Development and validation of RAYDOSE: a Geant4-based application for molecular radiotherapy

An automation of Monte Carlo workflow for dosimetry study of an Elekta LINAC delivery system in radiotherapy

Purpose

This study aims to automate the Monte Carlo (MC) workflow utilized for radiotherapy dosimetry, focusing on an Elekta LINAC delivery system. It addresses the challenge of integrating MC simulations into routine clinical practice, making this accurate yet complex method more accessible and efficient for radiotherapy dosimetry.

Methods and Materials

We developed a user-friendly software featuring a graphical user interface (GUI) that integrates EGSnrc for MC simulations. The software streamlines the process from retrieving Digital Imaging and Communications in Medicine (DICOM) data to executing dose calculations and comparing dose distributions. To validate our proposed tool, we compared its computed doses for IMRT and VMAT plans from the Pinnacle TPS for an Elekta Versa HD linear accelerator against MC simulation results. This comparison utilized our in-house software and GUI as the tool, covering various treatment sites and prescriptions.

Results

The automated MC workflow demonstrated high accuracy in dose calculations and streamlined integration with clinical workflows. The comparison between the MC-simulated and TPS-calculated doses revealed excellent agreement, highlighting the reliability of MC for independent dose verification in complex treatment scenarios.

Conclusions

The automated MC workflow developed represents a substantial improvement in the practicality and efficiency of MC simulations in radiotherapy. This advancement not only simplifies the dosimetry process but also ensures high accuracy, establishing it as a valuable tool for routine patient-specific quality assurance and the development of specialized treatment procedures.

A single centre intercomparison between commercial treatment planning systems for 90Y radioembolization using virtual and experimental phantoms

INTRODUCTION

Dedicated Treatment Planning Systems (TPSs) were developed to personalize 90Y-transarterial radioembolization. This study evaluated the agreement among four commercial TPSs assessing volumes of interest (VOIs) volumes and dose metrics.

METHODS

A homogeneous (EH) and an anthropomorphic phantom with hot and cold inserts (EA) filled with 99mTc-pertechnetate were acquired with a SPECT/CT scanner. Their virtual versions (VH and VA, respectively) and a phantom with activity inside a single voxel (VK) were generated by an in-house MATLAB script. Images and delineated VOIs were imported into the TPSs to compute voxel-based absorbed dose distributions with various dose deposition approaches: local deposition method (LDM) and dose kernel convolution (DKC) with/without local density correction (LDC). VOI volumes and mean absorbed doses were assessed against their median value across TPSs. Dose-volume histograms (DVHs) and VK-derived dose profiles were evaluated.

RESULTS

Small (<2.1 %) and large (up to 42.4 %) relative volume differences were observed on large (>500 ml) and small VOIs, respectively. Mean absorbed doses relative differences were < 3 % except for small VOIs with steep dose gradients (up to 89.1 % in the VA Cold Sphere VOI). Within the same TPS, LDC negligibly affected the mean absorbed dose, while DKC and LDM showed differences up to 63 %. DHVs were mostly overlapped in experimental phantoms, with some differences in the virtual versions. Dose profiles agreed within 1 %.

CONCLUSION

TPSs showed an overall good agreement except for small VOI volumes and mean absorbed doses of VOIs with steep dose gradients. These discrepancies should be considered in the dosimetry uncertainty assessment, thus requiring an appropriate harmonization.

Creation of waterproof, TLD probes for dose measurements to validate image-based radiopharmaceutical therapy dosimetry workflow

Voxel-level dosimetry based on nuclear medicine images offers patient-specific personalization of radiopharmaceutical therapy (RPT) treatments. Clinical evidence is emerging demonstrating improvements in treatment precision in patients when voxel-level dosimetry is used compared to MIRD. Voxel-level dosimetry requires absolute quantification of activity concentrations in the patient, but images from SPECT/CT scanners are not quantitative and require calibration using nuclear medicine phantoms. While phantom studies can validate a scanner's ability to recover activity concentrations, these studies provide only a surrogate for the true metric of interest: absorbed doses. Measurements using thermoluminescent dosimeters (TLDs) are a versatile and accurate method of measuring absorbed dose. In this work, a TLD probe was manufactured that can fit into currently available nuclear medicine phantoms for the measurement of absorbed dose of RPT agents. Next, 748 MBq of I-131 was administered to a 16 ml hollow source sphere placed in a 6.4 L Jaszczak phantom in addition to six TLD probes, each holding 4 TLD-100 1 × 1 × 1 mm TLD-100 (LiF:Mg,Ti) microcubes. The phantom then underwent a SPECT/CT scan in accordance with a standard SPECT/CT imaging protocol for I-131. The SPECT/CT images were then input into a Monte Carlo based RPT dosimetry platform named RAPID and a three dimensional dose distribution in the phantom was estimated. Additionally, a GEANT4 benchmarking scenario (denoted 'idealized') was created using a stylized representation of the phantom. There was good agreement for all six probes, the differences between measurement and RAPID ranged between -5.5% and 0.9%. The difference between the measured and the idealized GEANT4 scenario was calculated and ranged from -4.3% and -20.5%. This work demonstrates good agreement between TLD measurements and RAPID. In addition, it introduces a novel TLD probe that can be easily introduced into clinical nuclear medicine workflows to provide QA of image-based dosimetry for RPT treatments.

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.

Pre- and post-treatment image-based dosimetry in90Y-microsphere radioembolization using the TOPAS Monte Carlo toolkit

Objective. To evaluate the pre-treatment and post-treatment imaging-based dosimetry of patients treated with 90Y-microspheres, including accurate estimations of dose to tumor, healthy liver and lung. To do so, the Monte Carlo (MC) TOPAS platform is in this work extended towards its utilization in radionuclide therapy.Approach. Five patients treated at the Massachusetts General Hospital were selected for this study. All patients had data for both pre-treatment SPECT-CT imaging using 99mTc-MAA as a surrogate of the 90Y-microspheres treatment and SPECT-CT imaging immediately after the 90Y activity administration. Pre- and post-treatment doses were computed with TOPAS using the SPECT images to localize the source positions and the CT images to account for tissue inhomoegeneities. We compared our results with analytical calculations following the voxel-based MIRD scheme.Main results. TOPAS results largely agreed with the MIRD-based calculations in soft tissue regions: the average difference in mean dose to the liver was 0.14 Gy GBq^-1(2.6%). However, dose distributions in the lung differed considerably: absolute differences in mean doses to the lung ranged from 1.2 to 6.3 Gy GBq^-1and relative differences from 153% to 231%. We also found large differences in the intra-hepatic dose distributions between pre- and post-treatment imaging, but only limited differences in the pulmonary dose.Significance. Doses to lung were found to be higher using TOPAS with respect to analytical calculations which may significantly underestimate dose to the lung, suggesting the use of MC methods for 90Y dosimetry. According to our results, pre-treatment imaging may still be representative of dose to lung in these treatments.

Overview of commercial treatment planning systems for targeted radionuclide therapy

INTRODUCTION

Targeted Radionuclide Therapy (TRT) is a branch of cancer medicine dealing with the therapeutic use of radioisotopes associated with biological vectors accumulating in the tumors/targets, indicated as Molecular Radiotherapy (MRT), or directly injected into the arteries that supply blood to liver tumour vasculature, indicated as Selective RT (SRT). The aim of this work is to offer a panoramic view on the increasing number of commercially-available TRT treatment planning systems (TPSs).

