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smaller and larger tumors( Zweit, 1996; Ljungberg et al., 2016). It is recommended to use the appropriate cocktail of these radionuclides due to different size of metastatic tumors. Another important problem in radionuclide therapy is the prevention of critical organ radiotoxicity. Although a radionuclide may have a high efficacy but high absorbed dose in critical organs close to the target organ makes it an unsuitable selection for therapy( Kunikowska et al., 2011; Huizing et al., 2018). An important factor affecting critical organ dose is the size of that organ, which isdepended on BMI.
There are a few studies that compare the differences between the absorbed doses between 90 Y-DOTATATE and 177 Lu-DOTATAE( Segars et al., 2017). Most study in this field
use simple stylized phantom such as MIRD and do not consider anatomical details of organs( Mattsson, 2015). Human-simulating computational phantoms have become an important tool for dosimetric calculations in medical imaging and radiation therapy. Using these phantom in Monte Carlo simulation can lead to more convenient and also yield reliable results. In this study we used male CRP phantom to perform an accurate estimation of absorbed dose in organs of interest. A primary aim of this work was to assess differences in 177 Lu and 90 Y- DOTATATE dose coefficient( dose per unit cumulated
activity) named S-values estimations using various patient morphologies. S-values of 177 Lu and 90 Y- DOTATATE in different body mass indexes( BMIs) of ICRP( Pinto et al., 2020) male phantoms were calculated for pre-estimation of absorbed dose in critical target organs using GATE( GEANT4 Application to Tomographic Emission). We then investigated differences due to phantom format by comparing dose coefficients.
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Materials and methods
All simulations in this study were performed using GATE version 8.2 based on the Geant4( Geometry And Tracking, version 10.4).
We used the method developed by Segars et al.( 2017) to produce six total body ICRP male hantoms with different BMIs. Some parameters of phantom such as BMI are changeable. The reference mass and height are 60 kg and 163 cm respectively( BMI = 22.6 kg / cm 2). The adult mesh reference phantoms were reshaped and scaled to culminating in an additional 6 male phantomphantoms representing the 10th, 50th, and 90th percentiles of standing height and weight corresponding to BMI of 23, 24.9, 27.1, 28.4, 29.3, 34.5. The matrix size of each phantom was 128 128 300 with a voxel size of 3.125 mm 3.125 mm 3.125 mm to cover the desired region of the body( Fig. 1). Dosimetry calculations were performed for 20 MBq activity of 177 Lu-DOTATATE and 90 Y- DOTATATE that were distributed uniformly in each
voxels of the source organs.
The detailed decay data and emission spectra of selected radionuclides were taken from MIRD: radionuclide data and decay schemes( Goddu, 1997). For each radionuclide, the emitted energy spectra( gamma, X-ray, beta, internal conversion, Auger electrons and alpha) were considered independently to estimate the relative contribution of each radiation independently. The transitions for each radionuclide are summarized in Table 1.
Fig. 1. ICRP phantom that covers the interested organs.
Simulations were performed by Intel ® Core TM i5 Processors with 6 GB RAM. In total, 10 7 histories were simulated for each phantom. After dosimetry simulations with GATE, the output is two binary files containing the absorbed dose in voxels( in cGy) and the corresponding uncertainties respectively. Using MATLAB( version R2017b), results were converted to numeric values. We chose spine, bladder, kidneys, and salivary glands as the source organs according to the biodistribution of selected radiopharmacuticals and we calculated the S-value( in units of mGy / MBq. s) based on Medical International Radiation Dose( MIRD) committee guideline [ 13 ] in kidneys, spleen, liver, and bladder as the target organs. several target regions are defined in the ICRP phantom that comprise multiple sub regions( e. g. kidneys, comprising left and right renal pelvis, cortex, and medulla). The mass weighted combination of absorbed doses( in unit of mGy per MBq. s) were calculated as the mean dose to such regions according to the equation( 1).
S mreg ¼ X MrT∈mreg ð Þ
SrT∈mreg ð Þ ð1Þ rT∈mreg Mmreg ð Þ
where S mreg is the S-value for a multiregional target; M( rT ∈ mreg) is the mass of a region rT comprising the multiregional target; M( mreg) is the total mass of the