A treatment planning method for sequentially combining radiopharmaceutical therapy and external radiation therapy

Voxel-Level Dosimetry for Combined Iodine 131 Radiopharmaceutical Therapy and External Beam Radiation Therapy Treatment Paradigms for Head and Neck Cancer

PURPOSE

Targeted radiopharmaceutical therapy (RPT) in combination with external beam radiation therapy (EBRT) shows promise as a method to increase tumor control and mitigate potential high-grade toxicities associated with re-treatment for patients with recurrent head and neck cancer. This work establishes a patient-specific dosimetry framework that combines Monte Carlo-based dosimetry from the 2 radiation modalities at the voxel level using deformable image registration (DIR) and radiobiological constructs for patients enrolled in a phase 1 clinical trial combining EBRT and RPT.

METHODS AND MATERIALS

Serial single-photon emission computed tomography (SPECT)/computed tomography (CT) patient scans were performed at approximately 24, 48, 72, and 168 hours postinjection of 577.2 MBq/m2 (15.6 mCi/m2) CLR 131, an iodine 131-containing RPT agent. Using RayStation, clinical EBRT treatment plans were created with a treatment planning CT (TPCT). SPECT/CT images were deformably registered to the TPCT using the Elastix DIR module in 3D Slicer software and assessed by measuring mean activity concentrations and absorbed doses. Monte Carlo EBRT dosimetry was computed using EGSnrc. RPT dosimetry was conducted using RAPID, a GEANT4-based RPT dosimetry platform. Radiobiological metrics (biologically effective dose and equivalent dose in 2-Gy fractions) were used to combine the 2 radiation modalities.

RESULTS

The DIR method provided good agreement for the activity concentrations and calculated absorbed dose in the tumor volumes for the SPECT/CT and TPCT images, with a maximum mean absorbed dose difference of -11.2%. Based on the RPT absorbed dose calculations, 2 to 4 EBRT fractions were removed from patient EBRT treatments. For the combined treatment, the absorbed dose to target volumes ranged from 57.14 to 75.02 Gy. When partial volume corrections were included, the mean equivalent dose in 2-Gy fractions to the planning target volume from EBRT + RPT differed -3.11% to 1.40% compared with EBRT alone.

CONCLUSIONS

This work demonstrates the clinical feasibility of performing combined EBRT + RPT dosimetry on TPCT scans. Dosimetry guides treatment decisions for EBRT, and this work provides a bridge for the same paradigm to be implemented within the rapidly emerging clinical RPT space.

Lesion Dosimetry for [177Lu]Lu-PSMA-617 Radiopharmaceutical Therapy Combined with Stereotactic Body Radiotherapy in Patients with Oligometastatic Castration-Sensitive Prostate Cancer

A single-institution prospective pilot clinical trial was performed to demonstrate the feasibility of combining [177Lu]Lu-PSMA-617 radiopharmaceutical therapy (RPT) with stereotactic body radiotherapy (SBRT) for the treatment of oligometastatic castration-sensitive prostate cancer. Methods: Six patients with 9 prostate-specific membrane antigen (PSMA)-positive oligometastases received 2 cycles of [177Lu]Lu-PSMA-617 RPT followed by SBRT. After the first intravenous infusion of [177Lu]Lu-PSMA-617 (7.46 ± 0.15 GBq), patients underwent SPECT/CT at 3.2 ± 0.5, 23.9 ± 0.4, and 87.4 ± 12.0 h. Voxel-based dosimetry was performed with calibration factors (11.7 counts per second/MBq) and recovery coefficients derived from in-house phantom experiments. Lesions were segmented on baseline PSMA PET/CT (50% SUVmax). After a second cycle of [177Lu]Lu-PSMA-617 (44 ± 3 d; 7.50 ± 0.10 GBq) and an interim PSMA PET/CT scan, SBRT (27 Gy in 3 fractions) was delivered to all PSMA-avid oligometastatic sites, followed by post-PSMA PET/CT. RPT and SBRT voxelwise dose maps were scaled (α/β = 3 Gy; repair half-time, 1.5 h) to calculate the biologically effective dose (BED). Results: All patients completed the combination therapy without complications. No grade 3+ toxicities were noted. The median of the lesion SUVmax as measured on PSMA PET was 16.8 (interquartile range [IQR], 11.6) (baseline), 6.2 (IQR, 2.7) (interim), and 2.9 (IQR, 1.4) (post). PET-derived lesion volumes were 0.4-1.7 cm3 The median lesion-absorbed dose (AD) from the first cycle of [177Lu]Lu-PSMA-617 RPT (ADRPT) was 27.7 Gy (range, 8.3-58.2 Gy; corresponding to 3.7 Gy/GBq, range, 1.1-7.7 Gy/GBq), whereas the median lesion AD from SBRT was 28.1 Gy (range, 26.7-28.8 Gy). Spearman rank correlation, ρ, was 0.90 between the baseline lesion PET SUVmax and SPECT SUVmax (P = 0.005), 0.74 (P = 0.046) between the baseline PET SUVmax and the lesion ADRPT, and -0.81 (P = 0.022) between the lesion ADRPT and the percent change in PET SUVmax (baseline to interim). The median for the lesion BED from RPT and SBRT was 159 Gy (range, 124-219 Gy). ρ between the BED from RPT and SBRT and the percent change in PET SUVmax (baseline to post) was -0.88 (P = 0.007). Two cycles of [177Lu]Lu-PSMA-617 RPT contributed approximately 40% to the maximum BED from RPT and SBRT. Conclusion: Lesional dosimetry in patients with oligometastatic castration-sensitive prostate cancer undergoing [177Lu]Lu-PSMA-617 RPT followed by SBRT is feasible. Combined RPT and SBRT may provide an efficient method to maximize the delivery of meaningful doses to oligometastatic disease while addressing potential microscopic disease reservoirs and limiting the dose exposure to normal tissues.

