Effect of patient morphology on dosimetric calculations for internal irradiation as assessed by comparisons of Monte Carlo versus conventional methodologies.
Dosimetric calculations are performed with an increasing frequency before or after treatment in targeted radionuclide therapy, as well as for radiation protection purposes in diagnostic nuclear medicine. According to the MIRD committee formalism, the mean absorbed dose to a target is given by the product of the cumulated activity and a dose-conversion factor, known as the S factor. Standard S factors have been published for mathematic phantoms and for unit-density spheres. The accuracy of the results from the use of these S factors is questionable, because patient morphology can vary significantly. The aim of this work was to investigate differences between patient-specific dosimetric results obtained using Monte Carlo methodology and results obtained using S factors calculated on standard models.


METHODS
The CT images of 9 patients, who ranged in size, were used. Patient-specific S factors for 131I were calculated with the MCNPX2.5.0 Monte Carlo code using a tool for personalized internal dose assessment, OEDIPE; standard S factors from OLINDA/EXM were compared against the patient-specific S factors. Furthermore, realistic biodistributions and cumulated activities for normal organs and tumors were used, and mean organ- and tumor-absorbed doses calculated with OEDIPE and OLINDA/EXM were compared.


RESULTS
The ratio of the standard and the patient-specific S factors were between 0.49 and 1.84 for a target distant from the source for 4 organs and 2 tumors studied as source and targets. For the case of self-irradiation, the equivalent ratio ranged between 0.45 and 2.47 and between 1.00 and 1.06 when mass correction was applied. Differences in mean absorbed doses were as high as 140% when realistic cumulated activity values were used. These values decreased to less than 26% in all cases studied when mass correction was applied to the self-irradiation given by OLINDA/EXM.


CONCLUSIONS
Standard S factors can yield mean absorbed doses for normal organs or tumors with a reasonable accuracy (26% for the cases studied) as compared with absorbed doses calculated with Monte Carlo, provided that they have been corrected for mass.
Divoli
Antigoni
A
Joint Department of Physics, Institute of Cancer Research and the Royal Marsden NHS Foundation Trust, Sutton, United Kingdom. antigoni.divoli@icr.ac.uk
Chiavassa
Sophie
S
Ferrer
Ludovic
L
Barbet
Jacques
J
Flux
Glenn D
GD
Bardiès
Manuel
M
D000328:Adult; D000368:Aged; D001827:Body Height; D049628:Body Size; D001835:Body Weight; D006801:Humans; D008297:Male; D008875:Middle Aged; D008953:Models, Anatomic; D009010:Monte Carlo Method; D009369:Neoplasms / Q000000981:diagnostic imaging / Q000473:pathology / Q000532:radiotherapy; D009929:Organ Size; D019047:Phantoms, Imaging; D011874:Radiometry / Q000379:methods* / Q000706:statistics & numerical data; D011877:Radionuclide Imaging; D014057:Tomography, X-Ray Computed