MABDOSE. I: Characterization of a general purpose dose estimation code

Dosimetry in targeted alpha therapy. A systematic review: current findings and what is needed

Objective. A systematic review of dosimetry in Targeted Alpha Therapy (TAT) has been performed, identifying the common issues. Approach. The systematic review was performed in accordance with the PRISMA guidelines, and the literature was searched using the Scopus and PubMed databases. Main results. From the systematic review, three key points should be considered when performing dosimetry in TAT. (1) Biodistribution/Biokinetics: the accuracy of the biodistribution data is a limit to accurate dosimetry in TAT. The biodistribution of alpha-emitting radionuclides throughout the body is difficult to image directly, with surrogate radionuclide imaging, blood/faecal sampling, and animal studies able to provide information. (2) Daughter radionuclides: the decay energy of the alpha-emissions is sufficient to break the bond to the targeting vector, resulting in a release of free daughter radionuclides in the body. Accounting for daughter radionuclide migration is essential. (3) Small-scale dosimetry and microdosimetry: due to the short path length and heterogeneous distribution of alpha-emitters at the target site, small-scale/microdosimetry are important to account for the non-uniform dose distribution in a target region, organ or cell and for assessing the biological effect of alpha-particle radiation. Significance. TAT is a form of cancer treatment capable of delivering a highly localised dose to the tumour environment while sparing the surrounding healthy tissue. Dosimetry is an important part of treatment planning and follow up. Being able to accurately predict the radiation dose to the target region and healthy organs could guide the optimal prescribed activity. Detailed dosimetry models accounting for the three points mentioned above will help give confidence in and guide the clinical application of alpha-emitting radionuclides in targeted cancer therapy.

Comparison of commercial dosimetric software platforms in patients treated with 177 Lu-DOTATATE for peptide receptor radionuclide therapy

Purpose

The aim of this study was to quantitatively compare five commercial dosimetric software platforms based on the analysis of clinical datasets of patients who benefited from peptide receptor radionuclide therapy (PRRT) with ¹⁷⁷Lu‐DOTATATE (LUTATHERA^®).

Methods

The dosimetric analysis was performed on two patients during two cycles of PRRT with ¹⁷⁷Lu. Single photon emission computed tomography/computed tomography images were acquired at 4, 24, 72, and 192 h post injection. Reconstructed images were generated using Dosimetry Toolkit^® (DTK) from Xeleris™ and HybridRecon‐Oncology version_1.3_Dicom (HROD) from HERMES. Reconstructed images using DTK were analyzed using the same software to calculate time‐integrated activity coefficients (TIAC), and mean absorbed doses were estimated using OLINDA/EXM V1.0 with mass correction. Reconstructed images from HROD were uploaded into PLANET® OncoDose from DOSIsoft, STRATOS from Phillips, Hybrid Dosimetry Module™ from HERMES, and SurePlan™ MRT from MIM. Organ masses, TIACs, and mean absorbed doses were calculated from each application using their recommendations.

Results

The majority of organ mass estimates varied by <9.5% between all platforms. The highest variability for TIAC results between platforms was seen for the kidneys (28.2%) for the two patients and the two treatment cycles. Relative standard deviations in mean absorbed doses were slightly higher compared with those observed for TIAC, but remained of the same order of magnitude between all platforms.

Conclusions

When applying a similar processing approach, results obtained were of the same order of magnitude regardless of the platforms used. However, the comparison of the performances of currently available platforms is still difficult as they do not all address the same parts of the dosimetric analysis workflow. In addition, the way in which data are handled in each part of the chain from data acquisition to absorbed doses may be different, which complicates the comparison exercise. Therefore, the dissemination of commercial solutions for absorbed dose calculation calls for the development of tools and standards allowing for the comparison of the performances between dosimetric software platforms.

Dosimetry software Hermes Internal Radiation Dosimetry: from quantitative image reconstruction to voxel-level absorbed dose distribution

OBJECTIVE

The aim of this work is to validate a software package called Hermes Internal Radiation Dosimetry (HIRD) for internal dose assessment tailored for clinical practice. The software includes all the necessary steps to perform voxel-level absorbed dose calculations including quantitative reconstruction, image coregistration and volume of interest tools.

