A free database of radionuclide voxel S values for the dosimetry of nonuniform activity distributions

Characterizing the voxel-based approaches in radioembolization dosimetry with reDoseMC

BACKGROUND

Yttrium-90 (90 Y $^{90}{\rm {Y}}$ ) represents the primary radioisotope used in radioembolization procedures, while holmium-166 (166 Ho $^{166}{\rm {Ho}}$ ) is hypothesized to serve as a viable substitute for90 Y $^{90}{\rm {Y}}$ due to its comparable therapeutic potential and improved quantitative imaging. Voxel-based dosimetry for these radioisotopes relies on activity images obtained through PET or SPECT and dosimetry methods, including the voxel S-value (VSV) and the local deposition method (LDM). However, the evaluation of the accuracy of absorbed dose calculations has been limited by the use of non-ideal reference standards and investigations restricted to the liver. The objective of this study was to expand upon these dosimetry characterizations by investigating the impact of image resolutions, voxel sizes, target volumes, and tissue materials on the accuracy of90 Y $^{90}{\rm {Y}}$ and166 Ho $^{166}{\rm {Ho}}$ dosimetry techniques.

METHODS

A specialized radiopharmaceutical dosimetry software called reDoseMC was developed using the Geant4 Monte Carlo toolkit and validated by benchmarking the generated90 Y $^{90}{\rm {Y}}$ kernels with published data. The decay spectra of both90 Y $^{90}{\rm {Y}}$ and166 Ho $^{166}{\rm {Ho}}$ were also compared. Multiple VSV kernels were generated for the liver, lungs, soft tissue, and bone for isotropic voxel sizes of 1 mm, 2 mm, and 4 mm. Three theoretical phantom setups were created with 20 or 40 mm activity and mass density inserts for the same three voxel sizes. To replicate the limited spatial resolutions present in PET and SPECT images, image resolutions were modeled using a 3D Gaussian kernel with a Full Width at Half Maximum (FWHM) ranging from 0 to 16 mm and with no added noise. The VSV and LDM dosimetry methods were evaluated by characterizing their respective kernels and analyzing their absorbed dose estimates calculated on theoretical phantoms. The ground truth for these estimations was calculated using reDoseMC.

RESULTS

The decay spectra obtained through reDoseMC showed less than a 1% difference when compared to previously published experimental data for energies below 1.9 MeV in the case of90 Y $^{90}{\rm {Y}}$ and less than 1% for energies below 1.5 MeV for166 Ho $^{166}{\rm {Ho}}$ . Additionally, the validation kernels for90 Y $^{90}{\rm {Y}}$ VSV exhibited results similar to those found in published Monte Carlo codes, with source dose depositions having less than a 3% error margin. Resolution thresholds (FWHM thresh s ${\rm {FWHM}}_\mathrm{thresh}{\rm {s}}$ ), defined as resolutions that resulted in similar dose estimates between the LDM and VSV methods, were observed for90 Y $^{90}{\rm {Y}}$ . They were 1.5 mm for bone, 2.5 mm for soft tissue and liver, and 8.5 mm for lungs. For166 Ho $^{166}{\rm {Ho}}$ , the accuracy of absorbed dose deposition was found to be dependent on the contributions of absorbed dose from photons. Volume errors due to variations in voxel size impacted the final dose estimates. Larger target volumes yielded more accurate mean doses than smaller volumes. For both radioisotopes, the radial dose profiles for the VSV and LDM approximated but never matched the reference standard.

CONCLUSIONS

reDoseMC was developed and validated for radiopharmaceutical dosimetry. The accuracy of voxel-based dosimetry was found to vary widely with changes in image resolutions, voxel sizes, chosen target volumes, and tissue material; hence, the standardization of dosimetry protocols was found to be of great importance for comparable dosimetry analysis.

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.

Pre-treatment dosimetry in90Y-SIRT: Is it possible to optimise SPECT reconstruction parameters and calculation methods for accurate dosimetry?

PURPOSE

The aim of this study was (a) to optimise the99mTc-SPECT reconstruction parameters for the pre-treatment dosimetry of90Y-selective internal radiation therapy (SIRT) and (b) to compare the accuracy of clinical dosimetry methods with full Monte-Carlo dosimetry (fMCD) performed with Gate.

METHODS

To optimise the reconstruction parameters, two hundred reconstructions with different parameters were performed on a NEMA phantom, varying the number of iterations, subsets, and post-filtering. The accuracy of the dosimetric methods was then investigated using an anthropomorphic phantom. Absorbed dose maps were generated using (1) the Partition Model (PM), (2) the Dose Voxel Kernel (DVK) convolution, and (3) the Local Deposition Method (LDM) with known activity restricted to the whole phantom (WP) or to the liver and lungs (LL). The dose to the lungs was calculated using the "multiple DVK" and "multiple LDM" methods.

RESULTS

Optimal OSEM reconstruction parameters were found to depend on object size and dosimetric criterion chosen (Dmean or DVH-derived metric). The Dmean of all three dosimetric methods was close (≤ 10%) to the Dmean of fMCD simulations when considering large segmented volumes (whole liver, normal liver). In contrast, the Dmean to the small volume (∅=31) was systemically underestimated (12%-25%). For lungs, the "multiple DVK" and "multiple LDM" methods yielded a Dmean within 20% for the WP method and within 10% for the LL method.

CONCLUSIONS

All three methods showed a substantial degradation of the dose-volume histograms (DVHs) compared to fMCD simulations. The DVK and LDM methods performed almost equally well, with the "multiple DVK" method being more accurate in the lungs.

Updating 90Y Voxel S-Values including internal Bremsstrahlung: Monte Carlo study and development of an analytical model

PURPOSE

Internal Bremsstrahlung (IB) is a process accompanying β-decay but neglected in Voxel S-Values (VSVs) calculation. Aims of this work were to calculate, through Monte Carlo (MC) simulation, updated 90Y-VSVs including IB, and to develop an analytical model to evaluate 90Y-VSVs for any voxel size of practical interest.

METHODS

GATE (Geant4 Application for Tomographic Emission) was employed for simulating voxelized geometries of soft tissue, with voxels sides l ranging from 2 to 6 mm, in steps of 0.5 mm. The central voxel was set as a homogeneous source of 90Y when IB photons are not modelled. For each l, the VSVs were computed for 90Y decays alone and for 90Y + IB. The analytical model was then built through fitting procedures of the VSVs including IB contribution.

RESULTS

Comparing GATE-VSVs with and without IB, differences between + 25% and + 30% were found for distances from the central voxel larger than the maximum β-range. The analytical model showed an agreement with MC simulations within ± 5% in the central voxel and in the Bremsstrahlung tails, for any l value examined, and relative differences lower than ± 40%, for other distances from the source.

CONCLUSIONS

The presented 90Y-VSVs include for the first time the contribution due to IB, thus providing a more accurate set of dosimetric factors for three-dimensional internal dosimetry of 90Y-labelled radiopharmaceuticals and medical devices. Furthermore, the analytical model constitutes an easy and fast alternative approach for 90Y-VSVs estimation for non-standard voxel dimensions.

Voxel-S-value methods adapted to heterogeneous media for quantitative Y-90 microsphere radioembolization dosimetry

PURPOSE

The absorbed dose estimation from Voxel-S-Value (VSV) method in heterogeneous media is suboptimal as VSVs are calculated in homogeneous media. The aim of this study is to develop and evaluate new VSV methods in order to enhance the accuracy of Y-90 microspheres absorbed dose estimation in liver, lungs, tumors and lung-liver interface regions.

