  This paper describes the development and validation of a Monte Carlo (MC)
dose computing module dedicated to organ dose calculations of patients
undergoing nuclear medicine (NM) internal radiation exposures involving 18F-FDG
PET/CT examination. This new module extends the more-than-10-years-long ARCHER
project that developed a GPU-accelerated MC dose engine by adding dedicated NM
source-definition features. To validate the code, we compared dose
distributions from the 0.511-MeV point photon source calculated for a water
phantom as well as a patient PET/CT phantom against a well-tested MC code,
GATE. The water-phantom results show excellent agreement, suggesting that the
radiation physics module in the new NM code is adequate. To demonstrate the
clinical utility and advantage of ARCHER-NM, one set of PET/CT data for an
adult male NM patient is calculated using the new code. Radiosensitive organs
in the CT dataset are segmented using a CNN-based tool called DeepViewer. The
PET image intensity maps are converted to radioactivity distributions to allow
for MC radiation transport dose calculations at the voxel level. The dose rate
maps and corresponding statistical uncertainties were calculated for the
duration of PET image acquisition. The dose rate results of the 18F-FDG PET
imaging patient show that ARCHER-NM's results agree very well with those of the
GATE within 0.58% to 4.11%. Most impressively, ARCHER-NM obtains such results
in less than 0.5 minutes while it takes GATE as much as 376 minutes. This is
the first study presenting GPU-accelerated patient-specific MC internal
radiation dose rate calculations for clinically realistic 18F-FDG PET/CT
imaging cases involving auto-segmentation of whole-body PET/CT images. This
study suggests that modern computing tools -- ARCHER-NM and DeepViewer -- are
accurate and fast enough for routine internal dosimetry in NM clinics.
