Dosimetric comparison of Monte Carlo codes (EGS4, MCNP, MCNPX) considering external and internal exposures of the Zubal phantom to electron and photon sources

A microdosimetry model of kidney by GATE Monte Carlo simulation using a nonuniform activity distribution in digital phantom of nephron

OBJECTIVES

As the main pathway for the clearance of radiopharmaceutical from the body, kidney is a dose-limiting organ in medical application of radionuclides. Because of its unique physiology, radioactivity is seen to concentrate on kidney nonuniformly. This nonuniformity can be considered in nephron microstructures. A microdosimetry model of kidney is necessary to include the nonuniform distribution in internal radiation dosimetry.

METHOD

Implementing the microdosimetry model requires, first, a geometry phantom of nephrons. Stylized phantoms cannot distribute activities inside nephron compartments nonuniformly. A phantom of nephron was generated by a preliminary three-dimensional graphic model and was converted to a proper format of digital phantom. The phantom was fed to GATE Monte Carlo toolkits. Simulations were performed and S-values for five radionuclides (Tc-99m, In-111, Lu-177, Ac-225 and Bi-212) were calculated and compared with corresponding results published in the literature derived with a stylized phantom of nephron. Activity was distributed nonuniformly according to the kinetics of two mainly used diagnostic tracers (diethylenetriaminepetaacetate and ethylenedicysteine) and absorbed dose of nephron cells were calculated.

RESULTS

A good correlation was shown between the generated phantom microdosimetry model and stylized model and revealed the phantom can be used for future microdosimetry studies of kidney to evaluate radiobiological effects of internal radiation from various diagnostic and therapeutic radiopharmaceuticals. Absorbed dose of cells for nonuniform distribution showed that some cells in a nephron compartment receive higher dose than (more than two-fold) that of compartment average dose.

CONCLUSION

Average dose of nephron is not a reliable parameter for nephrotoxicity evaluation.

Impact of PET and MRI threshold-based tumor volume segmentation on patient-specific targeted radionuclide therapy dosimetry using CLR1404

Variations in tumor volume segmentation methods in targeted radionuclide therapy (TRT) may lead to dosimetric uncertainties. This work investigates the impact of PET and MRI threshold-based tumor segmentation on TRT dosimetry in patients with primary and metastatic brain tumors. In this study, PET/CT images of five brain cancer patients were acquired at 6, 24, and 48 h post-injection of ¹²⁴I-CLR1404. The tumor volume was segmented using two standardized uptake value (SUV) threshold levels, two tumor-to-background ratio (TBR) threshold levels, and a T1 Gadolinium-enhanced MRI threshold. The dice similarity coefficient (DSC), jaccard similarity coefficient (JSC), and overlap volume (OV) metrics were calculated to compare differences in the MRI and PET contours. The therapeutic ¹³¹I-CLR1404 voxel-level dose distribution was calculated from the ¹²⁴I-CLR1404 activity distribution using RAPID, a Geant4 Monte Carlo internal dosimetry platform. The TBR, SUV, and MRI tumor volumes ranged from 2.3-63.9 cc, 0.1-34.7 cc, and 0.4-11.8 cc, respectively. The average  ±  standard deviation (range) was 0.19  ±  0.13 (0.01-0.51), 0.30  ±  0.17 (0.03-0.67), and 0.75  ±  0.29 (0.05-1.00) for the JSC, DSC, and OV, respectively. The DSC and JSC values were small and the OV values were large for both the MRI-SUV and MRI-TBR combinations because the regions of PET uptake were generally larger than the MRI enhancement. Notable differences in the tumor dose volume histograms were observed for each patient. The mean (standard deviation) ¹³¹I-CLR1404 tumor doses ranged from 0.28-1.75 Gy GBq^-1 (0.07-0.37 Gy GBq^-1). The ratio of maximum-to-minimum mean doses for each patient ranged from 1.4-2.0. The tumor volume and the interpretation of the tumor dose is highly sensitive to the imaging modality, PET enhancement metric, and threshold level used for tumor volume segmentation. The large variations in tumor doses clearly demonstrate the need for standard protocols for multimodality tumor segmentation in TRT dosimetry.

Voxel dosimetry: Comparison of MCNPX and DOSXYZnrc Monte Carlo codes in patient specific phantom calculations

INTRODUCTION

Dose evaluation with two Monte Carlo codes using patient specific voxel phantom is presented in this paper. We employ both MCNPX and DOSXYZnrc to perform dosimetry for mathematical voxel phantoms generated by our in-house developed voxel phantom generator and EGSnrc/CTCreate respectively.

