  Purpose: To obtain individualized internal doses with a Monte Carlo method in
patients undergoing diagnostic FCH-PET studies and to compare such doses with
the MIRD method calculations. Methods: A patient cohort of 17 males were imaged
after intravenous administration of a mean FCH activity of 244.3 MBq. The
resulting PET/CT images were processed in order to generate individualized
input source and geometry files for dose computation with the MC tool GATE. The
resulting dose estimates were studied and compared to the MIRD method with two
different computational phantoms. Mass correction of the S-factors was applied
when possible. Potential sources of uncertainty were closely examined: the
effect of partial body images, urinary bladder emptying, and biokinetic
modeling.
  Results: Large differences in doses between our methodology and the MIRD
method were found, generally in the range of $\pm$25%, and up to $\pm$120% for
some cases. The mass scaling showed improvements, especially for non-walled and
high-uptake tissues. Simulations of the urinary bladder emptying showed
negligible effects on doses to other organs, with the exception of the
prostate. Dosimetry based on partial PET/CT images (excluding the legs)
resulted in an over-estimation of mean doses to bone, skin, and remaining
tissues, and minor differences in other organs/tissues. Estimated uncertainties
associated with the biokinetics of FCH introduce variations of cumulated
activities in the range of $\pm$10% in the high-uptake organs.
  Conclusions: The MC methodology allows for a higher degree of dosimetry
individualization than the MIRD methodology, which in some cases leads to
important differences in dose values. Dosimetry of FCH-PET based on a single
partial PET study seems viable due to the particular biokinetics of FCH, even
though some correction factors may need to be applied to estimate mean
skin/bone doses.