MATERIALS AND METHODS

A questionnaire was sent to manufacturers' representatives. Academic software were not considered. Questions were grouped as follows: general information, clinical workflow, calibration procedure, image processing/reconstruction, image registration and segmentation tools, time-activity curve (TAC) fitting and absorbed dose calculation.

RESULTS

All software reported have CE-marking. TPSs were divided between SRT-dedicated software [4] and MRT [5] dosimetry software. In SRT, since no kinetic process is involved, absorbed dose calculation does not require TAC fitting, and image registration is not fully developed in all TPS. All software requires a radionuclide-specific calibration. In SRT, a relative image calibration can be obtained by scaling the counts to a known activity. Automated VOI contouring and rigid/deformable propagation between different acquisitions time-points is implemented in most TPSs, although DICOM export is rare. Different TAC fits are available depending on the number of time-points. Voxel S-value and Local deposition methods are the most frequent dosimetric approaches; dose-voxel kernel convolution and semi-Monte Carlo method are also available.

CONCLUSIONS

Available TPSs allows performing personalized dosimetry in clinical practice. Individual variations in methodology/algorithms must be considered in the standardisation/harmonization processes.

Development of a Compartmental Pharmacokinetic Model for Molecular Radiotherapy with 131I-CLR1404

Pharmacokinetic modeling of the radiopharmaceuticals used in molecular radiotherapy is an important step towards accurate radiation dosimetry of such therapies. In this paper, we present a pharmacokinetic model for CLR1404, a phospholipid ether analog that, labeled with ¹²⁴I/¹³¹I, has emerged as a promising theranostic agent. We follow a systematic approach for the model construction based on a decoupling process applied to previously published experimental data, and using the goodness-of-fit, Sobol’s sensitivity analysis, and the Akaike Information Criterion to construct the optimal form of the model, investigate potential simplifications, and study factor prioritization. This methodology was applied to previously published experimental human time-activity curves for 9 organs. The resulting model consists of 17 compartments involved in the CLR1404 metabolism. Activity dynamics in most tissues are well described by a blood contribution plus a two-compartment system, describing fast and slow uptakes. The model can fit both clinical and pre-clinical kinetic data of ¹²⁴I/¹³¹I. In addition, we have investigated how simple fits (exponential and biexponential) differ from the complete model. Such fits, despite providing a less accurate description of time-activity curves, may be a viable alternative when limited data is available in a practical case.

Quantification of internal dosimetry in PET patients II: Individualized Monte Carlo-based dosimetry for [18F]fluorocholine PET

PURPOSE

To obtain individualized internal doses with a Monte Carlo (MC) method in patients undergoing diagnostic [18F]FCH-PET studies and to compare such doses with the MIRD method calculations.

METHODS

A patient cohort of 17 males were imaged after intravenous administration of a mean [18F]FCH activity of 244.3 MBq. The resulting PET/CT images were processed in order to generate individualized input source and geometry files for dose computation with the MC tool GATE. The resulting dose estimates were studied and compared to the MIRD method with two different computational phantoms. Mass correction of the S-factors was applied when possible. Potential sources of uncertainty were closely examined: the effect of partial body images, urinary bladder emptying, and biokinetic modeling.

RESULTS

Large differences in doses between our methodology and the MIRD method were found, generally in the range ±25%, and up to ±120% for some cases. The mass scaling showed improvements, especially for non-walled and high-uptake tissues. Simulations of the urinary bladder emptying showed negligible effects on doses to other organs, with the exception of the prostate. Dosimetry based on partial PET/CT images (excluding the legs) resulted in an overestimation of mean doses to bone, skin, and remaining tissues, and minor differences in other organs/tissues. Estimated uncertainties associated with the biokinetics of FCH introduce variations of cumulated activities in the range of ±10% in the high-uptake organs.

CONCLUSIONS

The MC methodology allows for a higher degree of dosimetry individualization than the MIRD methodology, which in some cases leads to important differences in dose values. Dosimetry of FCH-PET based on a single partial PET study seems viable due to the particular biokinetics of FCH, even though some correction factors may need to be applied to estimate mean skin/bone doses.

Managing radioiodine refractory thyroid cancer: the role of dosimetry and redifferentiation on subsequent I-131 therapy

Poor responses to iodine-131 (I-131) therapy can relate to either low iodine uptake and retention in thyroid cancer cells or to increased radioresistance. Both mechanisms are currently termed radioactive iodine (RAI)-refractory (RAI-R) thyroid cancer but the first reflects unsuitability for I-131 therapy that can be evaluated in advance of treatment, whereas the other can only be identified post hoc. Management of both represents a considerable challenge in clinical practice as failure of I-131 therapy, the most effective treatment of metastatic thyroid cancer, is associated with a poor overall prognosis. The development of targeted therapies has shown substantial promise in the treatment of RAI-R thyroid cancer in progressive patients. Recent studies show that selective tyrosine kinase inhibitors (TKIs) targeting B-type rapidly accelerated fibrosarcoma kinase (BRAF) and mitogen-activated protein kinase (MEK) can be used as redifferentiation agents to re-induce RAI uptake, thereby (re)enabling I-131 therapy. The use of dosimetry prior- and post-TKI treatment can assist in quantifying RAI uptake and improve identification of patients that will benefit from I-131 therapy. It also potentially offers the prospect of calculating individualized therapeutic administered activities to enhance efficacy and limit toxicity. In this review, we present an overview of the regulation of RAI uptake and clinically investigated redifferentiation agents, both reimbursed and in experimental setting, that induce renewed RAI uptake. We describe the role of dosimetry in redifferentiation and subsequent I-131 therapy in RAI-R thyroid cancer, explain different dosimetry approaches and discuss limitations and considerations in the field.

Individual dosimetry system for targeted alpha therapy based on PHITS coupled with microdosimetric kinetic model

BACKGROUND

An individual dosimetry system is essential for the evaluation of precise doses in nuclear medicine. The purpose of this study was to develop a system for calculating not only absorbed doses but also EQDX(α/β) from the PET-CT images of patients for targeted alpha therapy (TAT), considering the dose dependence of the relative biological effectiveness, the dose-rate effect, and the dose heterogeneity.

METHODS

A general-purpose Monte Carlo particle transport code PHITS was employed as the dose calculation engine in the system, while the microdosimetric kinetic model was used for converting the absorbed dose to EQDX(α/β). PHITS input files for describing the geometry and source distribution of a patient are automatically created from PET-CT images, using newly developed modules of the radiotherapy package based on PHITS (RT-PHITS). We examined the performance of the system by calculating several organ doses using the PET-CT images of four healthy volunteers after injecting 18F-NKO-035.

RESULTS

The deposition energy map obtained from our system seems to be a blurred image of the corresponding PET data because annihilation γ-rays deposit their energies rather far from the source location. The calculated organ doses agree with the corresponding data obtained from OLINDA 2.0 within 20%, indicating the reliability of our developed system. Test calculations by replacing the labeled radionuclide from 18F to 211At suggest that large dose heterogeneity in a target volume is expected in TAT, resulting in a significant decrease of EQDX(α/β) for higher-activity injection.