Personalized Dosimetry in the Context of Radioiodine Therapy for Differentiated Thyroid Cancer

The most frequent thyroid cancer is Differentiated Thyroid Cancer (DTC) representing more than 95% of cases. A suitable choice for the treatment of DTC is the systemic administration of 131-sodium or potassium iodide. It is an effective tool used for the irradiation of thyroid remnants, microscopic DTC, other nonresectable or incompletely resectable DTC, or all the cited purposes. Dosimetry represents a valid tool that permits a tailored therapy to be obtained, sparing healthy tissue and so minimizing potential damages to at-risk organs. Absorbed dose represents a reliable indicator of biological response due to its correlation to tissue irradiation effects. The present paper aims to focus attention on iodine therapy for DTC treatment and has developed due to the urgent need for standardization in procedures, since no unique approaches are available. This review aims to summarize new proposals for a dosimetry-based therapy and so explore new alternatives that could provide the possibility to achieve more tailored therapies, minimizing the possible side effects of radioiodine therapy for Differentiated Thyroid Cancer.

ASTRO's Framework for Radiopharmaceutical Therapy Curriculum Development for Trainees

In 2017, the American Society for Radiation Oncology (ASTRO) board of directors prioritized radiopharmaceutical therapy (RPT) as a leading area for new therapeutic development, and the ASTRO RPT workgroup was created. Herein, the workgroup has developed a framework for RPT curriculum development upon which education leaders can build to integrate this modality into radiation oncology resident education. Through this effort, the workgroup aims to provide a guide to ensure robust training in an emerging therapeutic area within the context of existing radiation oncology training in radiation biology, medical physics, and clinical radiation oncology. The framework first determines the core RPT knowledge required to select patients, prescribe, safely administer, and manage related adverse events. Then, it defines the most important topics for preparing residents for clinical RPT planning and delivery. This framework is designed as a tool to supplement the current training that exists for radiation oncology residents. The final document was approved by the ASTRO board of directors in the fall of 2021.

SIR-Spheres activity measurements reveal systematic miscalibration

Purpose: Accurate dosimetry-guided radiopharmaceutical therapy fundamentally relies on knowledge of the quantity of radioactivity administered to patients. The purpose of this work was to perform an independent and NIST-traceable activity measurement of ⁹⁰Y SIR-Spheres®. Methods: Gamma spectroscopic measurements of the ⁹⁰Y internal pair production decay mode were made using a high-purity germanium detector. Un-modified patient SIR-Spheres® vials were placed within a high-density polyethylene source holder positioned at a distance of 210 cm from the detector, with acquisition durations of 3 - 6 hours. Measured annihilation radiation detection rates were corrected for radioactive decay during acquisition, dead time, source attenuation, and source geometry effects. Detection efficiency was determined by two independent and NIST-traceable methods. Resulting ⁹⁰Y activity measurements were compared against the manufacturer activity calibration. Results: Measured SIR-Sphere® vials (n = 5) were found to contain more activity than specified by the manufacturer calibration - on average the ratio of measured activity to calibrated was 1.233 ± 0.030. Activity measurements made using two distinct efficiency calibration methods were found to agree within 1%. Uncertainty in individual measurements was dominated by counting statistical uncertainty (~2.5%), uncertainty in the internal pair production branching ratio of ⁹⁰Y (1.5%), and efficiency calibration (1.2% - 1.9%). Conclusion: The primary SIR-Spheres® activity calibration appears to be a significant underestimate of true activity. This mis-calibration has likely been consistent for as long as the SIR-Sphere® product has been available. This finding should be independently verified, and steps should be taken by the manufacturer to establish an accurate and traceable activity standard.