METHODS

The basics of voxel-level dosimetry methods and implementations to HIRD software are reviewed. Then, HIRD is validated using simulated SPECT/CT data and data from Lu-DOTATATE-treated patients by comparing absorbed kidney doses with OLINDA/EXM-based dosimetry. In addition, electron and photon dose components are studied separately in an example patient case.

RESULTS

The simulation study showed that HIRD can reproduce time-activity curves accurately and produce absorbed doses with less than 10% error for the kidneys, liver and spleen. From the patient data, the absorbed kidney doses calculated using HIRD and using OLINDA/EXM were highly correlated (Pearson's correlation coefficient, r=0.98). From Bland-Altman plot analysis, an average absorbed dose difference of -2% was found between the methods. In addition, we found that in Lu-DOTATATE-treated patients, photons can contribute over 10% of the kidney's total dose and is partly because of cross-irradiation from high-uptake lesions close to the kidneys.

CONCLUSION

HIRD is a straightforward voxel-level internal dosimetry software. Its clinical utility was verified with simulated and clinical Lu-DOTATATE-treated patient data. Patient studies also showed that photon contribution towards the total dose can be relatively high and voxel-level dose calculations can be valuable in cases where the target organ is in close proximity to high-uptake organs.

The NUKDOS software for treatment planning in molecular radiotherapy

The aim of this work was the development of a software tool for treatment planning prior to molecular radiotherapy, which comprises all functionality to objectively determine the activity to administer and the pertaining absorbed doses (including the corresponding error) based on a series of gamma camera images and one SPECT/CT or probe data. NUKDOS was developed in MATLAB. The workflow is based on the MIRD formalism For determination of the tissue or organ pharmacokinetics, gamma camera images as well as probe, urine, serum and blood activity data can be processed. To estimate the time-integrated activity coefficients (TIAC), sums of exponentials are fitted to the time activity data and integrated analytically. To obtain the TIAC on the voxel level, the voxel activity distribution from the quantitative 3D SPECT/CT (or PET/CT) is used for scaling and weighting the TIAC derived from the 2D organ data. The voxel S-values are automatically calculated based on the voxel-size of the image and the therapeutic nuclide ((90)Y, (131)I or (177)Lu). The absorbed dose coefficients are computed by convolution of the voxel TIAC and the voxel S-values. The activity to administer and the pertaining absorbed doses are determined by entering the absorbed dose for the organ at risk. The overall error of the calculated absorbed doses is determined by Gaussian error propagation. NUKDOS was tested for the operation systems Windows(®) 7 (64 Bit) and 8 (64 Bit). The results of each working step were compared to commercially available (SAAMII, OLINDA/EXM) and in-house (UlmDOS) software. The application of the software is demonstrated using examples form peptide receptor radionuclide therapy (PRRT) and from radioiodine therapy of benign thyroid diseases. For the example from PRRT, the calculated activity to administer differed by 4% comparing NUKDOS and the final result using UlmDos, SAAMII and OLINDA/EXM sequentially. The absorbed dose for the spleen and tumour differed by 7% and 8%, respectively. The results from the example from radioiodine therapy of benign thyroid diseases and the example given in the latest corresponding SOP were identical. The implemented, objective methods facilitate accurate and reproducible results. The software is freely available.

JADA: a graphical user interface for comprehensive internal dose assessment in nuclear medicine

PURPOSE

The main objective of this work was to design a comprehensive dosimetry package that would keep all aspects of internal dose calculation within the framework of a single software environment and that would be applicable for a variety of dose calculation approaches.

METHODS

Our MATLAB-based graphical user interface (GUI) can be used for processing data obtained using pure planar, pure SPECT, or hybrid planar/SPECT imaging. Time-activity data for source regions are obtained using a set of tools that allow the user to reconstruct SPECT images, load images, coregister a series of planar images, and to perform two-dimensional and three-dimensional image segmentation. Curve fits are applied to the acquired time-activity data to construct time-activity curves, which are then integrated to obtain time-integrated activity coefficients. Subsequently, dose estimates are made using one of three methods.

RESULTS

The organ level dose calculation subGUI calculates mean organ doses that are equivalent to dose assessment performed by OLINDA/EXM. Voxelized dose calculation options, which include the voxel S value approach and Monte Carlo simulation using the EGSnrc user code DOSXYZnrc, are available within the process 3D image data subGUI.