METHODS

Ten patients with Y-90 microspheres SPECT/CT and PET/CT data, six of whom had additional Tc-99m-macroaggregated albumin SPECT/CT data, were analyzed from the Deep Blue Data Repository. Seven existing VSV methods along with three newly proposed VSV methods were evaluated: liver and lung kernel with center voxel scaling (LiLuCK), liver kernel with density correction and lung kernel with center voxel scaling (LiKDLuCK), liver kernel with center voxel scaling and lung kernel with density correction (LiCKLuKD). Monte Carlo (MC) results were regarded as the gold standard. Absolute absorbed dose errors (%AADE) of these methods for the liver, lungs, tumors, upper liver, and lower lungs were assessed.

RESULTS

Liver and tumor's median %AADE of all methods were <3% for three types of imaging data. In the lungs, however, three recently proposed VSV methods provided median %AADEs of less than 7%, whereas the differences exceeded 20% for existing methods that did not use a lung kernel. LiCKLuKD could achieve median %AADE <2% in the liver, upper liver and tumors, and median %AADE <7% in the lungs and lower lungs in three types of data.

CONCLUSION

All methods are consistent with MC in the liver and tumors. Methods with tissue-specific kernel and effective correction achieve smaller errors in lungs. LiCKLuKD has comparable results with MC in absorbed dose estimation of Y-90 radioembolization for all target regions.

The American Brachytherapy Society consensus statement for permanent implant brachytherapy using Yttrium-90 microsphere radioembolization for liver tumors

PURPOSE

To develop a multidisciplinary consensus for high quality multidisciplinary implementation of brachytherapy using Yttrium-90 (90Y) microspheres transarterial radioembolization (90Y TARE) for primary and metastatic cancers in the liver.

METHODS AND MATERIALS

Members of the American Brachytherapy Society (ABS) and colleagues with multidisciplinary expertise in liver tumor therapy formulated guidelines for 90Y TARE for unresectable primary liver malignancies and unresectable metastatic cancer to the liver. The consensus is provided on the most recent literature and clinical experience.

RESULTS

The ABS strongly recommends the use of 90Y microsphere brachytherapy for the definitive/palliative treatment of unresectable liver cancer when recommended by the multidisciplinary team. A quality management program must be implemented at the start of 90Y TARE program development and follow-up data should be tracked for efficacy and toxicity. Patient-specific dosimetry optimized for treatment intent is recommended when conducting 90Y TARE. Implementation in patients on systemic therapy should account for factors that may enhance treatment related toxicity without delaying treatment inappropriately. Further management and salvage therapy options including retreatment with 90Y TARE should be carefully considered.

CONCLUSIONS

ABS consensus for implementing a safe 90Y TARE program for liver cancer in the multidisciplinary setting is presented. It builds on previous guidelines to include recommendations for appropriate implementation based on current literature and practices in experienced centers. Practitioners and cooperative groups are encouraged to use this document as a guide to formulate their clinical practices and to adopt the most recent dose reporting policies that are critical for a unified outcome analysis of future effectiveness studies.

Accuracy of two dosimetry software programs for 177Lu radiopharmaceutical therapy using voxel-based patient-specific phantoms

Purpose

Virtual dosimetry using voxel-based patient-specific phantoms and Monte Carlo (MC) simulations offer the advantage of having a gold standard against which absorbed doses may be benchmarked to establish the dosimetry accuracy. Furthermore, these reference values assist in investigating the accuracy of the absorbed dose methodologies from different software programs. Therefore, this study aimed to compare the accuracy of the absorbed doses computed using LundADose and OLINDA/EXM 1.0.

Methods

The accuracy was based on ¹⁷⁷Lu-DOTATATE distributions of three voxel-based phantoms. SPECT projection images were simulated for 1, 24, 96, and 168 h post-administration and reconstructed with LundADose using 3D OS-EM reconstruction. Mono-exponential curves were fitted to the bio-kinetic data for the kidneys, liver, spleen, and tumours resulting in SPECT time-integrated activity (SPECT-TIA). The SPECT-TIA were used to compute mean absorbed doses using LundADose (LND-D_SPECT) and OLINDA (OLINDA-D_SPECT) for the organs. Pre-defined true activity images, were used to obtain TRUE-TIA and, together with full MC simulations, computed the true doses (MC-D_True). The dosimetry accuracy was assessed by comparing LND-D_SPECT and OLINDA-D_SPECT to MC-D_True.

Results

Overall, the results presented an overestimation of the mean absorbed dose by LND-D_SPECT compared to the MC-D_True with a dosimetry accuracy ≤6.6%. This was attributed to spill-out activity from the reconstructed LND-D_SPECT, resulting in a higher dose contribution than the MC-D_True. There was a general underestimation (<8.1%) of OLINDA-D_SPECT compared to MC-D_True attributed to the geometrical difference in shape between the voxel-based phantoms and the OLINDA models. Furthermore, OLINDA-D_SPECT considers self-doses while MC-D_True reflects self-doses plus cross-doses.

Conclusion

The better than 10% accuracy suggests that the mean dose values obtained with LND-D_SPECT and OLINDA-D_SPECT approximate the true values. The mean absorbed doses of the two software programs, and the gold standard were comparable. This work shall be of use for optimising ¹⁷⁷Lu dosimetry for clinical applications.

EANM dosimetry committee recommendations for dosimetry of 177Lu-labelled somatostatin-receptor- and PSMA-targeting ligands

The purpose of the EANM Dosimetry Committee is to provide recommendations and guidance to scientists and clinicians on patient-specific dosimetry. Radiopharmaceuticals labelled with lutetium-177 (177Lu) are increasingly used for therapeutic applications, in particular for the treatment of metastatic neuroendocrine tumours using ligands for somatostatin receptors and prostate adenocarcinoma with small-molecule PSMA-targeting ligands. This paper provides an overview of reported dosimetry data for these therapies and summarises current knowledge about radiation-induced side effects on normal tissues and dose-effect relationships for tumours. Dosimetry methods and data are summarised for kidneys, bone marrow, salivary glands, lacrimal glands, pituitary glands, tumours, and the skin in case of radiopharmaceutical extravasation. Where applicable, taking into account the present status of the field and recent evidence in the literature, guidance is provided. The purpose of these recommendations is to encourage the practice of patient-specific dosimetry in therapy with 177Lu-labelled compounds. The proposed methods should be within the scope of centres offering therapy with 177Lu-labelled ligands for somatostatin receptors or small-molecule PSMA.

Personalized Dosimetry in Targeted Radiation Therapy: A Look to Methods, Tools and Critical Aspects

Targeted radiation therapy (TRT) is a strategy increasingly adopted for the treatment of different types of cancer. The urge for optimization, as stated by the European Council Directive (2013/59/EURATOM), requires the implementation of a personalized dosimetric approach, similar to what already happens in external beam radiation therapy (EBRT). The purpose of this paper is to provide a thorough introduction to the field of personalized dosimetry in TRT, explaining its rationale in the context of optimization and describing the currently available methodologies. After listing the main therapies currently employed, the clinical workflow for the absorbed dose calculation is described, based on works of the most experienced authors in the literature and recent guidelines. Moreover, the widespread software packages for internal dosimetry are presented and critical aspects discussed. Overall, a selection of the most important and recent articles about this topic is provided.

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.