Imaging and dosimetry for alpha-particle emitter radiopharmaceutical therapy: improving radiopharmaceutical therapy by looking into the black box

Radiopharmaceutical therapy using α-particle emitting radionuclides (αRPT) is a novel treatment modality that delivers highly potent alpha-particles to cancer cells or their environment. We review the advantages and challenges of imaging and dosimetry in implementing αRPT for cancer patients.

Dosimetry for Radiopharmaceutical Therapy: Current Practices and Commercial Resources

With the ongoing dramatic growth of radiopharmaceutical therapy, research and development in internal radiation dosimetry continue to advance both at academic medical centers and in industry. The basic paradigm for patient-specific dosimetry includes administration of a pretreatment tracer activity of the therapeutic radiopharmaceutical; measurement of its time-dependent biodistribution; definition of the pertinent anatomy; integration of the measured time-activity data to derive source-region time-integrated activities; calculation of the tumor, organ-at-risk, and/or whole-body absorbed doses; and prescription of the therapeutic administered activity. This paper provides an overview of the state of the art of patient-specific dosimetry for radiopharmaceutical therapy, including current methods and commercially available software and other resources.

[Treatment Planning and Dose Verification for Combined Internal and External Radiotherapy (CIERT)]

AIM

The combined internal and external radiotherapy (CIERT) take advantage of the benefits from radionuclide therapy and external beam irradiation. These include steep dose gradients and a low toxicity to normal tissue due to the use of unsealed radioisotopes as well as homogeneous dose distribution within the tumor due to external beam irradiation. For a combined irradiation planning, an infrastructure has to be developed that takes into account the dose contributions from both modalities. A physical verification of the absorbed dose distribution should follow by measurements using OSL detectors.

METHOD

Internal irradiation was performed using Re-188 in a cylindrical phantom with three inserts. SPECT images were acquired to calculate the internal dose using the software STRATOS. The dose distribution was exported as DICOM-RT data and imported in the software Pinnacle. Based on the internal dose distribution the external irradiation using 6 MV photons was planned. The dose contributions of both modalities separately as well as for combined irradiation was measured using OSL detectors made out of Beryllium oxide.

RESULTS

The planed doses of combined irradiation (1 Gy, 2 Gy, 4 Gy) could be verified within the uncertainty of the detectors. The mean energy response to Re-188 was (88.6 ± 2.4) % with respect to the calibration with 200 kV X-ray irradiation. The energy response to 6 MV photons was (146.0 ± 4.9) %.

CONCLUSION

A workflow for the treatment planning of combined internal and external radiotherapy has been developed and tested. Measurements verified the calculated doses. Therefore, the physical and technical basis for the dosimetry of combined irradiation were worked out.

Combination radionuclide therapy: A new paradigm

Targeted molecular radionuclide therapy (MRT) has shown its potential for the treatment of cancers of multiple origins. A combination therapy strategy employing two or more distinct therapeutic approaches in cancer management is aimed at circumventing tumor resistance by simultaneously targeting compensatory signaling pathways or bypassing survival selection mutations acquired in response to individual monotherapies. Combination radionuclide therapy (CRT) is a newer application of the concept, utilizing a combination of radiolabeled molecular targeting agents with chemotherapy and beam radiation therapy for enhanced therapeutic index. Encouraging results are reported with chemotherapeutic agents in combination with radiolabeled targeting molecules for cancer therapy. With increasing awareness of the various survival and stress response pathways activated after radionuclide therapy, different holistic combinations of MRT agents with radiosensitizers targeting such pathways have also been explored. MRT has also been studied in combination with beam radiotherapy modalities such as external beam radiation therapy and carbon ion radiation therapy to enhance the anti-tumor response. Nanotechnology aids in CRT by bringing together multiple monotherapies on a single nanostructure platform for treating cancers in a more precise or personalized way. CRT will be a key player in managing cancers if correctly tailored to the individual patient profile. The success of CRT lies in an in-depth understanding of the radiobiological principles and pathways activated in response.