CONCLUSIONS

The developed internal dosimetry software package provides an assortment of tools for every step in the dose calculation process, eliminating the need for manual data transfer between programs. This saves times and minimizes user errors, while offering a versatility that can be used to efficiently perform patient-specific internal dose calculations in a variety of clinical situations.

Patient-specific internal radionuclide dosimetry

The development of patient-specific treatment planning systems is of outmost importance in the development of radionuclide dosimetry, taking into account that quantitative three-dimensional nuclear medical imaging can be used in this regard. At present, the established method for dosimetry is based on the measurement of the biokinetics by serial gamma-camera scans, followed by calculations of the administered activity and the residence times, resulting in the radiation-absorbed doses of critical organs. However, the quantification of the activity in different organs from planar data is hampered by inaccurate attenuation and scatter correction as well as because of background and organ overlay. In contrast, dosimetry based on quantitative three-dimensional data can be more accurate and allows an individualized approach, provided that all effects that degrade the quantitative content of the images have been corrected for. In addition, inhomogeneous organ accumulation of the radionuclide can be detected and possibly taken into account. The aim of this work is to provide adequate information on internal emitter dosimetry and a state-of-the-art review of the current methodology and future trends.

Three-dimensional imaging-based radiobiological dosimetry

Targeted radionuclide therapy holds promise as a new treatment for cancer. Advances in imaging are making it possible for researchers to evaluate the spatial distribution of radioactivity in tumors and normal organs over time. Matched anatomical imaging, such as combined single-photon emission computed tomography/computed tomography and positron emission tomography/computed tomography, has also made it possible to obtain tissue density information in conjunction with the radioactivity distribution. Coupled with sophisticated iterative reconstruction algorithms, these advances have made it possible to perform highly patient-specific dosimetry that also incorporates radiobiological modeling. Such sophisticated dosimetry techniques are still in the research investigation phase. Given the attendant logistical and financial costs, a demonstrated improvement in patient care will be a prerequisite for the adoption of such highly-patient specific internal dosimetry methods.

A voxel-based mouse for internal dose calculations using Monte Carlo simulations (MCNP)

Murine models are useful for targeted radiotherapy pre-clinical experiments. These models can help to assess the potential interest of new radiopharmaceuticals. In this study, we developed a voxel-based mouse for dosimetric estimates. A female nude mouse (30 g) was frozen and cut into slices. High-resolution digital photographs were taken directly on the frozen block after each section. Images were segmented manually. Monoenergetic photon or electron sources were simulated using the MCNP4c2 Monte Carlo code for each source organ, in order to give tables of S-factors (in Gy Bq-1 s-1) for all target organs. Results obtained from monoenergetic particles were then used to generate S-factors for several radionuclides of potential interest in targeted radiotherapy. Thirteen source and 25 target regions were considered in this study. For each source region, 16 photon and 16 electron energies were simulated. Absorbed fractions, specific absorbed fractions and S-factors were calculated for 16 radionuclides of interest for targeted radiotherapy. The results obtained generally agree well with data published previously. For electron energies ranging from 0.1 to 2.5 MeV, the self-absorbed fraction varies from 0.98 to 0.376 for the liver, and from 0.89 to 0.04 for the thyroid. Electrons cannot be considered as 'non-penetrating' radiation for energies above 0.5 MeV for mouse organs. This observation can be generalized to radionuclides: for example, the beta self-absorbed fraction for the thyroid was 0.616 for I-131; absorbed fractions for Y-90 for left kidney-to-left kidney and for left kidney-to-spleen were 0.486 and 0.058, respectively. Our voxel-based mouse allowed us to generate a dosimetric database for use in preclinical targeted radiotherapy experiments.

Overview of dosimetry for systemic targeted radionuclide therapy (STaRT)

The purposes of systemic targeted radionuclide therapy dosimetry include compiling a database of normal organ radiation-absorbed doses that are carrier- and radionuclide-specific, and assuring that the normal organ radiation doses are within a safe range before therapy. Also of importance is quantitation of radiation delivery to tumors vs. normal tissues to correlate absorbed dose with tumor control. For agents with significant and variable excretion, estimates of individual patient distribution/clearance may be needed to optimize the dose-response relationship.