CONCLUSIONS

As an extension of RT-PHITS, an individual dosimetry system for nuclear medicine was developed based on PHITS coupled with the microdosimetric kinetic model. It enables us to predict the therapeutic and side effects of TAT based on the clinical data largely available from conventional external radiotherapy.

Monte Carlo 90Y PET/CT dosimetry of unexpected focal radiation-induced lung damage after hepatic radioembolisation

Transarterial radioembolization (TARE) with ⁹⁰Y-loaded microspheres is an established therapeutic option for inoperable hepatic tumors. Increasing knowledge regarding TARE hepatic dose-response and dose-toxicity correlation is available but few studies have investigated dose-toxicity correlation in extra-hepatic tissues. We investigated absorbed dose levels for the appearance of focal lung damage in a case of off-target deposition of ⁹⁰Y microspheres and compared them with the corresponding thresholds recommended to avoiding radiation induced lung injury following TARE. A 64-year-old male patient received 1.6 GBq of ⁹⁰Y-labelled glass microspheres for an inoperable left lobe hepatocellular carcinoma. A focal off-target accumulation of radiolabeled microspheres was detected in the left lung upper lobe at the post-treatment ⁹⁰Y-PET/CT, corresponding to a radiation-induced inflammatory lung lesion at the 3-months ¹⁸F-FDG PET/CT follow-up. ⁹⁰Y-PET/CT data were used as input for Monte-Carlo based absorbed dose estimations. Dose-volume-histograms were computed to characterize the heterogeneity of absorbed dose distribution. The dose level associated with the appearance of lung tissue damage was estimated as the median absorbed dose measured at the edge of the inflammatory nodule. To account for respiratory movements and possible inaccuracy of image co-registration, three different methods were evaluated to define the irradiated off-target volume. Monte Carlo-derived absorbed dose distribution showed a highly heterogeneous absorbed dose pattern at the site of incidental microsphere deposition (volume = 2.13 ml) with a maximum dose of 630 Gy. Absorbed dose levels ranging from 119 Gy to 133 Gy, were estimated at the edge of the inflammatory nodule, depending on the procedure used to define the target volume. This report describes an original Monte Carlo based patient-specific dosimetry methodology for the study of the radiation-induced damage in a focal lung lesion after TARE. In our patient, radiation-induced focal lung damage occurred at significantly higher absorbed doses than those considered for single administration or cumulative lung dose delivered during TARE.

Quantification of internal dosimetry in PET patients: individualized Monte Carlo vs generic phantom-based calculations

PURPOSE

The purpose of this work is to calculate individualized dose distributions in patients undergoing 18 F-FDG PET/CT studies through a methodology based on full Monte Carlo (MC) simulations and PET/CT patient images, and to compare such values with those obtained by employing nonindividualized phantom-based methods.

METHODS

We developed a MC-based methodology for individualized internal dose calculations, which relies on CT images (for organ segmentation and dose deposition), PET images (for organ segmentation and distributions of activities), and a biokinetic model (which works with information provided by PET and CT images) to obtain cumulated activities. The software vGATE version 8.1. was employed to carry out the Monte Carlo calculations. We also calculated deposited doses with nonindividualized phantom-based methods (Cristy-Eckerman, Stabin, and ICRP-133).

RESULTS

Median MC-calculated dose/activity values are within 0.01-0.03 mGy/MBq for most organs, with higher doses delivered especially to the bladder wall, major vessels, and brain (medians of 0.058, 0.060, 0.066 mGy/MBq, respectively). Comparison with values obtained with nonindividualized phantom-based methods has shown important differences in many cases (ranging from -80% to + 260%). These differences are significant (p < 0.05) for several organs/tissues, namely, remaining tissues, adrenals, bladder wall, bones, upper large intestine, heart, pancreas, skin, and stomach wall.

CONCLUSIONS

The methodology presented in this work is a viable and useful method to calculate internal dose distributions in patients undergoing medical procedures involving radiopharmaceuticals, individually, with higher accuracy than phantom-based methods, fulfilling the guidelines provided by the European Council directive 2013/59/Euratom.

Current Status of Radiopharmaceutical Therapy

In radiopharmaceutical therapy (RPT), a radionuclide is systemically or locally delivered with the goal of targeting and delivering radiation to cancer cells while minimizing radiation exposure to untargeted cells. Examples of current RPTs include thyroid ablation with the administration of ¹³¹I, treatment of liver cancer with ⁹⁰Y microspheres, the treatment of bony metastases with ²²³Ra, and the treatment of neuroendocrine tumors with ¹⁷⁷Lu-DOTATATE. New RPTs are being developed where radionuclides are incorporated into systemic targeted therapies. To assure that RPT is appropriately implemented, advances in targeting need to be matched with advances in quantitative imaging and dosimetry methods. Currently, radiopharmaceutical therapy is administered by intravenous or locoregional injection, and the treatment planning has typically been implemented like chemotherapy, where the activity administered is either fixed or based on a patient's body weight or body surface area. RPT pharmacokinetics are measurable by quantitative imaging and are known to vary across patients, both in tumors and normal tissues. Therefore, fixed or weight-based activity prescriptions are not currently optimized to deliver a cytotoxic dose to targets while remaining within the tolerance dose of organs at risk. Methods that provide dose estimates to individual patients rather than to reference geometries are needed to assess and adjust the injected RPT dose. Accurate doses to targets and organs at risk will benefit the individual patients and decrease uncertainties in clinical trials. Imaging can be used to measure activity distribution in vivo, and this information can be used to determine patient-specific treatment plans where the dose to the targets and organs at risk can be calculated. The development and adoption of imaging-based dosimetry methods is particularly beneficial in early clinical trials. In this work we discuss dosimetric accuracy needs in modern radiation oncology, uncertainties in the dosimetry in RPT, and best approaches for imaging and dosimetry of internal radionuclide therapy.

Comparison of different calculation techniques for absorbed dose assessment in patient specific peptide receptor radionuclide therapy

Aim

The present work concerns the comparison of the performances of three systems for dosimetry in RPT that use different techniques for absorbed dose calculation (organ-level dosimetry, voxel-level dose kernel convolution and Monte Carlo simulations). The aim was to assess the importance of the choice of the most adequate calculation modality, providing recommendations about the choice of the computation tool.

Methods

The performances were evaluated both on phantoms and patients in a multi-level approach. Different phantoms filled with a ¹⁷⁷Lu-radioactive solution were used: a homogeneous cylindrical phantom, a phantom with organ-shaped inserts and two cylindrical phantoms with inserts different for shape and volume. A total of 70 patients with NETs treated by PRRT with ¹⁷⁷Lu-DOTATOC were retrospectively analysed.

Results

The comparisons were performed mainly between the mean values of the absorbed dose in the regions of interest. A general better agreement was obtained between Dose kernel convolution and Monte Carlo simulations results rather than between either of these two and organ-level dosimetry, both for phantoms and patients. Phantoms measurements also showed the discrepancies mainly depend on the geometry of the inserts (e.g. shape and volume). For patients, differences were more pronounced than phantoms and higher inter/intra patient variability was observed.