Dosimetry for Optimized, Personalized Radiopharmaceutical Therapy

Radiopharmaceutical therapy (RPT) has grown rapidly over the last decade for treatment of numerous cancer types. Dosimetric guidance, as with other radiotherapy modalities, has benefitted patients by reducing the incidence of side effects and improving overall survival in populations treated under this paradigm. Development of tools and techniques for dosimetry-guided therapy is ongoing, with numerous the Food and Drug Administration-cleared products reaching the U.S. market in 2019. Safe use of commercial dosimetry platforms requires a deep understanding of the underlying physical principles and thoroughly vetted input data. Likewise, interpretation of dosimetry results relies on an understanding of radiobiological principles, and the principles of uncertainty propagation. In this article, we review strategies commonly employed for dosimetry-guided RPT - including quantitative imaging, dose calculation methods, and modeling of dose across time-points. Additionally, we review recent literature evidence (2013-2020) demonstrating the efficacy of personalized RPT.

Radiopharmaceutical therapy in cancer: clinical advances and challenges

Radiopharmaceutical therapy (RPT) is emerging as a safe and effective targeted approach to treating many types of cancer. In RPT, radiation is systemically or locally delivered using pharmaceuticals that either bind preferentially to cancer cells or accumulate by physiological mechanisms. Almost all radionuclides used in RPT emit photons that can be imaged, enabling non-invasive visualization of the biodistribution of the therapeutic agent. Compared with almost all other systemic cancer treatment options, RPT has shown efficacy with minimal toxicity. With the recent FDA approval of several RPT agents, the remarkable potential of this treatment is now being recognized. This Review covers the fundamental properties, clinical development and associated challenges of RPT.

Improving 131I Radioiodine Therapy By Hybrid Polymer-Grafted Gold Nanoparticles

BACKGROUND

Human trials combining external radiotherapy (RT) and metallic nanoparticles are currently underway in cancer patients. For internal RT, in which a radioisotope such as radioiodine is systemically administered into patients, there is also a need for enhancing treatment efficacy, decreasing radiation-induced side effects and overcoming radio-resistance. However, if strategies vectorising radioiodine through nanocarriers have been documented, sensitizing the neoplasm through the use of nanotherapeutics easily translatable to the clinic in combination with the standard systemic radioiodine treatment has not been assessed yet.

METHOD AND MATERIALS

The present study explored the potential of hybrid poly(methacrylic acid)-grafted gold nanoparticles to improve the performances of systemic 131I-mediated RT on cancer cells and in tumor-bearing mice. Such nanoparticles were chosen based on their ability previously described by our group to safely withstand irradiation doses while exhibiting good biocompatibility and enhanced cellular uptake.

RESULTS

In vitro clonogenic assays performed on melanoma and colorectal cancer cells showed that poly(methacrylic acid)-grafted gold nanoparticles (PMAA-AuNPs) could efficiently lead to a marked tumor cell mortality when combined to a low activity of radioiodine, which alone appeared to be essentially ineffective on tumor cells. In vivo, tumor enrichment with PMAA-AuNPs significantly enhanced the killing potential of a systemic radioiodine treatment.

CONCLUSION

This is the first report of a simple and reliable nanomedicine-based approach to reduce the dose of radioiodine required to reach curability. In addition, these results open up novel perspectives for using high-Z metallic NPs in additional molecular radiation therapy demonstrating heterogeneous dose distributions.

Leveraging the power of non-radium radionuclide treatments in bone metastases

This review overviews the current status and clinical results of unsealed radionuclide therapies in skeletal metastasis. The other modes of treatment such as external bean radiotherapy and the newer receptor targeted radiopharmaceuticals tagged to alpha and beta particle emitting radionuclides have also been touched upon. With the advent of the latter in recent years, the intravenously administered radiopharmaceuticals that can be employed in the setting of skeletal metastases can be broadly categorized into (i) bone-seeking and (ii) receptor targeted specific tumor-seeking radiopharmaceuticals. The second category conceptualizes the "radionuclide based theranostics" and "precision oncology" and has the additional advantage of targeting both skeletal and non-skeletal disease and being the preferred therapy befitting the contemporary paradigm of clinical oncology.