Quantitative SPECT/CT for dosimetry of peptide receptor radionuclide therapy

Neuroendocrine tumors (NETs) are uncommon malignancies of increasing incidence and prevalence. As these slow growing tumors usually overexpress somatostatin receptors (SSTRs), the use of ⁶⁸Ga-DOTA-peptides (gallium-68 chelated with dodecane tetra-acetic acid to somatostatin), which bind to the SSTRs, allows for PET based imaging and selection of patients for peptide receptor radionuclide therapy (PRRT). PRRT with radiolabeled somatostatin analogues such as ¹⁷⁷Lu-DOTATATE (lutetium-177-[DOTA,Tyr³]-octreotate), is mainly used for the treatment of metastatic or inoperable NETs. However, PRRT is generally administered at a fixed injected activity in order not to exceed dose limits in critical organs, which is suboptimal given the variability in radiopharmaceutical uptake among patients. Advances in SPECT (single photon emission computed tomography) imaging enable the absolute quantitative measure of the true radiopharmaceutical distribution providing for PRRT dosimetry in each patient. Personalized PRRT based on patient-specific dosimetry could improve therapeutic efficacy by optimizing effective tumor absorbed dose while limiting treatment related radiotoxicity.

Efficacy of voxel-based dosimetry map for predicting response to trans-arterial radioembolization therapy for hepatocellular carcinoma: a pilot study

OBJECTIVE

Typical clinical dosimetry models for trans-arterial radioembolization (TARE) assume uniform dose distribution in each tissue compartment. We performed simple voxel-based dosimetry using post-treatment 90Y PET following TARE with 90Y-resin microspheres and investigated its prognostic value in a pilot cohort.

METHOD

Ten patients with 14 hepatocellular carcinoma lesions who underwent TARE with 90Y-resin microspheres were retrospectively included. The partition model-based expected target tumor dose (TDp) was calculated using a pretreatment 99mTc-macroaggregated albumin scan. From post-treatment 90Y-microsphere PET and voxel-wise S-value kernels, voxel-based dose maps were produced and the absorbed dose of each lesion (TDv) was calculated. Heterogeneity of intratumoral absorbed doses was assessed using the SD and coefficient of variation of voxel doses. The response of each lesion was determined based on contrast-enhanced MRI or CT, or both. Lesion responses were classified as local control success or failure. Prognostic values of dosimetry parameters and clinicopathological factors were evaluated in terms of progression-free survival (PFS) of each lesion.

RESULTS

TDv was significantly different between local control success and failure groups, whereas tumor size, TDp and intratumoral dose heterogeneity were not. Univariate survival analysis identified serum aspartate transaminase level ≥40 IU/L, tumor size ≥66 mm and TDv <81 Gy as significant prognostic factors for PFS. However, only TDv was an independent predictive factor in the multivariate analysis (P = 0.022). There was a significant correlation between TDv and PFS (P = 0.009; r = 0.669).

CONCLUSIONS

In TARE, voxel-based dose index TDv can be estimated on post-treatment 90Y PET using a simple method. TDv was a more effective prognostic factor for TARE than TDp and clinicopathologic factors in this pilot study. Further studies are warranted on the role of voxel-based dose and dose distribution in TARE.

EANM dosimetry committee series on standard operational procedures: a unified methodology for 99mTc-MAA pre- and 90Y peri-therapy dosimetry in liver radioembolization with 90Y microspheres

The aim of this standard operational procedure is to standardize the methodology employed for the evaluation of pre- and post-treatment absorbed dose calculations in 90Y microsphere liver radioembolization. Basic assumptions include the permanent trapping of microspheres, the local energy deposition method for voxel dosimetry, and the patient-relative calibration method for activity quantification.The identity of 99mTc albumin macro-aggregates (MAA) and 90Y microsphere biodistribution is also assumed. The large observed discrepancies in some patients between 99mTc-MAA predictions and actual 90Y microsphere distributions for lesions is discussed. Absorbed dose predictions to whole non-tumoural liver are considered more reliable and the basic predictors of toxicity. Treatment planning based on mean absorbed dose delivered to the whole non-tumoural liver is advised, except in super-selective treatments.Given the potential mismatch between MAA simulation and actual therapy, absorbed doses should be calculated both pre- and post-therapy. Distinct evaluation between target tumours and non-tumoural tissue, including lungs in cases of lung shunt, are vital for proper optimization of therapy. Dosimetry should be performed first according to a mean absorbed dose approach, with an optional, but important, voxel level evaluation. Fully corrected 99mTc-MAA Single Photon Emission Computed Tomography (SPECT)/computed tomography (CT) and 90Y TOF PET/CT are regarded as optimal acquisition methodologies, but, for institutes where SPECT/CT is not available, non-attenuation corrected 99mTc-MAA SPECT may be used. This offers better planning quality than non dosimetric methods such as Body Surface Area (BSA) or mono-compartmental dosimetry. Quantitative 90Y bremsstrahlung SPECT can be used if dedicated correction methods are available.The proposed methodology is feasible with standard camera software and a spreadsheet. Available commercial or free software can help facilitate the process and improve calculation time.

Impact of the dosimetry approach on the resulting 90Y radioembolization planned absorbed doses based on 99mTc-MAA SPECT-CT: is there agreement between dosimetry methods?

BACKGROUND

Prior radioembolization, a simulation using ^99mTc-macroaggregated albumin as ⁹⁰Y-microspheres surrogate is performed. Gamma scintigraphy images (planar, SPECT, or SPECT-CT) are acquired to evaluate intrahepatic ⁹⁰Y-microspheres distribution and detect possible extrahepatic and lung shunting. These images may be used for pre-treatment dosimetry evaluation to calculate the ⁹⁰Y activity that would get an optimal tumor response while sparing healthy tissues. Several dosimetry methods are available, but there is still no consensus on the best methodology to calculate absorbed doses. The goal of this study was to retrospectively evaluate the impact of using different dosimetry approaches on the resulting ⁹⁰Y-radioembolization pre-treatment absorbed dose evaluation based on ^99mTc-MAA images.

METHODS

Absorbed doses within volumes of interest resulting from partition model (PM) and 3D voxel dosimetry methods (3D-VDM) (dose-point kernel convolution and local deposition method) were evaluated. Additionally, a new "Multi-tumor Partition Model" (MTPM) was developed. The differences among dosimetry approaches were evaluated in terms of mean absorbed dose and dose volume histograms within the volumes of interest.

RESULTS

Differences in mean absorbed dose among dosimetry methods are higher in tumor volumes than in non-tumoral ones. The differences between MTPM and both 3D-VDM were substantially lower than those observed between PM and any 3D-VDM. A poor correlation and concordance were found between PM and the other studied dosimetry approaches. DVH obtained from either 3D-VDM are pretty similar in both healthy liver and individual tumors. Although no relevant global differences, in terms of absorbed dose in Gy, between both 3D-VDM were found, important voxel-by-voxel differences have been observed.

CONCLUSIONS

Significant differences among the studied dosimetry approaches for ⁹⁰Y-radioembolization treatments exist. Differences do not yield a substantial impact in treatment planning for healthy tissue but they do for tumoral liver. An individual segmentation and evaluation of the tumors is essential. In patients with multiple tumors, the application of PM is not optimal and the 3D-VDM or the new MTPM are suggested instead. If a 3D-VDM method is not available, MTPM is the best option. Furthermore, both 3D-VDM approaches may be indistinctly used.

Absorbed dose distributions from beta-decaying radionuclides: Experimental validation of Monte Carlo tools for radiopharmaceutical dosimetry

PURPOSE

This study aims to experimentally validate the Monte Carlo generated absorbed doses from the beta particles emitted by ⁹⁰ Y and ¹⁷⁷ Lu using radiochromic EBT3 film-based dosimetry.