Internal radionuclide therapy: The ULMDOS software for treatment planning

Before therapy with unsealed radionuclides, a dosimetry assessment must be performed for each patient. We present the interactive software tool ULMDOS, which facilitates dosimetric calculations, enhances traceability, and adequate documentation. ULMDOS is developed in IDL 6.1 (Interactive Data Language) under Windows XP/2000. First the patient data, the radiotracer data, and optionally urine and serum data are entered. After loading planar gamma camera images and drawing regions of interest, the residence times can be calculated using fits of the time activity data to exponential functions. Data can be saved in ASCII format for retrospective examination and further processing. ULMDOS allows one to process the dosimetric calculations within a standardized environment, spares the time-consuming transfer of data between different software tools, enables the documentation of ROI and raw data, and reduces intraindividual variability. ULMDOS satisfies the required conditions for traceability and documentation as a prerequisite to routine use in clinical settings.

The two types of correction of absorbed dose estimates for internal emitters

BACKGROUND

Two types of correction for absorbed dose (D) estimates are described for clinical applications of internal emitters. The first is appropriate for legal and scientific reasons involving phantom-based estimates; the second is patient-specific and primarily intended for radioimmunotherapy (RIT).

METHODS

The Medical Internal Radiation Dose (MIRD) relationship (D) = S A is used, where S is a geometric matrix factor and A is the integral of source organ activities. Internal consistency of the data and the size of organ systems in the humanoid phantom must be assured in both types of estimation.

RESULTS

The first dose estimate correction (I) is one whereby computations refer to one or another standard (e.g., MIRD-type) phantom. In this case the S value remains as given, but the measured patient A data must be standardized. The correction factor is the phantom's ratio of organ mass to whole-body mass divided by the same ratio for the volunteer or patient. The second dose estimate correction (II) is patient-specific. While the A value is unchanged for this application, a correction term is provided for the phantom-derived S matrix. The dominant (nonpenetrating radiation) component of this correction factor can be obtained via the ratio of the patient to phantom organ masses. In both corrections, we recommend that true organ sizes, necessary in each method of estimation, be determined in a separate sequence of anatomic images.

CONCLUSIONS

In both dose estimation corrections, true sizes of the patient's or volunteer's internal organs must be obtained. Correction due to organ mass size can be severalfold and is probably the dominant uncertainty in the internal emitter absorbed dose calculation process.

Biokinetics of yttrium-90--labeled huBrE-3 monoclonal antibody

BACKGROUND

This study reports summary biokinetics for 17 patients treated with huBrE-3 antibody labeled with indium-111 ((111)In) and yttrium-90 ((90)Y) in a Phase I dose escalation trial.

METHODS

Patients were infused with huBrE-3 antibody conjugated to 1-p-isothiocyanatobenzyl 3-methyl- and 1-p-isothiocyanatobenzyl 4-methyl-diethylenetriamine pentaacetic acid (MX-DTPA). The huBrE-3 was labeled with increasing amounts of (90)Y radioactivity according to the following activity regimen: 10 mCi/m(2), 20 mCi/m(2), 33 mCi/m(2), 50 mCi/m(2), and 70 mCi/m(2). In addition to the (90)Y activity, 3--5 mCi of (111)In was labeled to huBrE-3 to serve as an imaging agent. In characterizing the biokinetics of huBrE-3, serial urine and blood samples were acquired. Additionally, whole-body imaging using a scintillation camera was performed at four time points postinfusion.

RESULTS

Cumulative urine data yielded a plot of total-body biologic excretion that was relatively flat. Total body regions of interest derived from nuclear medicine scintigrams decreased according to a monoexponential function with a slope slightly greater than the rate of physical decay. When physical decay was combined with the urine biologic excretion rate, the calculated rate of activity decrease was indistinguishable from the scintigraphic rate of decrease in total-body activity.