Radiopharmaceutical Therapy

Radiopharmaceutical therapy involves the use of radionuclides that are either conjugated to tumor-targeting agents (e.g., nanoscale constructs, antibodies, peptides, and small molecules) or that concentrate in tumors through natural physiological mechanisms that occur predominantly in neoplastic cells. In the latter category, radioiodine therapy of thyroid cancer is the prototypical and most widely implemented radiopharmaceutical therapy. In the category of radionuclide-ligand conjugates, antibody and peptide conjugates have been studied extensively. The efficacy of radiopharmaceutical therapy relies on the ability to deliver cytotoxic radiation to tumor cells without causing prohibitive normal tissue toxicity. After some 30 y of preclinical and clinical research, a number of recent developments suggest that radiopharmaceutical therapy is poised to emerge as an important and widely recognized therapeutic modality. These developments include the substantial investment in antibodies by the pharmaceutical industry and the compelling rationale to build upon this already existing and widely tested platform. In addition, the growing recognition that the signaling pathways responsible for tumor cell survival and proliferation are less easily and durably inhibited than originally envisioned has also provided a rationale for identifying agents that are cytotoxic rather than inhibitory. A number of radiopharmaceutical agents are currently undergoing clinical trial investigation; these include beta-particle emitters, such as Lu, that are being used to label antisomatostatin receptor peptides for neuroendocrine cancers and also prostate-specific membrane antigen targeting small molecules for prostate cancer. Alpha-particle-emitting radionuclides have also been studied for radiopharmaceutical therapy; these include At for glioblastoma, Ac for leukemias and prostate cancer, Pb for breast cancer, and Ra for prostate cancer. The alpha emitters have tended to show particular promise, and there is substantial interest in further developing these agents for therapy of cancers that are particularly difficult to treat.

Strengths and Weaknesses of a Planar Whole-Body Method of (153)Sm Dosimetry for Patients with Metastatic Osteosarcoma and Comparison with Three-Dimensional Dosimetry

PURPOSE

Dosimetric accuracy depends directly upon the accuracy of the activity measurements in tumors and organs. The authors present the methods and results of a retrospective tumor dosimetry analysis in 14 patients with a total of 28 tumors treated with high activities of (153)Sm-ethylenediaminetetramethylenephosphonate ((153)Sm-EDTMP) for therapy of metastatic osteosarcoma using planar images and compare the results with three-dimensional dosimetry.

MATERIALS AND METHODS

Analysis of phantom data provided a complete set of parameters for dosimetric calculations, including buildup factor, attenuation coefficient, and camera dead-time compensation. The latter was obtained using a previously developed methodology that accounts for the relative motion of the camera and patient during whole-body (WB) imaging. Tumor activity values calculated from the anterior and posterior views of WB planar images of patients treated with (153)Sm-EDTMP for pediatric osteosarcoma were compared with the geometric mean value. The mean activities were integrated over time and tumor-absorbed doses were calculated using the software package OLINDA/EXM.

RESULTS

The authors found that it was necessary to employ the dead-time correction algorithm to prevent measured tumor activity half-lives from often exceeding the physical decay half-life of (153)Sm. Measured half-lives so long are unquestionably in error. Tumor-absorbed doses varied between 0.0022 and 0.27 cGy/MBq with an average of 0.065 cGy/MBq; however, a comparison with absorbed dose values derived from a three-dimensional analysis for the same tumors showed no correlation; moreover, the ratio of three-dimensional absorbed dose value to planar absorbed dose value was 2.19. From the anterior and posterior activity comparisons, the order of clinical uncertainty for activity and dose calculations from WB planar images, with the present methodology, is hypothesized to be about 70%.

CONCLUSION

The dosimetric results from clinical patient data indicate that absolute planar dosimetry is unreliable and dosimetry using three-dimensional imaging is preferable, particularly for tumors, except perhaps for the most sophisticated planar methods. The relative activity and patient kinetics derived from planar imaging show a greater level of reliability than the dosimetry.

Development and evaluation of convergent and accelerated penalized SPECT image reconstruction methods for improved dose-volume histogram estimation in radiopharmaceutical therapy

PURPOSE

Three-dimensional (3D) dosimetry has the potential to provide better prediction of response of normal tissues and tumors and is based on 3D estimates of the activity distribution in the patient obtained from emission tomography. Dose-volume histograms (DVHs) are an important summary measure of 3D dosimetry and a widely used tool for treatment planning in radiation therapy. Accurate estimates of the radioactivity distribution in space and time are desirable for accurate 3D dosimetry. The purpose of this work was to develop and demonstrate the potential of penalized SPECT image reconstruction methods to improve DVHs estimates obtained from 3D dosimetry methods.

METHODS

The authors developed penalized image reconstruction methods, using maximum a posteriori (MAP) formalism, which intrinsically incorporate regularization in order to control noise and, unlike linear filters, are designed to retain sharp edges. Two priors were studied: one is a 3D hyperbolic prior, termed single-time MAP (STMAP), and the second is a 4D hyperbolic prior, termed cross-time MAP (CTMAP), using both the spatial and temporal information to control noise. The CTMAP method assumed perfect registration between the estimated activity distributions and projection datasets from the different time points. Accelerated and convergent algorithms were derived and implemented. A modified NURBS-based cardiac-torso phantom with a multicompartment kidney model and organ activities and parameters derived from clinical studies were used in a Monte Carlo simulation study to evaluate the methods. Cumulative dose-rate volume histograms (CDRVHs) and cumulative DVHs (CDVHs) obtained from the phantom and from SPECT images reconstructed with both the penalized algorithms and OS-EM were calculated and compared both qualitatively and quantitatively. The STMAP method was applied to patient data and CDRVHs obtained with STMAP and OS-EM were compared qualitatively.