METHODS

Line sources of ⁹⁰ Y and ¹⁷⁷ Lu were inserted longitudinally through blocks of low-density polyethylene and tissue-equivalent slabs of cortical bone and lung equivalent plastics. Radiochromic film (Gafchromic EBT3) was laser cut to accommodate orthogonal line sources of radioactivity, and the film was sandwiched intimately between the rectangular blocks to achieve charged particle equilibrium. Line sources consisted of plastic capillary tube of length (13 ± 0.1) cm, with 0.42-mm inner diameter and a wall thickness of 0.21 mm. ⁹⁰ Y line sources were prepared from a solution of dissolved ⁹⁰ Y resin microspheres. ¹⁷⁷ Lu line sources were prepared from an aliquot of ¹⁷⁷ Lu-DOTATATE. Film exposures were conducted for durations ranging from 10 min to 38 h. Radiochromic film calibration was performed by irradiation with 6-MV-bremsstrahlung x rays from a calibrated linear accelerator, in accordance with literature recommendations. Experimental geometries were precisely simulated within the GATE Monte Carlo toolkit, which has previously been used for the generation of dose point kernels.

RESULTS

The mean percentage difference between measured and simulated absorbed doses were 5.04% and 7.21% for ⁹⁰ Y and ¹⁷⁷ Lu beta absorbed dose in the range of (0.1-10) Gy. Additionally, 1D gamma analysis using a local 10%/1 mm gamma criterion was performed to compare the absorbed dose distributions. The percentage of measurement points passing the gamma criterion, averaged over all tests, was 93.5%.

CONCLUSIONS

We report the experimental validation of Monte Carlo derived beta absorbed dose distributions for ⁹⁰ Y and ¹⁷⁷ Lu, solidifying the validity of using Monte Carlo-based methods for estimating absorbed dose from beta emitters. Overall, excellent agreement was observed between the experimental beta absorbed doses in the linear region of the radiochromic film and the GATE Monte Carlo simulations demonstrating that radiochromic film dosimetry has sufficient sensitivity and spatial resolution to be used as a tool for measuring beta decay absorbed dose distributions.

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.

Transarterial Radioembolization for Hepatocellular Carcinoma and Hepatic Metastases: Clinical Aspects and Dosimetry Models

Transarterial radioembolization (TARE) with Yttrium-90 (⁹⁰Y) microspheres is a liver-directed therapy for primary and metastatic disease. This manuscript provides a review of the clinical literature on TARE indications and efficacy with overviews of patient-selection and toxicity. Current dosimetry models used in practice are safe, relatively simple, and easy for clinicians to use. Planning currently relies on the imperfect surrogate, ^99mTc macroaggregated albumin. Post-therapy quantitative imaging (⁹⁰Y SPECT/CT or ⁹⁰Y PET/CT) of microspheres can be used to calculate the macroscopic in vivo absorbed dose distribution. Similar to the evolution of other brachytherapy dose calculations, TARE is moving toward more patient-specific dosimetry that includes calculating and reporting nonuniform dose distributions throughout tumors and normal uninvolved liver.

Technical Note: Rapid multiexponential curve fitting algorithm for voxel-based targeted radionuclide dosimetry

BACKGROUND

Dosimetry in nuclear medicine often relies on estimating pharmacokinetics based on sparse temporal data. As analysis methods move toward image-based three-dimensional computation, it becomes important to interpolate and extrapolate these data without requiring manual intervention; that is, in a manner that is highly efficient and reproducible. Iterative least-squares solvers are poorly suited to this task because of the computational overhead and potential to optimize to local minima without applying tight constraints at the outset.

METHODOLOGY

This work describes a fully analytical method for solving three-phase exponential time-activity curves based on three measured time points in a manner that may be readily employed by image-based dosimetry tools. The methodology uses a series of conditional statements and a piecewise approach for solving exponential slope directly through measured values in most instances. The proposed algorithm is tested against a purpose-designed iterative fitting technique and linear piecewise method followed by single exponential in a cohort of ten patients receiving ¹⁷⁷ Lu-DOTA-Octreotate therapy.

RESULTS

Tri-exponential time-integrated values are shown to be comparable to previously published methods with an average difference between organs when computed at the voxel level of 9.8 ± 14.2% and -3.6 ± 10.4% compared to iterative and interpolated methods, respectively. Of the three methods, the proposed tri-exponential algorithm was most consistent when regional time-integrated activity was evaluated at both voxel- and whole-organ levels. For whole-body SPECT imaging, it is possible to compute 3D time-integrated activity maps in <5 min processing time. Furthermore, the technique is able to predictably and reproducibly handle artefactual measurements due to noise or spatial misalignment over multiple image times.

CONCLUSIONS

An efficient, analytical algorithm for solving multiphase exponential pharmacokinetics is reported. The method may be readily incorporated into voxel-dose routines by combining with widely available image registration and radiation transport tools.

Radiation doses from 161Tb and 177Lu in single tumour cells and micrometastases

Background

Targeted radionuclide therapy (TRT) is gaining importance. For TRT to be also used as adjuvant therapy or for treating minimal residual disease, there is a need to increase the radiation dose to small tumours. The aim of this in silico study was to compare the performances of ¹⁶¹Tb (a medium-energy β⁻ emitter with additional Auger and conversion electron emissions) and ¹⁷⁷Lu for irradiating single tumour cells and micrometastases, with various distributions of the radionuclide.

Methods

We used the Monte Carlo track-structure (MCTS) code CELLDOSE to compute the radiation doses delivered by ¹⁶¹Tb and ¹⁷⁷Lu to single cells (14 μm cell diameter with 10 μm nucleus diameter) and to a tumour cluster consisting of a central cell surrounded by two layers of cells (18 neighbours). We focused the analysis on the absorbed dose to the nucleus of the single tumoral cell and to the nuclei of the cells in the cluster. For both radionuclides, the simulations were run assuming that 1 MeV was released per μm³ (1436 MeV/cell). We considered various distributions of the radionuclides: either at the cell surface, intracytoplasmic or intranuclear.

Results

For the single cell, the dose to the nucleus was substantially higher with ¹⁶¹Tb compared to ¹⁷⁷Lu, regardless of the radionuclide distribution: 5.0 Gy vs. 1.9 Gy in the case of cell surface distribution; 8.3 Gy vs. 3.0 Gy for intracytoplasmic distribution; and 38.6 Gy vs. 10.7 Gy for intranuclear location. With the addition of the neighbouring cells, the radiation doses increased, but remained consistently higher for ¹⁶¹Tb compared to ¹⁷⁷Lu. For example, the dose to the nucleus of the central cell of the cluster was 15.1 Gy for ¹⁶¹Tb and 7.2 Gy for ¹⁷⁷Lu in the case of cell surface distribution of the radionuclide, 17.9 Gy for ¹⁶¹Tb and 8.3 Gy for ¹⁷⁷Lu for intracytoplasmic distribution and 47.8 Gy for ¹⁶¹Tb and 15.7 Gy for ¹⁷⁷Lu in the case of intranuclear location.

Conclusion

¹⁶¹Tb should be a better candidate than ¹⁷⁷Lu for irradiating single tumour cells and micrometastases, regardless of the radionuclide distribution.

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.

EANM Dosimetry Committee series on standard operational procedures for internal dosimetry for 131I mIBG treatment of neuroendocrine tumours

The purpose of the EANM Dosimetry Committee Series on "Standard Operational Procedures for Dosimetry" (SOP) is to provide advice to scientists and clinicians on how to perform patient-specific absorbed dose assessments. This SOP describes image and data acquisition parameters and dosimetry calculations to determine the absorbed doses delivered to whole-body, tumour and normal organs following a therapeutic administration of 131I mIBG for the treatment of neuroblastoma or adult neuroendocrine tumours. Recommendations are based on evidence in recent literature where available and on expert opinion within the community. This SOP is intended to promote standardisation of practice within the community and as such is based on the facilities and expertise that should be available to any centre able to perform specialised treatments with radiopharmaceuticals and patient-specific dosimetry. A clinical example is given to demonstrate the application of the absorbed dose calculations.