Conclusion

This study suggests that voxel-level techniques for dosimetry calculation are potentially more accurate and personalized than organ-level methods. In particular, a voxel-convolution method provides good results in a short time of calculation, while Monte Carlo based computation should be conducted with very fast calculation systems for a possible use in clinics, despite its intrinsic higher accuracy. Attention to the calculation modality is recommended in case of clinical regions of interest with irregular shape and far from spherical geometry, in which Monte Carlo seems to be more accurate than voxel-convolution methods.

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.

Preclinical Voxel-Based Dosimetry in Theranostics: a Review

Due to the increasing use of preclinical targeted radionuclide therapy (TRT) studies for the development of novel theranostic agents, several studies have been performed to accurately estimate absorbed doses to mice at the voxel level using reference mouse phantoms and Monte Carlo (MC) simulations. Accurate dosimetry is important in preclinical theranostics to interpret radiobiological dose-response relationships and to translate results for clinical use. Direct MC (DMC) simulation is believed to produce more realistic voxel-level dose distribution with high precision because tissue heterogeneities and nonuniform source distributions in patients or animals are considered. Although MC simulation is considered to be an accurate method for voxel-based absorbed dose calculations, it is time-consuming, computationally demanding, and often impractical in daily practice. In this review, we focus on the current status of voxel-based dosimetry methods applied in preclinical theranostics and discuss the need for accurate and fast voxel-based dosimetry methods for pretherapy absorbed dose calculations to optimize the dose computation time in preclinical TRT.

Absorbed dose measurements from a 90Y radionuclide liquid solution using LiF:Mg,Cu,P thermoluminescent dosimeters

In the last few years there has been an increasing interest in the measurement of the absorbed dose from radionuclides, with special attention devoted to molecular radiotherapy treatments. In particular, the determination of the absorbed dose from beta emitting radionuclides in liquid solution poses a number of issues when dose measurements are performed using thermoluminescent dosimeters (TLD). Finite volume effect, i.e. the exclusion of radioactivity from the volume occupied by the TLD is one of these. Furthermore, TLDs need to be encapsulated into some kind of waterproof envelope that unavoidably contributes to beta particle attenuation during the measurement. The purpose of this study is twofold: I) to measure the absorbed dose to water, D_w, using LiF:Mg,Cu,P chips inside a PMMA cylindrical phantom filled with a homogenous ⁹⁰YCl₃ aqueous solution II) to assess the uncertainty budget related to D_w measurements. To this purpose, six cylindrical PMMA phantoms were manufactured at ENEA. Each phantom can host a waterproof PMMA stick containing 3 TLD chips encapsulated by a polystyrene envelope. The cylindrical phantoms were manufactured so that the radioactive liquid environment surrounds the whole stick. Finally, D_w measurements were compared with Monte Carlo (MC) calculations. The measurement of absorbed dose to water from ⁹⁰YCl₃ radionuclide solution using LiF:Mg,Cu,P TLDs turned out to be a viable technique, provided that all necessary correction factors are applied. Using this method, a relative combined standard uncertainty in the range 3.1-3.7% was obtained on each D_w measurement. The major source of uncertainty was shown to be TLDs calibration, with associated uncertainties in the range 0.7-2.2%. Comparison of measured and MC-calculated absorbed dose per emitted beta particle provided good results, with the two quantities being in the ratio 1.08.

BIGDOSE: software for 3D personalized targeted radionuclide therapy dosimetry

Background

Advance 3D quantitative radionuclide imaging techniques boost the accuracy of targeted radionuclide therapy (TRT) dosimetry to voxel level. The goal of this work is to develop a comprehensive 3D dosimetric software, BIGDOSE, with new features of image registration and virtual CT for patient-specific dosimetry.

Methods

BIGDOSE includes a portable graphical user interface written in Python, integrating (I) input of sequential ECT/CT images; (II) segmentation; (III) non-rigid image registration; (IV) curve fitting and voxel-based integration; (V) dose conversion and (VI) 3D dose analysis. The accuracy of the software was evaluated using a simulation study with 9 XCAT phantoms. We simulated SPECT/CT acquisitions at 1, 12, 24, 72 and 144-hrs post In-111 Zevalin injection with inter-scans misalignments using an analytical projector for medium energy general purpose (MEGP) collimator, modeling attenuation, scatter and collimator-detector response. The SPECT data were reconstructed using quantitative OS-EM method. A CT organ-based registration was performed before the dose calculation. Organ absorbed doses for the corresponding Y-90 therapeutic agent were calculated on target organs and compared with those obtained from OLINDA/EXM, using dose measured from GATE as the gold standard. One patient with In-111 DTPAOC injection as well as two patients with Y-90 microsphere embolization were used to demonstrate the clinical effectiveness of our software.

Results

In the simulation, the organ dose errors of BIGDOSE were -9.59%±9.06%, -8.36±5.82%, -23.41%±6.67%, -6.05%±2.06% for liver, spleen, kidneys and lungs, while they were -25.72%±12.52%, -14.93%±10.91%, -28.63%±12.97% and -45.30%±5.84% for OLINDA/EXM. Cumulative dose volume histograms, dose maps and iso-dose contours provided 3D dose distribution information on the simulated and patient data.

Conclusions

BIGDOSE provides a one-stop platform for voxel-based dose estimation with enhanced functions. It is a promising tool to streamline the current clinical TRT dosimetric practice with high accuracy, incorporating 3D personalized imaging information for improved treatment outcome.

Particle filter de-noising of voxel-specific time-activity-curves in personalized 177Lu therapy

BACKGROUND

Currently, there is a high interest in ¹⁷⁷Lu targeted radionuclide therapies, which could be attributed to favorable results obtained from ¹⁷⁷Lu compounds targeting neuro-endocrine and prostate tumors. SPECT based dosimetry could be used for deriving dose values for individual voxels, as is the standard in external-beam radiation-therapy (EBRT). For this a time-activity-curve (TAC) at voxel resolution and also a voxel-wise modeling of radiation energy deposition are necessary. But a voxel-wise determination of TACs is problematic, since several confounding factors exist, such as e.g. poor count-statistics or registration inaccuracies, which add noise to the observed activity states. A particle filter (PF) is a class of methods which applies regularization based on a model of the temporal evolution of activity states. The aim of this study is to introduce the application of PFs for de-noising of per-voxel time-activity curves.

METHODS

We applied a PF for de-noising the TACs of 26 patients, who underwent ¹⁷⁷Lu-DOTATOC or -PSMA therapy. The TACs were obtained from fully-quantitative, serial SPECT(/CT) data, acquired at 4h, 24h, 48h, 72h p.i. The model used in the PF was a mono-exponential decay and its free parameters were determined based on objective criteria. The time-integrated activities (TIA) resulting from the PF (PFF) were compared to the results of a mono-exponential fit (SF) of individual voxels in several volumes of interest (kidneys, spleen, tumors). Additionally, an organ-averaged TIA was derived from whole-organ VOIs and subsequent curve-fitting. This whole-organ TIA was also compared to the whole-organ TIAs obtained from summation of the voxel-wise TIAs from PFF and SF.