RESULTS

The results showed that the penalized algorithms substantially improved the CDRVH and CDVH estimates for large organs such as the liver compared to optimally postfiltered OS-EM. For example, the mean squared errors (MSEs) of the CDRVHs for the liver at 5 h postinjection obtained with CTMAP and STMAP were about 15% and 17%, respectively, of the MSEs obtained with optimally filtered OS-EM. For the CDVH estimates, the MSEs obtained with CTMAP and STMAP were about 16% and 19%, respectively, of the MSEs from OS-EM. For the kidneys and renal cortices, larger residual errors were observed for all algorithms, likely due to partial volume effects. The STMAP method showed promising qualitative results when applied to patient data.

CONCLUSIONS

Penalized image reconstruction methods were developed and evaluated through a simulation study. The study showed that the MAP algorithms substantially improved CDVH estimates for large organs such as the liver compared to optimally postfiltered OS-EM reconstructions. For small organs with fine structural detail such as the kidneys, a large residual error was observed for both MAP algorithms and OS-EM. While CTMAP provided marginally better MSEs than STMAP, given the extra effort needed to handle misregistration of images at different time points in the algorithm and the potential impact of residual misregistration, 3D regularization methods, such as that used in STMAP, appear to be a more practical choice.

Improving external beam radiotherapy by combination with internal irradiation

The efficacy of external beam radiotherapy (EBRT) is dose dependent, but the dose that can be applied to solid tumour lesions is limited by the sensitivity of the surrounding tissue. The combination of EBRT with systemically applied radioimmunotherapy (RIT) is a promising approach to increase efficacy of radiotherapy. Toxicities of both treatment modalities of this combination of internal and external radiotherapy (CIERT) are not additive, as different organs at risk are in target. However, advantages of both single treatments are combined, for example, precise high dose delivery to the bulk tumour via standard EBRT, which can be increased by addition of RIT, and potential targeting of micrometastases by RIT. Eventually, theragnostic radionuclide pairs can be used to predict uptake of the radiotherapeutic drug prior to and during therapy and find individual patients who may benefit from this treatment. This review aims to highlight the outcome of pre-clinical studies on CIERT and resultant questions for translation into the clinic. Few clinical data are available until now and reasons as well as challenges for clinical implementation are discussed.

Tumor and red bone marrow dosimetry: comparison of methods for prospective treatment planning in pretargeted radioimmunotherapy

Background

Red bone marrow (RBM) toxicity is dose-limiting in (pretargeted) radioimmunotherapy (RIT). Previous blood-based and two-dimensional (2D) image-based methods have failed to show a clear dose-response relationship. We developed a three-dimensional (3D) image-based RBM dosimetry approach using the Monte Carlo-based 3D radiobiological dosimetry (3D-RD) software and determined its additional value for predicting RBM toxicity.

Methods

RBM doses were calculated for 13 colorectal cancer patients after pretargeted RIT with the two-step administration of an anti-CEA × anti-HSG bispecific monoclonal antibody and a ¹⁷⁷Lu-labeled di-HSG-peptide. 3D-RD RBM dosimetry was based on the lumbar vertebrae, delineated on single photon emission computed tomography (SPECT) scans acquired directly, 3, 24, and 72 h after ¹⁷⁷Lu administration. RBM doses were correlated to hematologic effects, according to NCI-CTC v3 and compared with conventional 2D cranium-based and blood-based dosimetry results. Tumor doses were calculated with 3D-RD, which has not been possible with 2D dosimetry. Tumor-to-RBM dose ratios were calculated and compared for ¹⁷⁷Lu-based pretargeted RIT and simulated pretargeted RIT with ⁹⁰Y.

Results

3D-RD RBM doses of all seven patients who developed thrombocytopenia were higher (range 0.43 to 0.97 Gy) than that of the six patients without thrombocytopenia (range 0.12 to 0.39 Gy), except in one patient (0.47 Gy) without thrombocytopenia but with grade 2 leucopenia. Blood and 2D image-based RBM doses for patients with grade 1 to 2 thrombocytopenia were in the same range as in patients without thrombocytopenia (0.14 to 0.29 and 0.11 to 0.26 Gy, respectively). Blood-based RBM doses for two grade 3 to 4 patients were higher (0.66 and 0.51 Gy, respectively) than the others, and the cranium-based dose of only the grade 4 patient was higher (0.34 Gy). Tumor-to-RBM dose ratios would increase by 25% on average when treating with ⁹⁰Y instead of ¹⁷⁷Lu.