Dose point kernels for 2,174 radionuclides

PURPOSE

Rapid adoption of targeted radionuclide therapy as an oncologic intervention has motivated the development of patient-specific voxel-wise approaches to radiation dosimetry. These approaches often rely on pretabulated dose point kernels for convolution-based calculations; however, these dose kernels are sparse in literature and often have suboptimal characteristics. The purpose of this work was to generate an extensive library of dose point kernels with sufficient size and resolution for general clinical application of voxel-wise dosimetry.

METHODS

Nuclear data were acquired for 2174 radionuclides from the National Nuclear Data Center (Brookhaven National Laboratory, accessed March 2018). Based on these data, isotropic point sources of radioactivity in water were simulated using Monte Carlo N-Particle transport v6.2 (MCNP6.2, Los Alamos National Laboratory). Simulations were separated by emission type for each radionuclide - photons (γ-rays, x rays), beta particles (positrons, electrons); and discrete electrons (conversion electrons, Auger electrons, Coster-Kronig electrons). Dose was tallied in concentric spherical shells about the point source using an energy deposition pulse-height tally (MCNP *F8 tally). Bins were spaced every 0.1 mm until a radius of 10 cm, and every 1 mm until a radius of 2 m. Positron emissions where treated as electrons for transport, with annihilation photons generated at the origin within the photon simulation. Alpha particle emissions were not simulated since their energy is deposited within ~0.2 mm of the source. Neutron and spallation effects were not considered. A subset of the resultant dose point kernels (¹¹ C, ¹⁸ F, ³² P, ^52g Mn, ⁶⁴ Cu, ⁶⁷ Ga, ⁸⁹ Sr, ⁸⁹ Zr, ⁹⁰ Y, ^99m Tc, ¹¹¹ In, ^117m Sn, ¹²³ I, ¹²⁴ I, ¹²⁵ I, ¹³¹ I, ¹⁵³ Sm, ¹⁷⁷ Lu, ¹⁸⁶ Re, ¹⁸⁸ Re, ²¹¹ As, ²¹² Pb, ²¹³ Bi, ²²³ Ra, and ²²⁵ Ac) were evaluated for accuracy based on conservation of energy, comparison to kernels in the literature, and statistical precision.

RESULTS

Among dose point kernels that were manually reviewed, good agreement with previously published dose point kernels was observed. Energy within the kernels was found to be conserved to within 1% of the value expected from nuclear data, suggesting that a radius of 2 m was sufficient to capture the almost all of the energy released during decay for all isotopes considered. Local dosimetric uncertainty, evaluated at the radius of 99% energy deposition, was found to be less than 9% for all radioisotopes evaluated. Rebinning data more coarsely by a factor of 10, similar to what would be done for a clinical dose calculation, results in all evaluated kernels having a relative error of less than 1.1% at R_50% , 1.5% at R_90% , and 2.7% at R_99% (the radius corresponding to 50%, 90%, and 99% of total energy deposition, respectively). The kernels produced in this work have been made freely available (https://zenodo.org/record/2564036).

CONCLUSIONS

An extensive library of high-resolution radial dose kernels was generated and validated against published data. In addition to enabling patient-specific voxel-wise internal dosimetry by convolution superposition, the generated dose point kernels data may prove useful to the wider health physics community.

PET-CT post therapy dosimetry in radioembolization with resin 90Y microspheres: Comparison with pre-treatment SPECT-CT 99mTc-MAA results

Resin microspheres radioembolization is an effective treatment for liver tumors when the surgical option is not feasible. Doses delivered to tumor and normal liver can be assess in the pre-therapy phase by means of a ^99mTc-MAA SPECT-CT simulation and after the treatment with ⁹⁰Y PET-CT acquisition. The optimal therapeutic ⁹⁰Y activity is determined on ^99mTc-MAA SPECT-CT dose results in order to avoid healthy parenchyma toxicity and to effectively irradiate the tumor. The assumption of identical radiopharmaceutical distribution between simulation and verification is still under debate and literature data showed controversial results. In this study 10 HCC patient's dosimetry performed on ^99mTc SPECT-CT and ⁹⁰Y PET-CT were compared. Patients were selected when a good agreement between the pre and post-therapy distribution was observed in order to investigate the intrinsic dosimetric variations between the two imaging modalities. Mean doses (MIRD and Voxel approaches) showed a good correlation (Pearson's coefficient r > 0.90) both for tumor and normal liver. Dose Volume Histogram curves were compared with a good agreement particularly for normal liver (D50). Goal doses were achieved for 90% of patients. Bland-Altman analysis indicates lower variations for healthy parenchyma than for tumor (1.96 SD equal to 9.1 Gy and 68 Gy respectively) confirming the robustness of the dose-toxicity approach. PET-CT dosimetry well correlates with SPECT-CT doses (under assumption of same catheter position and ⁹⁰Y activity). Better agreement was showed for 7/10 and 8/10 patients for T and NL respectively, confirming dosimetry as effective tool to optimize and individualize the treatment.

90 Y SPECT/CT quantitative study and comparison of uptake with pretreatment 99 m Tc-MAA SPECT/CT in radiomicrosphere therapy

INTRODUCTION

Yttrium-90 (90 Y) microsphere post-treatment imaging reflects the true distribution characteristics of microspheres in the tumor and liver compartments. However, due to its decay spectra profile lacking a pronounced photopeak, the bremsstrahlung imaging for 90 Y has inherent limitations. The absorbed dose calculations for 90 Y microspheres radiomicrosphere therapy (RMT) sustain a limitation due to the poor quality of 90 Y imaging. The aim of this study was to develop quantitative methods to improve the post-treatment 90 Y bremsstrahlung single photon emission tomography (SPECT)/computed tomography (CT) image analysis for dosimetric purposes and to perform a quantitative comparison with the 99m Tc-MAA SPECT/CT images, which is used for theranostics purposes for liver and tumor dosimetry.

METHODS

Pre and post-treatment SPECT/CT data of patients who underwent RMT for primary or metastatic liver cancer were acquired. A Jasczak phantom with eight spherical inserts of various sizes was used to obtain optimal iteration number for the contrast recovery algorithm for improving 90 Y bremsstrahlung SPECT/CT images. Comparison of uptake on 99m Tc-MAA and 90 Y microsphere SPECT/CT images was assessed using tumor to healthy liver ratios (TLRs). The voxel dosimetry technique was used to estimate absorbed doses. Absorbed doses within the tumor and healthy part of the liver were also investigated for correlation with administered activity.

RESULTS

Improvement in CNR and contrast recovery coefficients on patient and phantom 90 Y bremsstrahlung SPECT/CT images respectively were achieved. The 99m Tc-MAA and 90 Y microspheres SPECT/CT images showed significant uptake correlation (r = 0.9, P = 0.05) with mean TLR of 9.4 ± 9.2 and 5.0 ± 2.2, respectively. The correlation between the administered activity and tumor absorbed dose was weak (r = 0.5, P > 0.05), however, healthy liver absorbed dose increased with administered activity (r = 0.8, P = 0.0).

CONCLUSIONS

This study demonstrated correlation in mean TLR between 99m Tc-MAA and 90 Y microsphere SPECT/CT.

Biodistribution, pharmacokinetics, and organ-level dosimetry for 188Re-AHDD-Lipiodol radioembolization based on quantitative post-treatment SPECT/CT scans

Background

Rhenium-188-labelled-Lipiodol radioembolization is a safe and cost-effective treatment for primary liver cancer. In order to determine correlations between treatment doses and patient response to therapy, accurate patient-specific dosimetry is required. Up to date, the reported dosimetry of ¹⁸⁸Re-Lipiodol has been based on whole-body (WB) planar imaging only, which has limited quantitative accuracy. The aim of the present study is to determine the in vivo pharmacokinetics, bio-distribution, and organ-level dosimetry of ¹⁸⁸Re-AHDD-Lipiodol radioembolization using a combination of post-treatment planar and quantitative SPECT/CT images. Furthermore, based on the analysis of the pharmacokinetic data, a practical and relatively simple imaging and dosimetry method that could be implemented in clinics for ¹⁸⁸Re-AHDD-Lipiodol radioembolization is proposed.