RESULTS

The number of particles was set to 1000. Optimal values for noise of observations and noise of the model were 0.25 and 0.5, respectively. The deviation of whole-organ TIAs from conventional organ-based dosimetry and the summation of the voxel-wise TIAs was substantial for SF (kidneys -22.3%, spleen -49.6%, tumor -60.0%), as well as for PFF (kidneys -37.1%, spleen -57.9%, tumor -70.9%). The distribution of voxel-wise half-lives resulting from the PFF method was considerably closer to the organ-averaged value, and the number of implausibly long half-lives (>physical HL) was reduced.

CONCLUSION

The PFF leads to voxel-wise half-lives, which are more plausible than those resulting from SF. However, one has to admit that voxel-wise fitting generally leads to considerable deviations from the organ-averaged TIA as obtained by conventional whole-organ evaluation. Unfortunately, we did not have ground-truth TIA of our patient data and proper ground-truth could even be impossible to obtain. Nevertheless, there are strong indicators that particle filtering can be used for reducing voxel-wise TAC noise.

Development of Targeted Alpha Particle Therapy for Solid Tumors

Targeted alpha-particle therapy (TAT) aims to selectively deliver radionuclides emitting α-particles (cytotoxic payload) to tumors by chelation to monoclonal antibodies, peptides or small molecules that recognize tumor-associated antigens or cell-surface receptors. Because of the high linear energy transfer (LET) and short range of alpha (α) particles in tissue, cancer cells can be significantly damaged while causing minimal toxicity to surrounding healthy cells. Recent clinical studies have demonstrated the remarkable efficacy of TAT in the treatment of metastatic, castration-resistant prostate cancer. In this comprehensive review, we discuss the current consensus regarding the properties of the α-particle-emitting radionuclides that are potentially relevant for use in the clinic; the TAT-mediated mechanisms responsible for cell death; the different classes of targeting moieties and radiometal chelators available for TAT development; current approaches to calculating radiation dosimetry for TATs; and lead optimization via medicinal chemistry to improve the TAT radiopharmaceutical properties. We have also summarized the use of TATs in pre-clinical and clinical studies to date.

Influence of the SPECT calibration source position on the absorbed dose calculation for 131I-NaI therapy using GATE simulations

Many research groups have studied nuclear medicine image quantification to improve its accuracy in dose estimation. This work aims to evaluate the influence of the source calibration position for absorbed dose calculation for a ¹³¹I-NaI therapy using Monte Carlo (MC) simulations. The calibration approach consisted of a cylindrical phantom filled with water. A cylindrical ¹³¹I source with 361.1 ± 3.6 kBq ml^-1 was positioned at the center of the phantom and its outer part. Images were acquired with 150 00 counts per projection image acquired with SPECT detector (high counts density-HCD) and 3000 counts per projection (low counts density-LCD). MC simulations, performed with GATE code, were validated by comparing the S values of a water sphere uniformly filled with ¹³¹I, as from the sphere model of OLINDA/EXM 1.1. Calibration factors deviation between central and peripheral calibrations is more significant for HCD (18.3%) than for LCD images (3.7%). The 3D dose distribution map obtained from GATE resulted in a dose factor equal to 1.5 × 10^-3 mGy/(MBq.s). For both HCD and LCD images, the commonly used approach, which employs the central source calibration to obtain the dose from a peripheral source, resulted in dose overestimation. Results suggest that organ dose calculation can be improved considering the organ position in the field of view. Finally, patients' radiation protection in dosimetry studies could be improved considering the calibration source position, due to the superior accuracy in dose calculation.

Development and Validation of RAPID: A Patient-Specific Monte Carlo Three-Dimensional Internal Dosimetry Platform

This work describes the development and validation of a patient-specific Monte Carlo internal dosimetry platform called RAPID (Radiopharmaceutical Assessment Platform for Internal Dosimetry). RAPID utilizes serial PET/CT or SPECT/CT images to calculate voxelized three-dimensional (3D) internal dose distributions with the Monte Carlo code Geant4. RAPID's dosimetry calculations were benchmarked against previously published S-values and specific absorbed fractions (SAFs) calculated for monoenergetic photon and electron sources within the Zubal phantom and for S-values calculated for a variety of radionuclides within spherical tumor phantoms with sizes ranging from 1 to 1000 g. The majority of the S-values and SAFs calculated in the Zubal Phantom were within 5% of the previously published values with the exception of a few 10 keV photon SAFs that agreed within 10%, and one value within 16%. The S-values calculated in the spherical tumor phantoms agreed within 2% for ¹⁷⁷Lu, ¹³¹I, ¹²⁵I, ¹⁸F, and ⁶⁴Cu, within 3.5% for ²¹¹At and ²¹³Bi, within 6.5% for ¹⁵³Sm, ¹¹¹In, ⁸⁹Zr, and ²²³Ra, and within 9% for ⁹⁰Y, ⁶⁸Ga, and ¹²⁴I. In conclusion, RAPID is capable of calculating accurate internal dosimetry at the voxel-level for a wide variety of radionuclides and could be a useful tool for calculating patient-specific 3D dose distributions.

CPU time optimization and precise adjustment of the Geant4 physics parameters for a VARIAN 2100 C/D gamma radiotherapy linear accelerator simulation using GAMOS

We have verified the GAMOS/Geant4 simulation model of a 6 MV VARIAN Clinac 2100 C/D linear accelerator by the procedure of adjusting the initial beam parameters to fit the percentage depth dose and cross-profile dose experimental data at different depths in a water phantom. Thanks to the use of a wide range of field sizes, from 2  ×  2 cm² to 40  ×  40 cm², a small phantom voxel size and high statistics, fine precision in the determination of the beam parameters has been achieved. This precision has allowed us to make a thorough study of the different physics models and parameters that Geant4 offers. The three Geant4 electromagnetic physics sets of models, i.e. Standard, Livermore and Penelope, have been compared to the experiment, testing the four different models of angular bremsstrahlung distributions as well as the three available multiple-scattering models, and optimizing the most relevant Geant4 electromagnetic physics parameters. Before the fitting, a comprehensive CPU time optimization has been done, using several of the Geant4 efficiency improvement techniques plus a few more developed in GAMOS.

Phantom validation of quantitative Y-90 PET/CT-based dosimetry in liver radioembolization

Background

PET/CT has recently been shown to be a viable alternative to traditional post-infusion imaging methods providing good quality images of ⁹⁰Y-laden microspheres after selective internal radiation therapy (SIRT). In the present paper, first we assessed the quantitative accuracy of ⁹⁰Y-PET using an anthropomorphic phantom provided with lungs, liver, spine, and a cylindrical homemade lesion located into the hepatic compartment. Then, we explored the accuracy of different computational approaches on dose calculation, including (I) direct Monte Carlo radiation transport using Raydose, (II) Kernel convolution using Philips Stratos, (III) local deposition algorithm, (IV) Monte Carlo technique (MCNP) considering a uniform activity distribution, and (V) MIRD (Medical Internal Radiation Dose) analytical approach. Finally, calculated absorbed doses were compared with those obtained performing measurements with LiF:Mg,Cu,P TLD chips in a liquid environment.