Conclusions

3D dosimetry identifies patients at risk of developing any grade of RBM toxicity more accurately than blood- or 2D image-based methods. It has the added value to enable calculation of tumor-to-RBM dose ratios.

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.

Improving the dose-myelotoxicity correlation in radiometabolic therapy of bone metastases with 153Sm-EDTMP

PURPOSE

(153)Sm-ethylene diamine tetramethylene phosphonic acid ((153)Sm-EDTMP) is widely used to palliate pain from bone metastases, and is being studied for combination therapy beyond palliation. Conceptually, red marrow (RM) dosimetry allows myelotoxicity to be predicted, but the correlation is poor due to dosimetric uncertainty, individual sensitivity and biological effects from previous treatments. According to EANM guidelines, basic dosimetric procedures have been studied to improve the correlation between dosimetry and myelotoxicity in (153)Sm-EDTMP therapy.

METHODS

RM dosimetry for 33 treatments of bone metastases from breast, prostate and lung tumours was performed prospectively (with (99m)Tc-MDP) and retrospectively, acquiring whole-body scans early and late after injection. The (153)Sm-EDTMP activity was calculated by prospective dosimetry based on measured skeletal uptake and full physical retention, with the RM absorbed dose not exceeding 3.8 Gy. Patient-specific RM mass was evaluated by scaling in terms of body weight (BW), lean body mass (LBM) and trabecular volume (TV) estimated from CT scans of the L2–L4 vertebrae. Correlations with toxicity were determined in a selected subgroup of 27 patients, in which a better correlation between dosimetry and myelotoxicity was expected.

RESULTS

Skeletal uptakes of (99m)Tc and (153)Sm (Tc% and Sm%) were well correlated. The median Sm% was higher in prostate cancer (75.3 %) than in lung (60.5%, p = 0.005) or breast (60.8%, p = 0.008). PLT and WBC nadirs were not correlated with administered activity, but were weakly correlated with uncorrected RM absorbed doses, and the correlation improved after rescaling in terms of BW, LBM and TV. Most patients showed transient toxicity (grade 1–3), which completely and spontaneously recovered over a few days. Using TV, RM absorbed dose was in the range 2–5 Gy, with a median of 312 cGy for PLT in patients with toxicity and 247 cGy in those with no toxicity (p = 0.019), and 312 cGy for WBC in those with toxicity and 232 cGy in those with no toxicity (p = 0.019). ROC curves confirmed the correlations, yielding toxicity absorbed dose thresholds of 265 cGy for PLT and 232 cGy for WBC.

CONCLUSION

The best predictor of myelotoxicity and blood cells nadir was obtained scaling the RM absorbed dose in terms of the estimated TV. It seems clear that the increase in skeletal uptake due to the presence of bone metastases and the assumption of full physical retention cause an overestimation of the RM absorbed dose. Nevertheless, an improvement of the dose–toxicity correlation is easily achievable by simple methods, also leading to possible improvement in multifactorial analyses of myelotoxicity.

Planning combined treatments of external beam radiation therapy and molecular radiotherapy

Molecular radiotherapy (MRT) with radiolabeled molecules has being constantly evolving, leading to notable results in cancer treatment. In some cases, the absorbed doses delivered to tumors by MRT are sufficient to obtain complete responses; in other cases, instead, to be effective, MRT needs to be combined with other therapeutic approaches. Recently, several studies proposed the combination of MRT with external beam radiation therapy (EBRT). Some describe the theoretical basis within radiobiological models, others report the results of clinical phase I-II studies aimed to assess the feasibility and tolerability. The latter includes the treatment of various tumors, such as meningiomas, paragangliomas, non-Hodgkin's lymphomas, bone, brain, hepatic, and breast lesions. The underlying principle of combined MRT and EBRT is the possibility of exploiting the full potential of each modality, given the different organs at risk. Target tissues can indeed receive a higher irradiation, while respecting the threshold limits of more than one critical tissue. Nevertheless, clinical trials are empirical and optimization is still a theoretical issue. This article describes the state of the art of combined MRT and EBRT regarding the rationale and the results of clinical studies, with special focus on the possibility of treatment improvement.