Thirteen patients with histologically proven hepatocellular carcinoma underwent ¹⁸⁸Re-AHDD-Lipiodol radioembolization. A series of 2–3 WB planar images and one SPECT/CT scan were acquired over 48 h after the treatment. The time-integrated activity coefficients (TIACs, also known as residence-times) and absorbed doses of tumors and organs at risk (OARs) were determined using a hybrid WB/SPECT imaging method.

Results

Whole-body imaging showed that ¹⁸⁸Re-AHDD-Lipiodol accumulated mostly in the tumor and liver tissue but a non-negligible amount of the pharmaceutical was also observed in the stomach, lungs, salivary glands, spleen, kidneys, and urinary bladder. On average, the measured effective half-life of ¹⁸⁸Re-AHDD-Lipiodol was 12.5 ± 1.9 h in tumor. The effective half-life in the liver and lungs (the two organs at risk) was 12.6 ± 1.7 h and 12.0 ± 1.9 h, respectively. The presence of ¹⁸⁸Re in other organs was probably due to the chemical separation and subsequent release of the free radionuclide from Lipiodol.

The average doses per injected activity in the tumor, liver, and lungs were 23.5 ± 40.8 mGy/MBq, 2.12 ± 1.78 mGy/MBq, and 0.11 ± 0.05 mGy/MBq, respectively. The proposed imaging and dosimetry method, consisting of a single SPECT/CT for activity determination followed by ¹⁸⁸Re-AHDD-Lipiodol clearance with the liver effective half-life of 12.6 h, resulted in TIACs estimates (and hence, doses) mostly within ± 20% from the reference TIACs (estimated using three WB images and one SPECT/CT).

Conclusions

The large inter-patient variability of the absorbed doses in tumors and normal tissue in ¹⁸⁸Re-HDD-Lipiodol radioembolization patients emphasizes the importance of patient-specific dosimetry calculations based on quantitative post-treatment SPECT/CT imaging.

Electronic supplementary material

The online version of this article (10.1186/s40658-018-0227-6) contains supplementary material, which is available to authorized users.

Dose volume histogram-based optimization of image reconstruction parameters for quantitative 90 Y-PET imaging

PURPOSE

⁹⁰ Y-microsphere radioembolization or selective internal radiation therapy is increasingly being used as a treatment option for tumors that are not candidates for surgery and external beam radiation therapy. Recently, volumetric ⁹⁰ Y-dosimetry techniques have been implemented to explore tumor dose-response on the basis of 3D ⁹⁰ Y-activity distribution from PET imaging. Despite being a theranostic study, the optimization of quantitative ⁹⁰ Y-PET image reconstruction still uses the mean activity concentration recovery coefficient (RC) as the objective function, which is more relevant to diagnostic and detection tasks than is to dosimetry. The aim of this study was to optimize ⁹⁰ Y-PET image reconstruction by minimizing errors in volumetric dosimetry via the dose volume histogram (DVH). We propose a joint optimization of the number of equivalent iterations (the product of the iterations and subsets) and the postreconstruction filtration (FWHM) to improve the accuracy of voxel-level ⁹⁰ Y dosimetry.

METHODS

A modified NEMA IEC phantom was used to emulate clinically relevant ⁹⁰ Y-PET imaging conditions through various combinations of acquisition durations, activity concentrations, sphere-to-background ratios, and sphere diameters. PET data were acquired in list mode for 300 min in a single-bed position; we then rebinned the list mode PET data to 60, 45, 30, 15, and 5 min per bed, with 10 different realizations. Errors in the DVH were calculated as root mean square errors (RMSE) of the differences in the image-based DVH and the expected DVH. The new optimization approach was tested in a phantom study, and the results were compared with the more commonly used objective function of the mean activity concentration RC.

RESULTS

In a wide range of clinically relevant imaging conditions, using 36 equivalent iterations with a 5.2-mm filtration resulted in decreased systematic errors in volumetric ⁹⁰ Y dosimetry, quantified as image-based DVH, in ⁹⁰ Y-PET images reconstructed using the ordered subset expectation maximization (OSEM) iterative reconstruction algorithm with time of flight (TOF) and point spread function (PSF) modeling. Our proposed objective function of minimizing errors in DVH, which allows for joint optimization of ⁹⁰ Y-PET iterations and filtration for volumetric quantification of the ⁹⁰ Y dose, was shown to be superior to conventional RC-based optimization approaches for image-based absorbed dose quantification.

CONCLUSION

Our proposed objective function of minimizing errors in DVH, which allows for joint optimization of iterations and filtration to reduce errors in the PET-based volumetric quantification ⁹⁰ Y dose, is relevant to dosimetry in therapy procedures. The proposed optimization method using DVH as the objective function could be applied to any imaging modality used to assess voxel-level quantitative information.

Technical Note: Virtual CT for reducing CT dose in targeted radionuclide therapy dosimetry

PURPOSE

Previously we have shown that using sequential CT images is superior to sequential SPECT for nonrigid registration in three-dimensional (3D) targeted radionuclide therapy (TRT) dosimetry. However, sequential CTs are often not available due to radiation concerns. In this paper, we propose a virtual CT (vCT) method for attenuation and scatter correction, image registration, and segmentation for improved dosimetric accuracy with single CT acquisition.

METHODS

We used a population of nine XCAT phantoms with different In-111 Zevalin biokinetics and anatomical variations for the simulations. An analytical projector was used to simulate sequential SPECT/CT acquisitions for a medium energy general purpose collimator at 1, 12, 24, 72, and 144 h postinjection, modeling attenuation, scatter, and geometric collimator-detector response. The corresponding sequential attenuation maps of the phantoms served as real CT (rCT) images. For vCT generation, we investigated three registration methods, that is, (a) SPECT to SPECT; (b) SPECT to CT, and (c) CT to SPECT, and the optimal time point for single CT acquisition. Difference images and average normalized mean square errors (NMSE) were calculated between different vCTs and their corresponding rCTs. Absorbed dose and dose-volume histograms (DVHs) for critical organs were computed for the rCT, optimized vCT, and conventional single CT (1CT) protocols, respectively, for dosimetric analyses.

RESULTS

For vCT generation, SPECT to SPECT registration with a single CT acquired at the first time point shows the smallest difference and NMSE. For organ absorbed doses, the results of vCT were similar to those of rCT and were superior to 1CT, that is, -0.24 ± 1.56% vs -0.49 ± 1.76% vs -6.37 ± 5.63% for the liver, -1.05 ± 2.89% vs -0.69 ± 2.74% vs -4.87 ± 4.35% for kidneys, respectively. The results of DVHs also showed improvement for all organs using vCTs as compared to the conventional 1CT protocol.

CONCLUSION

The optimized vCT method can effectively increase the TRT dosimetric results if there is only a single CT available in the sequential imaging protocol, reducing the substantial increase in radiation burden from repeated CT scans.

Dosimetry methods and clinical applications in peptide receptor radionuclide therapy for neuroendocrine tumours: a literature review

Background

The main challenge for systemic radiation therapy using radiopharmaceuticals (SRT) is to optimise the dose delivered to the tumour, while minimising normal tissue irradiation. Dosimetry could help to increase therapy response and decrease toxicity after SRT by individual treatment planning. Peptide receptor radionuclide therapy (PRRT) is an accepted SRT treatment option for irresectable and metastatic neuroendocrine tumours (NET). However, dosimetry in PRRT is not routinely performed, mainly due to the lack of evidence in literature and clinical implementation difficulties. The goal of this review is to provide insight in dosimetry methods and requirements and to present an overview of clinical aspects of dosimetry in PRRT for NET.