CONCLUSIONS

The authors concluded from these observations that the radioactivity remains essentially inside the patient, that biologic excretion of activity from the total body is not appreciable, and that the path for biologic excretion of activity that does occur is via the urine. The half-time associated with the beta (slow) phase for extraction from the blood averages 40.5 hours. Since large amounts of radioactivity do not appear in the urine, and total-body activity is decreased approximately according to physical decay (64.1 hours), activity must pool elsewhere after leaving the blood. The logical place is the skeleton, with possible selective binding to the bone marrow. Bone marrow biopsies from 4 of 7 patients who consented to serial biopsies were supportive of this conclusion.

Estimate of absorbed dose based on two-dimensional autoradiographic information in internal radionuclide therapy

In radiation therapies using radionuclides emitting short-range particles, such as radioimmunotherapy or boron neutron capture therapy, the biological effects are strongly affected by the heterogeneity of the absorbed dose distribution delivered to tumor cells. The three-dimensional (3D) information of the source distribution at the cellular level is required to accurately determine the absorbed dose distribution to the individual tumor cells. Two-dimensional distribution of cell and nuclide with a resolution of 1 microm can be obtained from individual tissue sections by microautoradiography. To obtain such information in 3D, an ideal approach would be to align the serial tissue sections from a block and analyze all of them. This is straightforward in theory, but extremely difficult in practice. Furthermore, every section in the block has to be processed and analyzed, and the usage of the data from this laborious work is very inefficient. An approach presented here is to estimate the absorbed dose based on individual sections without 3D reconstruction. It is realistically workable since it avoids the most difficult task of alignment for the serial tissue sections. In addition, the absorbed dose can be estimated based on a limited number of noncontiguous sections. The validity of this approach has been tested by a Monte Carlo simulation for two representative radionuclide configurations: (a) a uniform distribution of sources and (b) a cell membrane bound source distribution. With only a limited number of sampling sections, the uncertainties in the dose estimation were estimated to approximately 15% for short-range particles.

Absorbed fractions in a voxel-based phantom calculated with the MCNP-4B code

A new approach for calculating internal dose estimates was developed through the use of a more realistic computational model of the human body. The present technique shows the capability to build a patient-specific phantom with tomography data (a voxel-based phantom) for the simulation of radiation transport and energy deposition using Monte Carlo methods such as in the MCNP-4B code. MCNP-4B absorbed fractions for photons in the mathematical phantom of Snyder et al. agreed well with reference values. Results obtained through radiation transport simulation in the voxel-based phantom, in general, agreed well with reference values. Considerable discrepancies, however, were found in some cases due to two major causes: differences in the organ masses between the phantoms and the occurrence of organ overlap in the voxel-based phantom, which is not considered in the mathematical phantom.

MABDOSE. II: Validation of a general purpose dose estimation code

The MABDOSE software represents a general tool to assess internal radiation dose. A suite of tests are described that validate the dosimetry system's implementation.

METHODS

The validation suite is divided among tests that verify target digitalization, tumor digitalization and organ replacement, cumulated activity calculation, random number generation, radiation transport, and dose calculation.

RESULTS

A comparison between Reference Man organ volumes and MABDOSE organ volumes at (5 mm)3 resolution demonstrates volume correspondence within 10% save for ten organs having dimensions smaller than the target lattice resolution. An accounting of normal organ volume replaced by an arbitrary tumor volume indicates mass is conserved. A comparison between cumulated activities generated by MABDOSE and solutions obtained analytically demonstrates exact correspondence for curve-fitting algorithms. For mathematical modeling algorithms, cumulated activity solutions converge to their correct values provided sufficient data of high precision are input, accompanied by reasonable initial estimates of rate constants. A comparison of MABDOSE results with the MIRD 3 report demonstrates good agreement (<8% difference) in absorbed fractions for spheres at energies from 20 keV to 2.75 MeV. A comparison of MABDOSE results with the Cristy-Eckerman report demonstrates marginal agreement (specific absorbed fractions within a factor of 2 for all Reference Man organs) at simulation energies of 20, 50, and 100 keV. Lack of exact correspondence is attributed to volume digitalization errors, and to differences in cross-section libraries, interpolation schemes between cross-section data points, and random number generators. Finally, the doses reported by MABDOSE correspond to the correct algebraic combination of paired cumulated activities and "S" values.

CONCLUSIONS

The MABDOSE program has been validated as a general purpose computation tool for use in internal radionuclide dosimetry.