Lung exhibited the
most variation (54 HU) but minimally affected dose distribution (<0.5% dose
differences between phantoms). The mean difference between FLASH and CONV from
the first replicate to the second decreased from 4.3% to 1.2%, and the mean
difference from the prescribed dose decreased from 3.6% to 2.5% for CONV and
6.4% to 2.7% for FLASH. The framework presented here is promising for
credentialing of multi-institutional studies of FLASH preclinical research to
maximize the reproducibility of biological findings.

Medical Physics