Methods

A PubMed query including the search criteria dosimetry, radiation dose, peptide receptor radionuclide therapy, and radionuclide therapy was performed. Articles were selected based on title and abstract, and description of dosimetric approach.

Results

A total of 288 original articles were included. The most important dosimetry methods, their main advantages and limitations, and implications in the clinical setting are discussed. An overview of dosimetry in clinical studies regarding PRRT treatment for NET is provided.

Conclusion

Clinical dosimetry in PRRT is feasible and can result in improved treatment outcomes. Current clinical dosimetry studies focus on safety and apply non-voxel-based dosimetry methods. Personalised treatment using sophisticated dosimetry methods to assess tumour and normal tissue uptake in clinical trials is the next step towards routine dosimetry in PRRT for NET.

Electronic supplementary material

The online version of this article (10.1186/s13550-018-0443-z) contains supplementary material, which is available to authorized users.

Bone lesion absorbed dose profiles in patients with metastatic prostate cancer treated with molecular radiotherapy

OBJECTIVE

The aim of this study was to calculate the range of absorbed doses that could potentially be delivered by a variety of radiopharmaceuticals and typical fixed administered activities used for bone pain palliation in a cohort of patients with metastatic castration-resistant prostate cancer (mCRPC). The methodology for the extrapolation of the biodistribution, pharmacokinetics and absorbed doses from a given to an alternative radiopharmaceutical is presented.

METHODS

Sequential single photon emission CT images from 22 patients treated with 5 GBq of 186Re-HEDP were used to extrapolate the time-activity curves for various radiopharmaceuticals. Cumulated activity distributions for the delivered and extrapolated treatment plans were converted into absorbed dose distributions using the convolution dosimetry method. The lesion absorbed doses obtained for the different treatments were compared using the patient population distributions and cumulative dose-volume histograms.

RESULTS

The median lesion absorbed doses across the patient cohort ranged from 2.7 Gy (range: 0.6-11.8 Gy) for 1100 MBq of 166Ho-DOTMP to 21.8 Gy (range: 4.5-117.6 Gy) for 150 MBq of 89Sr-dichloride. 32P-Na3PO4, 153Sm-EDTMP, 166Ho-DOTMP, 177Lu-EDTMP and 188Re-HEDP would have delivered 41, 32, 85, 20 and 64% lower absorbed doses, for the typical administered activities as compared to 186Re-HEDP, respectively, whilst 89Sr-dichloride would have delivered 25% higher absorbed doses.

CONCLUSION

For the patient cohort studied, a wide range of absorbed doses would have been delivered for typical administration protocols in mCRPC. The methodology presented has potential use for emerging theragnostic agents. Advances in knowledge: The same patient cohort can receive a range of lesion absorbed doses from typical molecular radiotherapy treatments for patients with metastatic prostate cancer, highlighting the need to establish absorbed dose response relationships and to treat patients according to absorbed dose instead of using fixed administered activities.

Evaluation of sequential SPECT and CT for targeted radionuclide therapy dosimetry

PURPOSE

In targeted radionuclide therapy (TRT), a prior knowledge of the absorbed dose biodistribution is essential for pre-therapy treatment planning. Previously, we showed that non-rigid organ-by-organ registration in sequential quantitative SPECT images improved dose estimation. This study aims to investigate if sequential CT can further improve TRT dosimetric accuracy.

METHODS

We simulated SPECT/CT acquisitions at 1, 12, 24, 72 and 144 h In-111 Zevalin post-injection using an analytical MEGP projector, modeling attenuation, scatter and collimator-detector response. We later recruited a patient injected with 222 MBq In-111 DTPAOC imaged at 3 SPECT/CT sessions for clinical evaluations. Four registration schemes were evaluated: whole-body-based registration performed on sequential (1) SPECT (WB-SPECT) or (2) CT (WB-CT) images; organ-based registration applied on organs individually segmented from sequential (3) SPECT (O-SPECT) or (4) CT (O-CT) images. Voxel-by-voxel integration was performed followed by Y-90 voxel-S-kernel convolution. Organ-absorbed doses, iso-dose curves, dose-volume histograms (DVHs) were generated for targeted organs for analysis.

RESULTS

In simulation study, organ-absorbed dose errors were (- 8.66 ± 2.83)%, (- 2.51 ± 3.69)%, (- 9.23 ± 3.28)%, (- 7.17 ± 2.53)% for liver, (- 14.81 ± 4.91)%, (- 3.60 ± 4.37)%, (- 18.13 ± 4.44)%, (- 11.34 ± 4.22)% for spleen, for O-SPECT, O-CT, WB-SPECT and WB-CT registrations, respectively. For all organs, O-CT showed superior results. Results of iso-dose contour, DVHs were in accordance with the organ-absorbed doses. In clinical studies, the results were also consistent which showed O-CT method deviated the most from the result with no registration.

CONCLUSIONS

We conclude that if both sequential SPECT/CT scans are available, CT organ-based registration method can more effectively improve the 3D dose estimation. Sequential low-dose CT scans might be considered to be included in the standard TRT protocol.

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.

Specific Absorbed Fractions of Internal Photon and Electron Emitters in a Human Voxel-based Phantom: A Monte Carlo Study

The specific absorbed fraction (SAF) of energy is an essential element of internal dose assessment. Here reported a set of SAFs calculated for selected organs of a human voxel-based phantom. The Monte Carlo transport code GATE version 6.1 was used to simulate monoenergetic photons and electrons with energies ranging from 10 keV to 2 MeV. The particles were emitted from three source organs: kidneys, liver, and spleen. SAFs were calculated for three target regions in the body (kidneys, liver, and spleen) and compared with the results obtained using the MCNP4B and GATE/GEANT4 Monte Carlo codes. For most photon energies, the self-irradiation is higher, and the cross-irradiation is lower in the GATE results compared to the MCNP4B. The results show generally good agreement for photons and high-energy electrons with discrepancies within − 2% ±3%. Nevertheless, significant differences were found for cross-irradiation of photons of lower energy and electrons of higher energy due to statistical uncertainties larger than 10%. The comparisons of the SAF values for the human voxel phantom do not show significant differences, and the results also demonstrated the usefulness and applicability of GATE Monte Carlo package for voxel level dose calculations in nonuniform media. The present SAFs calculation for the Zubal voxel phantom is validated by the intercomparison of the results obtained by other Monte Carlo codes.

Personalized Dosimetry for Radionuclide Therapy Using Molecular Imaging Tools

For treatment of systemic malignancies, when external radiation therapy is not applicable, radionuclide therapy can be an alternative. In this form of therapy, radionuclides are administered to the patient, often in a form where the radionuclide is labelled to a molecule that plays the active part in the localization of the tumor. Since the aim is to impart lethal damage to tumor cells while maintaining possible side-effects to normal tissues at tolerable levels, a proper and accurate personalized dosimetry should be a pre-requisite. In radionuclide therapy, there is a need to measure the distribution of the radiopharmaceutical in vivo, as well as its re-distribution over time, in order estimate the total energy released in radioactive decays and subsequent charged-particle interactions, governing the absorbed dose to different organs and tumors. Measurements are usually performed by molecular imaging, more specifically planar and SPECT (Single-Photon Emission Computed Tomography) imaging, combined with CT. This review describes the different parts in the dosimetry chain of radionuclide therapy. Emphasis is given to molecular imaging tools and the requirements for determining absorbed doses from quantitative planar and SPECT images. As example solutions to the different problems that need to be addressed in such a dosimetric chain, we describe our tool, Lundadose, which is a set of methods that we have developed for personalized dosimetry.