Results

Our results indicate that despite ⁹⁰Y-PET being likely to provide high-resolution images, the ⁹⁰Y low branch ratio, along with other image-degrading factors, may produce non-uniform activity maps, even in the presence of uniform activity. A systematic underestimation of the recovered activity, both for the tumor insert and for the liver background, was found. This is particularly true if no partial volume correction is applied through recovery coefficients. All dose algorithms performed well, the worst case scenario providing an agreement between absorbed dose evaluations within 20%. Average absorbed doses determined with the local deposition method are in excellent agreement with those obtained using the MIRD and the kernel-convolution dose calculation approach.

Finally, absorbed dose assessed with MC codes are in good agreement with those obtained using TLD in liquid solution, thus confirming the soundness of both calculation approaches. This is especially true for Raydose, which provided an absorbed dose value within 3% of the measured dose, well within the stated uncertainties.

Conclusions

Patient-specific dosimetry is possible even in a scenario with low true coincidences and high random fraction, as in ⁹⁰Y–PET imaging, granted that accurate absolute PET calibration is performed and acquisition times are sufficiently long. Despite Monte Carlo calculations seeming to outperform all dose estimation algorithms, our data provide a strong argument for encouraging the use of the local deposition algorithm for routine ⁹⁰Y dosimetry based on PET/CT imaging, due to its simplicity of implementation.

Improved outcome of 131I-mIBG treatment through combination with external beam radiotherapy in the SK-N-SH mouse model of neuroblastoma

PURPOSE

To assess the efficacy of different schedules for combining external beam radiotherapy (EBRT) with molecular radiotherapy (MRT) using 131I-mIBG in the management of neuroblastoma.

MATERIALS AND METHODS

BALB/c nu/nu mice bearing SK-N-SH neuroblastoma xenografts were assigned to five treatment groups: 131I-mIBG 24h after EBRT, EBRT 6days after 131I-mIBG, EBRT alone, 131I-mIBG alone and control (untreated). A total of 56 mice were assigned to 3 studies. Study 1: Vessel permeability was evaluated using dynamic contrast-enhanced (DCE)-MRI (n=3). Study 2: Tumour uptake of 131I-mIBG in excised lesions was evaluated by γ-counting and autoradiography (n=28). Study 3: Tumour volume was assessed by longitudinal MR imaging and survival was analysed (n=25). Tumour dosimetry was performed using Monte Carlo simulations of absorbed fractions with the radiation transport code PENELOPE.

RESULTS

Given alone, both 131I-mIBG and EBRT resulted in a seven-day delay in tumour regrowth. Following EBRT, vessel permeability was evaluated by DCE-MRI and showed an increase at 24h post irradiation that correlated with an increase in 131I-mIBG tumour uptake, absorbed dose and overall survival in the case of combined treatment. Similarly, EBRT administered seven days after MRT to coincide with tumour regrowth, significantly decreased the tumour volume and increased overall survival.

CONCLUSIONS

This study demonstrates that combining EBRT and MRT has an enhanced therapeutic effect and emphasizes the importance of treatment scheduling according to pathophysiological criteria such as tumour vessel permeability and tumour growth kinetics.

Practical Nuclear Medicine and Utility of Phantoms for Internal Dosimetry: XCAT Compared with Zubal

PURPOSE

The absorbed doses for two radioisotopes, 99mTc and 131I, between previously validated Zubal phantom and the recently developed XCAT phantom were compared.

MATERIALS AND METHODS

GATE Monte Carlo code was used to simulate the statistical process of radiation. A XCAT phantom with voxel and matrix sizes similar to a standard Zubal phantom was generated. According to Medical International Radiation Dose formalism, specific absorbed fraction (SAF) values for photons and S-factors for beta particles were tabulated. The amounts of absorbed doses were calculated and compared in different organs.

RESULTS

The differences of gamma radiation doses, SAFs, between Zubal and XCAT are >50% in adrenal from adrenal, pancreas from pancreas and thyroid from thyroid, in lung from kidney, kidneys from lungs and in kidneys from thyroid and thyroid from kidneys. The beta radiation doses differences between Zubal and XCAT are >50% in thyroid from thyroid, bladder from bladder, kidney from kidney, liver from bladder, thyroid from bladder and kidney from thyroid. The size and distances of the organs were different between XCAT and Zubal phantoms. Denoted differences of SAF and S-factors correspond to the different organ geometries in phantoms.

CONCLUSION

The results of absorbed doses in Zubal and XCAT phantoms are different. The variations prohibit easy comparison or interchangeability of dosimetry between these phantoms.

Three-Dimensional Dosimetry for Radiation Safety Estimates from Intrathecal Administration

Intrathecal administration is of growing interest for drug delivery, and its utility is being increasingly investigated through imaging. In this work, the 3-dimensional Voxel-Based Internal Dosimetry Application (VIDA) and 4D Extended Cardiac Torso Phantom (XCAT) were extended to provide radiation safety estimates specific to intrathecal administration. Methods: The 3-dimensional VIDA dosimetry application Monte Carlo simulation was run using a modified XCAT phantom with additional and edited cerebrospinal fluid (CSF) regions to produce voxel-level absorbed dose per unit cumulated activity maps for 9 selected source regions. Simulation validation was performed to compare absorbed dose estimates for common organs in a preexisting dosimetry tool (OLINDA/EXM). Dynamic planar imaging data were acquired in 6 healthy subjects using administered volumes of 5 or 15 mL (n = 3 each) of 185 MBq of ^99mTc-diethylenetriaminepentaacetic acid. Absorbed dose was estimated for each subject using the intrathecal-specific dosimetry application. Results: Simulation results were within 6% of OLINDA estimates for common organs. Absorbed dose estimates were highest (0.3-0.8 mGy/MBq) in the lumbar CSF space. A whole-body effective dose estimate of 0.003 mSv/MBq was observed. An administered volume dependency was observed with a 15-mL volume, resulting in lower absorbed dose estimates for several intrathecal and nonintrathecal regions. Conclusion: The intrathecal-specific VIDA implementation enables tailored dosimetry estimation for regions most relevant in intrathecal administration. Absorbed doses are highly localized to CSF and spinal regions and should be taken into consideration when designing intrathecal imaging studies. A potentially interesting relationship was observed between absorbed dose and administered volume, which merits further investigation.

An Expanded Multi-scale Monte Carlo Simulation Method for Personalized Radiobiological Effect Estimation in Radiotherapy: a feasibility study

A novel and versatile "bottom-up" approach is developed to estimate the radiobiological effect of clinic radiotherapy. The model consists of multi-scale Monte Carlo simulations from organ to cell levels. At cellular level, accumulated damages are computed using a spectrum-based accumulation algorithm and predefined cellular damage database. The damage repair mechanism is modeled by an expanded reaction-rate two-lesion kinetic model, which were calibrated through replicating a radiobiological experiment. Multi-scale modeling is then performed on a lung cancer patient under conventional fractionated irradiation. The cell killing effects of two representative voxels (isocenter and peripheral voxel of the tumor) are computed and compared. At microscopic level, the nucleus dose and damage yields vary among all nucleuses within the voxels. Slightly larger percentage of cDSB yield is observed for the peripheral voxel (55.0%) compared to the isocenter one (52.5%). For isocenter voxel, survival fraction increase monotonically at reduced oxygen environment. Under an extreme anoxic condition (0.001%), survival fraction is calculated to be 80% and the hypoxia reduction factor reaches a maximum value of 2.24. In conclusion, with biological-related variations, the proposed multi-scale approach is more versatile than the existing approaches for evaluating personalized radiobiological effects in radiotherapy.

