Effect of Patient Morphology on Dosimetric Calculations for Internal Irradiation as Assessed by Comparisons of Monte Carlo Versus Conventional Methodologies

Abstract

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 131 I 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. Conclusion: 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.

Prediction of Therapy Tumor-Absorbed Dose Estimates in I-131 Radioimmunotherapy Using Tracer Data Via a Mixed-Model Fit to Time Activity

“…Because of the limitations of this model, alternative approaches to tumor dosimetry using dose-point kernel or Monte Carlobased methods have been used in some recent internal emitter therapy studies. 19,20 In the present study, we used SPECT/CT-imaging data coupled with the Dose-Planning Method (DPM) Monte Carlo algorithm 21 to carryout patientspecific estimation of the mean radiation-absorbed dose to tumor. The calculation also provided 3D dose distributions and results of the 3D dose measures, including dose response, were reported in a previous publication 3 and are not the focus of the present work.…”

Power output and efficiency of beta-emitting microspheres

“…These models are generally formulated using either solutions of the transport equations [40,41], Monte Carlo calculations [39,[42][43], or convolutions of tabulated data from experimental sources such as thin foil [44] or PET/SPECT measurements [45,46]. Each of these methods have their advantages and limitations.…”

“…Solving the transport equations, as is used here for instance, is a fast and robust method but without the previously mentioned mass attenuation coefficients and analytical solutions for the spherical geometry, requires significant numerical calculations [41]. Similarly, Monte Carlo methods have been very successfully used in dosimetry, but require substantial computational effort to generate sufficient data sets [42,43]. Fortunately codes such as FLUKA [47] and Geant4 [48] are becoming increasingly robust and sophisticated and are being adapted for this purpose.…”

Comparison of I-131 Radioimmunotherapy Tumor Dosimetry: Unit Density Sphere Model Versus Patient-Specific Monte Carlo Calculations

“…16 The underestimation of tumor dose due to neglecting this contribution was found to be in the range 10%-20%, which is comparable to the results of the present study. In a more recent study, focusing mainly on normal organ dosimetry Divoli et al 17 reported on the differences between results obtained using patient-specific Monte Carlo methodology and results obtained using the reference S-values implemented in OLINDA. Individual patient's CT images (total of 9 patients) were used to define the normal organs, whereas published biodistribution data were used to assign cumulated activities.…”

“…Previous reports on such comparisons have either focused on organ dosimetry or were limited to simulated tumor geo metries. 16,17 The goal of the present study is to compare mean tumor absorbed dose estimates using the unit sphere model incorporated in OLINDA with dose estimates from previously reported Monte Carlo calculations for NHL patients. Unlike previous reports, the present study uses actual patient SPECT/CT imaging data and tumors of varying size, shape, and uptake relative to background for this comparison.…”