  Purpose: The physical properties of yttrium-90 (90Y) allow for imaging with
positron emission tomography/computed tomography (PET/CT). The increased
sensitivity of long axial field-of-view (LAFOV) PET/CT scanners possibly allows
to overcome the small branching ratio for positron production from 90Y decays
and to improve for the post-treatment dosimetry of 90Y of selective internal
radiation therapy.
  Methods: For the challenging case of an image quality body phantom, we
compare a full Monte Carlo (MC) dose calculation with the results from the two
commercial software packages Simplicit90Y and Hermes. The voxel dosimetry
module of Hermes relies on the 90Y images taken with a LAFOV PET/CT, while the
MC and Simplicit90Y dose calculations are image independent.
  Results: The resulting doses from the MC calculation and Simplicit90Y agree
well within the error margins. The image-based dose calculation with Hermes,
however, consistently underestimates the dose. This is due to the mismatch of
the activity distribution in the PET images and the size of the volume of
interest. Furthermore, there are likely limitations of Hermes' dose calculation
algorithm for 90Y. We found that only for the smallest phantom sphere there is
a statistically significant dependence of the Hermes dose on the image
reconstruction parameters and scan time.
  Conclusion: Our study shows that Simplicit90Y's local deposition model can
provide a reliable dose estimate. On the other hand, the image based dose
calculation requires further benchmarks and verification in order to take full
advantage of LAFOV PET/CT systems.