Impact of SPECT corrections on 3D-dosimetry for liver transarterial radioembolization using the patient relative calibration methodology

PURPOSE

Many centers aim to plan liver transarterial radioembolization (TARE) with dosimetry, even without CT-based attenuation correction (AC), or with unoptimized scatter correction (SC) methods. This work investigates the impact of presence vs absence of such corrections, and limited spatial resolution, on 3D dosimetry for TARE.

METHODS

Three voxelized phantoms were derived from CT images of real patients with different body sizes. Simulations of (99m)Tc-SPECT projections were performed with the SIMIND code, assuming three activity distributions in the liver: uniform, inside a "liver's segment," or distributing multiple uptaking nodules ("nonuniform liver"), with a tumoral liver/healthy parenchyma ratio of 5:1. Projection data were reconstructed by a commercial workstation, with OSEM protocol not specifically optimized for dosimetry (spatial resolution of 12.6 mm), with/without SC (optimized, or with parameters predefined by the manufacturer; dual energy window), and with/without AC. Activity in voxels was calculated by a relative calibration, assuming identical microspheres and (99m)Tc-SPECT counts spatial distribution. 3D dose distributions were calculated by convolution with (90)Y voxel S-values, assuming permanent trapping of microspheres. Cumulative dose-volume histograms in lesions and healthy parenchyma from different reconstructions were compared with those obtained from the reference biodistribution (the "gold standard," GS), assessing differences for D95%, D70%, and D50% (i.e., minimum value of the absorbed dose to a percentage of the irradiated volume). γ tool analysis with tolerance of 3%/13 mm was used to evaluate the agreement between GS and simulated cases. The influence of deep-breathing was studied, blurring the reference biodistributions with a 3D anisotropic gaussian kernel, and performing the simulations once again.

RESULTS

Differences of the dosimetric indicators were noticeable in some cases, always negative for lesions and distributed around zero for parenchyma. Application of AC and SC reduced systematically the differences for lesions by 5%-14% for a liver segment, and by 7%-12% for a nonuniform liver. For parenchyma, the data trend was less clear, but the overall range of variability passed from -10%/40% for a liver segment, and -10%/20% for a nonuniform liver, to -13%/6% in both cases. Applying AC, SC with preset parameters gave similar results to optimized SC, as confirmed by γ tool analysis. Moreover, γ analysis confirmed that solely AC and SC are not sufficient to obtain accurate 3D dose distribution. With breathing, the accuracy worsened severely for all dosimetric indicators, above all for lesions: with AC and optimized SC, -38%/-13% in liver's segment, -61%/-40% in the nonuniform liver. For parenchyma, D50% resulted always less sensitive to breathing and sub-optimal correction methods (difference overall range: -7%/13%).

CONCLUSIONS

Reconstruction protocol optimization, AC, SC, PVE and respiratory motion corrections should be implemented to obtain the best possible dosimetric accuracy. On the other side, thanks to the relative calibration, D50% inaccuracy for the healthy parenchyma from absence of AC was less than expected, while the optimization of SC was scarcely influent. The relative calibration therefore allows to perform TARE planning, basing on D50% for the healthy parenchyma, even without AC or with suboptimal corrections, rather than rely on nondosimetric methods.

3D absorbed dose distribution estimated by Monte Carlo simulation in radionuclide therapy with a monoclonal antibody targeting synovial sarcoma

Backround

Radiolabeled OTSA101, a monoclonal antibody targeting synovial sarcoma (SS) developed by OncoTherapy Science, was used to treat relapsing SS metastases following a theranostic procedure: in case of significant ¹¹¹In-OTSA101 tumor uptake and favorable biodistribution, patient was randomly treated with 370/1110 MBq ⁹⁰Y-OTSA101. Monte Carlo-based 3D dosimetry integrating time-activity curves in VOI was performed on ¹¹¹In-OTSA101 repeated SPECT/CT. Estimated absorbed doses (AD) in normal tissues were compared to biological side effects and to the admitted maximal tolerated absorbed dose (MTD) in normal organs. Results in the tumors were also compared to disease evolution.

Results

Biodistribution and tracer quantification were analyzed on repeated SPECT/CT acquisitions performed after injection of ¹¹¹In-OTSA101 in 19/20 included patients. SPECT images were warped to a common coordinates system with deformable registration. Volumes of interest (VOI) for various lesions and normal tissues were drawn on the first CT acquisition and reported to all the SPECT images. Tracer quantification and residence time of ¹¹¹In-OTSA101 in VOI were used to evaluate the estimated absorbed doses per MBq of ⁹⁰Y-OTSA101 by means of Monte Carlo simulations (GATE). A visual scale analysis was applied to assess tumor uptake (grades 0 to 4) and results were compared to the automated quantification. Results were then compared to biological side effects reported in the selected patients treated with ⁹⁰Y-OTSA101 but also to disease response to treatment.

After screening, 8/20 patients were treated with 370 or 1110 MBq ⁹⁰Y-OTSA101. All demonstrated medullary toxicity, only one presented with transient grade 3 liver toxicity due to disease progression, and two patients presented with transient grade 1 renal toxicity. Median absorbed doses were the highest in the liver (median, 0.64 cGy/MBq; [0.27 −1.07]) being far lower than the 20 Gy liver MTD, and the lowest in bone marrow (median, 0.09 cGy/MBq; [0.02 −0.18]) being closer to the 2 Gy bone marrow MTD. Most of the patients demonstrated progressive disease on RECIST criteria during patient follow-up. ¹¹¹In-OTSA101 tumors tracer uptake visually appeared highly heterogeneous in inter- and intra-patient analyses, independently of tumor sizes, with variable kinetics. The majority of visual grades corresponded to the automated computed ones. Estimated absorbed doses in the 95 supra-centimetric selected lesions ranged from 0.01 to 0.71 cGy per injected MBq (median, 0.22 cGy/MBq). The maximal tumor AD obtained was 11.5 Gy.

Conclusions

3D dosimetry results can explain the observed toxicity and tumors response. Despite an intense visual ¹¹¹In-OTSA101 liver uptake, liver toxicity was not the dose limiting factor conversely to bone marrow toxicity. Even though tumors ¹¹¹In-OTSA101 avidity was visually obvious for treated patients, the low estimated tumors AD obtained by 3D dosimetry explain the lack of tumor response.

Model-based versus specific dosimetry in diagnostic context: comparison of three dosimetric approaches

PURPOSE

The dosimetric assessment of novel radiotracers represents a legal requirement in most countries. While the techniques for the computation of internal absorbed dose in a therapeutic context have made huge progresses in recent years, in a diagnostic scenario the absorbed dose is usually extracted from model-based lookup tables, most often derived from International Commission on Radiological Protection (ICRP) or Medical Internal Radiation Dose (MIRD) Committee models. The level of approximation introduced by these models may impact the resulting dosimetry. The aim of this work is to establish whether a more refined approach to dosimetry can be implemented in nuclear medicine diagnostics, by analyzing a specific case.