MATERIAL AND METHOD

Our case study was a 2.5 × 2.4 × 2.4 cm3 tumor in the middle lobe of right lung of a male patient exposed to 6MV parallel beam. In order to compare these Monte Carlo codes with together gross tumor volume (GTV) and organ at risks (OAR) doses and dose volume histograms (DVH) were calculated.

RESULTS

Comparing the mean absorbed dose results (in Gy) from both codes indicates that gross tumor volume, heart and spinal cord have 2% to 10% difference. The 10% difference between the codes were from the spinal cord region where was not in the therapy beam and it just received the scatter radiation. The dose volume DVH obtained from DOSXYZnrc results demonstrate a milder slope compared with MCNPX DVHs.

CONCLUSION

It was revealed that MCNPX has some advantages in comparison to DOSXYZnrc, but it is important to consider that for equal precision in voxel dosimetry calculation, DOSXYZnrc runs faster than MCNPX and it is a great advantage.

Paired organs-should they be treated jointly or separately in internal dosimetry?

PURPOSE

Size, shape, and the position of paired organs are different in abdomen. However, the counterpart organs are conventionally treated jointly together in internal dosimetry. This study was performed to quantify the difference of specific absorbed fraction of organs in considering paired organs jointly like single organs or as two separate organs.

METHODS

Zubal phantom and GATE Monte Carlo package were used to calculate the SAF for the self-absorption and cross-irradiation of the lungs, kidneys, adrenal glands (paired organs), liver, spleen, stomach, and pancreas (single organs). The activity was assumed uniformly distributed in the organs, and simulation was performed for monoenergetic photons of 10, 50, 100, 500, 1000 keV and mono-energetic electrons of 350, 500, 690, 935, 1200 keV.

RESULTS

The results demonstrated that self-absorption of left and right counterpart organs may be different depending upon the differences in their masses. The cross-irradiations between left-to-right and right-to-left counterpart organs are always equal irrespective of difference in their masses. Cross-irradiation from the left and right counterpart organs to other organs are different (4-24 times in Zubal phantom) depending on the photon energy and organs. The irradiation from a single source organ to the left and right counterpart paired organs is always different irrespective of activity concentration.

CONCLUSIONS

Left and right counterpart organs always receive different absorbed doses from target organs and deliver different absorbed doses to target organs. Therefore, in application of radiopharmaceuticals in which the dose to the organs plays a role, counterpart organs should be treated separately as two separate organs.© 2011 American Association of Physicists in Medicine.

The minimum crystal size needed for a complete diffraction data set

In this work, classic intensity formulae were united with an empirical spot-fading model in order to calculate the diameter of a spherical crystal that will scatter the required number of photons per spot at a desired resolution over the radiation-damage-limited lifetime. The influences of molecular weight, solvent content, Wilson B factor, X-ray wavelength and attenuation on scattering power and dose were all included. Taking the net photon count in a spot as the only source of noise, a complete data set with a signal-to-noise ratio of 2 at 2 A resolution was predicted to be attainable from a perfect lysozyme crystal sphere 1.2 microm in diameter and two different models of photoelectron escape reduced this to 0.5 or 0.34 microm. These represent 15-fold to 700-fold less scattering power than the smallest experimentally determined crystal size to date, but the gap was shown to be consistent with the background scattering level of the relevant experiment. These results suggest that reduction of background photons and diffraction spot size on the detector are the principal paths to improving crystallographic data quality beyond current limits.

Current developments at IRSN on computational tools dedicated to assessing doses for both internal and external exposure

The paper presents the OEDIPE (French acronym that stands for tool for personalised internal dose assessment) and SESAME (for simulation of external source accident with medical images) computational tools, dedicated to internal and external dose assessment, respectively, and currently being developed at the Institute for Radiological Protection and Nuclear Safety. The originality of OEDIPE and SESAME, by using voxel phantoms in association with Monte Carlo codes, lies in their ability to construct personalised voxel phantoms from medical images and automatically generate the Monte Carlo input file and visualise the expected results. OEDIPE simulates in vivo measurements to improve their calibration, and calculates the dose distribution taking both internal contamination and internal radiotherapy cases into account. SESAME enables radiological overexposure doses to be reconstructed, as also victim, source and accident environment modelling. The paper presents the principles on which these tools function and an overview of specificities and results linked to their fields of application.