Whole-remnant and maximum-voxel SPECT/CT dosimetry in 131 I-NaI treatments of differentiated thyroid cancer

PURPOSE

To investigate the possible differences between SPECT/CT based whole-remnant and maximum-voxel dosimetry in patients receiving radio-iodine ablation treatment of differentiated thyroid cancer (DTC).

METHODS

Eighteen DTC patients were administered 1.11 GBq of 131 I-NaI after near-total thyroidectomy and rhTSH stimulation. Two patients had two remnants, so in total dosimetry was performed for 20 sites. Three SPECT/CT scans were performed for each patient at 1, 2, and 3-7 days after administration. The activity, the remnant mass, and the maximum-voxel activity were determined from these images and from a recovery-coefficient curve derived from experimental phantom measurements. The cumulated activity was estimated using trapezoidal-exponential integration. Finally, the absorbed dose was calculated using S-values for unit-density spheres in whole-remnant dosimetry and S-values for voxels in maximum-voxel dosimetry.

RESULTS

The mean absorbed dose obtained from whole-remnant dosimetry was 40 Gy (range 2-176 Gy) and from maximum-voxel dosimetry 34 Gy (range 2-145 Gy). For any given patient, the activity concentrations for each of the three time-points were approximately the same for the two methods. The effective half-lives varied (R = 0.865), mainly due to discrepancies in estimation of the longer effective half-lives. On average, absorbed doses obtained from whole-remnant dosimetry were 1.2 ± 0.2 (1 SD) higher than for maximum-voxel dosimetry, mainly due to differences in theS-values. The method-related differences were however small in comparison to the wide range of absorbed doses obtained in patients.

CONCLUSIONS

Simple and consistent procedures for SPECT/CT based whole-volume and maximum-voxel dosimetry have been described, both based on experimentally determined recovery coefficients. Generally the results from the two approaches are consistent, although there is a small, systematic difference in the absorbed dose due to differences in the S-values, and some variability due to differences in the estimated effective half-lives, especially when the effective half-life is long. Irrespective of the method used, the patient absorbed doses obtained span over two orders of magnitude.

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.

Fast voxel-level dosimetry for (177)Lu labelled peptide treatments

In peptide receptor radionuclide therapy (PRRT), voxel-level radiation absorbed dose calculations can be performed using several different methods. Each method has it strengths and weaknesses; however, Monte Carlo (MC) simulation is presently considered the most accurate method at providing absorbed dose distributions. Unfortunately MC simulation is time-consuming and often impractical to carry out in a clinical practice. In this work, a fast semi-Monte Carlo (sMC) absorbed dose calculation method for (177)Lu PRRT dosimetry is presented. The sMC method is based on a local electron absorption assumption and fast photon MC simulations. The sMC method is compared against full MC simulation code built on PENELOPE (vxlPen) using digital phantoms to assess the accuracy of these assumptions.Due to the local electron absorption assumption of sMC, the potential errors in cross-fire dose from electrons and photons emitted by (177)Lu were first evaluated using an ellipsoidal kidney model by comparing vxlPen and sMC. The photon cross-fire dose from background to kidney and kidney to background with varying kidney-to-background activity concentration ratios were calculated. In addition, kidney to kidney photon and electron cross-dose with different kidney to kidney distances were studied. Second, extended cardiac-torso (XCAT) phantoms were created with liver lesions and with realistic activity distributions and tissue densities. The XCAT phantoms were used to simulate SPECT projections and 3D activity distribution images were reconstructed using an OSEM algorithm. Image-based dose rate distributions were calculated using vxlPen and sMC. Total doses and dose rate volume histograms (DrVH) produced by the two methods were compared.The photon cross-fire dose from the kidney increased the background's absorbed dose by 5% or more up to 5.8 cm distance with 20 : 1 kidney to background activity concentration ratio. On the other hand, the photon cross-fire dose from the background to the kidney volume was negligible. The vxlPen results showed that the cross fire dose between two similar kidney volumes relative to the source kidney's self-dose were 0.5% and 0.02% for photon and electrons, respectively, when source and target kidneys were modelled next to each other. The photon cross-dose decreased as function of distance, and electron doses were zero at distances larger than 4 mm. The difference between sMC and vxlPen kidney total doses in the XCAT phantom study was -0.4% while the electron dose DrVHs were identical between the methods. There was a systematic 5% difference in photon doses in soft tissue between the codes due to different simulations parameters. However, the photons produced only 4% of the kidney's total dose, thus the difference was not considered significant for total dose calculations.The comparison studies show that the absorbed doses calculated using the sMC differ only slightly from dedicated MC simulator results, while the dose estimates can be obtained in a fraction of the dedicated simulator's calculation time. Results imply that there is no need for electron MC simulation for (177)Lu absorption calculations with current SPECT systems. However, the photon cross-fire dose should be taken into account in healthy tissues, which have a relatively low uptake especially in cases where there are high uptake volumes are nearby.

Comparing voxel-based absorbed dosimetry methods in tumors, liver, lung, and at the liver-lung interface for (90)Y microsphere selective internal radiation therapy

Background

To assess differences between four different voxel-based dosimetry methods (VBDM) for tumor, liver, and lung absorbed doses following ⁹⁰Y microsphere selective internal radiation therapy (SIRT) based on ⁹⁰Y bremsstrahlung SPECT/CT, a secondary objective was to estimate the sensitivity of liver and lung absorbed doses due to differences in organ segmentation near the liver-lung interface.

Methods

Investigated VBDM were Monte Carlo (MC), soft-tissue kernel with density correction (SKD), soft-tissue kernel (SK), and local deposition (LD). Seventeen SIRT cases were analyzed. Mean absorbed doses (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \overline{AD} $$\end{document}AD¯) were calculated for tumor, non-tumoral liver (NL), and right lung (RL). Simulations with various SPECT spatial resolutions (FHWMs) and multiple lung shunt fractions (LSs) estimated the accuracy of VBDM at the liver-lung interface. Sensitivity of patient RL and NL \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \overline{AD} $$\end{document}AD¯ on segmentation near the interface was assessed by excluding portions near the interface.

Results

SKD, SK, and LD were within 5 % of MC for tumor and NL \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \overline{AD} $$\end{document}AD¯. LD and SKD overestimated RL \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \overline{AD} $$\end{document}AD¯ compared to MC on average by 17 and 20 %, respectively; SK underestimated RL \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \overline{AD} $$\end{document}AD¯ on average by −60 %. Simulations (20 mm FWHM, 20 % LS) showed that SKD, LD, and MC were within 10 % of the truth deep (>39 mm) in the lung; SK significantly underestimated the absorbed dose deep in the lung by approximately −70 %. All VBDM were within 10 % of truth deep (>12 mm) in the liver. Excluding 1, 2, and 3 cm of RL near the interface changed the resulting RL \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \overline{AD} $$\end{document}AD¯ by −22, −38, and −48 %, respectively, for all VBDM. An average change of −7 % in the NL \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \overline{AD} $$\end{document}AD¯ was realized when excluding 3 cm of NL from the interface. \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \overline{AD} $$\end{document}AD¯ was realized when excluding 3 cm of NL from the interface.

Conclusions

SKD, SK, and LD are equivalent to MC for tumor and NL \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \overline{AD} $$\end{document}AD¯. SK underestimates RL \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \overline{AD} $$\end{document}AD¯ relative to MC whereas LD and SKD overestimate. RL \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \overline{AD} $$\end{document}AD¯ is strongly influenced by the liver-lung interface.