METHODS

The authors calculated absorbed doses to various organs in six healthy volunteers administered with flutemetamol ((18)F) injection. Each patient underwent from 8 to 10 whole body 3D PET/CT scans. This dataset was analyzed using a Monte Carlo (MC) application developed in-house using the toolkit gate that is capable to take into account patient-specific anatomy and radiotracer distribution at the voxel level. They compared the absorbed doses obtained with GATE to those calculated with two commercially available software: OLINDA/EXM and STRATOS implementing a dose voxel kernel convolution approach.

RESULTS

Absorbed doses calculated with gate were higher than those calculated with OLINDA. The average ratio between gate absorbed doses and OLINDA's was 1.38 ± 0.34 σ (from 0.93 to 2.23). The discrepancy was particularly high for the thyroid, with an average GATE/OLINDA ratio of 1.97 ± 0.83 σ for the six patients. Differences between STRATOS and GATE were found to be higher. The average ratio between GATE and STRATOS absorbed doses was 2.51 ± 1.21 σ (from 1.09 to 6.06).

CONCLUSIONS

This study demonstrates how the choice of the absorbed dose calculation algorithm may introduce a bias when gamma radiations are of importance, as is the case in nuclear medicine diagnostics.

Multi-scale hybrid models for radiopharmaceutical dosimetry with Geant4

The accuracy of radiopharmaceutical absorbed dose distributions computed through Monte Carlo (MC) simulations is mostly limited by the low spatial resolution of 3D imaging techniques used to define the simulation geometry. This issue also persists with the implementation of realistic hybrid models built using polygonal mesh and/or NURBS as they require to be simulated in their voxel form in order to reduce computation times. The existing trade-off between voxel size and simulation speed leads on one side, in an overestimation of the size of small radiosensitive structures such as the skin or hollow organs walls and, on the other, to unnecessarily detailed voxelization of large, homogeneous structures.We developed a set of computational tools based on VTK and Geant4 in order to build multi-resolution organ models. Our aim is to use different voxel sizes to represent anatomical regions of different clinical relevance: the MC implementation of these models is expected to improve spatial resolution in specific anatomical structures without significantly affecting simulation speed. Here we present the tools developed through a proof of principle example. Our approach is validated against the standard Geant4 technique for the simulation of voxel geometries.

VIDA: a voxel-based dosimetry method for targeted radionuclide therapy using Geant4

We have developed the Voxel-Based Internal Dosimetry Application (VIDA) to provide patient-specific dosimetry in targeted radionuclide therapy performing Monte Carlo simulations of radiation transport with the Geant4 toolkit. The code generates voxel-level dose rate maps using anatomical and physiological data taken from individual patients. Voxel level dose rate curves are then fit and integrated to yield a spatial map of radiation absorbed dose. In this article, we present validation studies using established dosimetry results, including self-dose factors (DFs) from the OLINDA/EXM program for uniform activity in unit density spheres and organ self- and cross-organ DFs in the Radiation Dose Assessment Resource (RADAR) reference adult phantom. The comparison with reference data demonstrated agreement within 5% for self-DFs to spheres and reference phantom source organs for four common radionuclides used in targeted therapy ((131)I, (90)Y, (111)In, (177)Lu). Agreement within 9% was achieved for cross-organ DFs. We also present dose estimates to normal tissues and tumors from studies of two non-Hodgkin Lymphoma patients treated by (131)I radioimmunotherapy, with comparison to results generated independently with another dosimetry code. A relative difference of 12% or less was found between methods for mean absorbed tumor doses accounting for tumor regression.

Dosimetry for radiopharmaceutical therapy

Radiopharmaceutical therapy (RPT) involves the use of radionuclides that are either conjugated to tumor-targeting agents (e.g., nanoscale constructs, antibodies, peptides, and small molecules) or concentrated in tissue through natural physiological mechanisms that occur predominantly in neoplastic or otherwise targeted cells (e.g., Graves disease). The ability to collect pharmacokinetic data by imaging and use this to perform dosimetry calculations for treatment planning distinguishes RPT from other systemic treatment modalities such as chemotherapy, wherein imaging is not generally used. Treatment planning has not been widely adopted, in part, because early attempts to relate dosimetry to outcome were not successful. This was partially because a dosimetry methodology appropriate to risk evaluation rather than efficacy and toxicity was being applied to RPT. The weakest links in both diagnostic and therapeutic dosimetry are the accuracy of the input and the reliability of the radiobiological models used to convert dosimetric data to the relevant biologic end points. Dosimetry for RPT places a greater demand on both of these weak links. To date, most dosimetric studies have been retrospective, with a focus on tumor dose-response correlations rather than prospective treatment planning. In this regard, transarterial radioembolization also known as intra-arterial radiation therapy, which uses radiolabeled ((90)Y) microspheres of glass or resin to treat lesions in the liver holds much promise for more widespread dosimetric treatment planning. The recent interest in RPT with alpha-particle emitters has highlighted the need to adopt a dosimetry methodology that specifically accounts for the unique aspects of alpha particles. The short range of alpha-particle emitters means that in cases in which the distribution of activity is localized to specific functional components or cell types of an organ, the absorbed dose will be equally localized and dosimetric calculations on the scale of organs or even voxels (~5mm) are no longer sufficient. This limitation may be overcome by using preclinical models to implement macromodeling to micromodeling. In contrast to chemotherapy, RPT offers the possibility of evaluating radiopharmaceutical distributions, calculating tumor and normal tissue absorbed doses, and devising a treatment plan that is optimal for a specific patient or specific group of patients.

Use of the FLUKA Monte Carlo code for 3D patient-specific dosimetry on PET-CT and SPECT-CT images

Patient-specific absorbed dose calculation for nuclear medicine therapy is a topic of increasing interest. 3D dosimetry at the voxel level is one of the major improvements for the development of more accurate calculation techniques, as compared to the standard dosimetry at the organ level. This study aims to use the FLUKA Monte Carlo code to perform patient-specific 3D dosimetry through direct Monte Carlo simulation on PET-CT and SPECT-CT images. To this aim, dedicated routines were developed in the FLUKA environment. Two sets of simulations were performed on model and phantom images. Firstly, the correct handling of PET and SPECT images was tested under the assumption of homogeneous water medium by comparing FLUKA results with those obtained with the voxel kernel convolution method and with other Monte Carlo-based tools developed to the same purpose (the EGS-based 3D-RD software and the MCNP5-based MCID). Afterwards, the correct integration of the PET/SPECT and CT information was tested, performing direct simulations on PET/CT images for both homogeneous (water) and non-homogeneous (water with air, lung and bone inserts) phantoms. Comparison was performed with the other Monte Carlo tools performing direct simulation as well. The absorbed dose maps were compared at the voxel level. In the case of homogeneous water, by simulating 10(8) primary particles a 2% average difference with respect to the kernel convolution method was achieved; such difference was lower than the statistical uncertainty affecting the FLUKA results. The agreement with the other tools was within 3–4%, partially ascribable to the differences among the simulation algorithms. Including the CT-based density map, the average difference was always within 4% irrespective of the medium (water, air, bone), except for a maximum 6% value when comparing FLUKA and 3D-RD in air. The results confirmed that the routines were properly developed, opening the way for the use of FLUKA for patient-specific, image-based dosimetry in nuclear medicine.