The role of preclinical models in radiopharmaceutical therapy

Radiopharmaceutical therapy (RPT) is a treatment modality that involves the use of radioactively labeled targeting agents to deliver a cytotoxic dose of radiation to tumor while sparing normal tissue. The biologic function of the target and the biologic action of the targeting agent is largely irrelevant as long as the targeting agent delivers cytotoxic radiation to the tumor. Preclinical RPT studies use imaging and ex vivo evaluation of radioactivity concentration in target and normal tissues to obtain biodistribution and pharmacokinetic data that can be used to evaluate radiation absorbed doses. Since the efficacy and toxicity of RPT depend on radiation absorbed dose, this quantity can be used to translate results from preclinical studies to human studies. The absorbed dose can also be used to customize therapy to account for pharmacokinetic and other differences among patients so as to deliver a prespecified absorbed dose to the tumor or to dose-limiting tissue. The combination of RPT with other agents can be investigated and optimized by identifying the effect of other agents on tumor or normal tissue radiosensitivity and also on how other agents change the absorbed dose to these tissues. RPT is a distinct therapeutic modality whose mechanism of action is well understood. Measurements can be made in preclinical models to help guide clinical implementation of RPT and optimize combination therapy using RPT.

Redefining relative biological effectiveness in the context of the EQDX formalism: implications for alpha-particle emitter therapy

Alpha-particle radiopharmaceutical therapy (αRPT) is currently enjoying increasing attention as a viable alternative to chemotherapy for targeting of disseminated micrometastatic disease. In theory, αRPT can be personalized through pre-therapeutic imaging and dosimetry. However, in practice, given the particularities of α-particle emissions, a dosimetric methodology that accurately predicts the thresholds for organ toxicity has not been reported. This is in part due to the fact that the biological effects caused by α-particle radiation differ markedly from the effects caused by traditional external beam (photon or electron) radiation or β-particle emitting radiopharmaceuticals. The concept of relative biological effectiveness (RBE) is used to quantify the ratio of absorbed doses required to achieve a given biological response with alpha particles versus a reference radiation (typically a beta emitter or external beam radiation). However, as conventionally defined, the RBE varies as a function of absorbed dose and therefore a single RBE value is limited in its utility because it cannot be used to predict response over a wide range of absorbed doses. Therefore, efforts are underway to standardize bioeffect modeling for different fractionation schemes and dose rates for both nuclear medicine and external beam radiotherapy. Given the preponderant use of external beams of radiation compared to nuclear medicine in cancer therapy, the more clinically relevant quantity, the 2 Gy equieffective dose, EQD2(α/β), has recently been proposed by the ICRU. In concert with EQD2(α/β), we introduce a new, redefined RBE quantity, named RBE2(α/β), as the ratio of the two linear coefficients that characterize the α particle absorbed dose-response curve and the low-LET megavoltage photon 2 Gy fraction equieffective dose-response curve. The theoretical framework for the proposed new formalism is presented along with its application to experimental data obtained from irradiation of a breast cancer cell line. Radiobiological parameters are obtained using the linear quadratic model to fit cell survival data for MDA-MB-231 human breast cancer cells that were irradiated with either α particles or a single fraction of low-LET (137)Cs γ rays. From these, the linear coefficient for both the biologically effective dose (BED) and the EQD2(α/β) response lines were derived for fractionated irradiation. The standard RBE calculation, using the traditional single fraction reference radiation, gave RBE values that ranged from 2.4 for a surviving fraction of 0.82-6.0 for a surviving fraction of 0.02, while the dose-independent RBE2(4.6) value was 4.5 for all surviving fraction values. Furthermore, bioeffect modeling with RBE2(α/β) and EQD2(α/β) demonstrated the capacity to predict the surviving fraction of cells irradiated with acute and fractionated low-LET radiation, α particles and chronic exponentially decreasing dose rates of low-LET radiation. RBE2(α/β) is independent of absorbed dose for α-particle emitters and it provides a more logical framework for data reporting and conversion to equieffective dose than the conventional dose-dependent definition of RBE. Moreover, it provides a much needed foundation for the ongoing development of an α-particle dosimetry paradigm and will facilitate the use of tolerance dose data available from external beam radiation therapy, thereby helping to develop αRPT as a single modality as well as for combination therapies.

Bone-seeking radiopharmaceuticals as targeted agents of osteosarcoma: samarium-153-EDTMP and radium-223

Osteosarcoma is a cancer characterized by formation of bone by malignant cells. Routine bone scan imaging with Tc-99m-MDP is done at diagnosis to evaluate primary tumor uptake and check for bone metastases. At time of relapse the Tc-99m-MDP bone scan also provides a specific means to assess formation of bone by malignant osteosarcoma cells and the potential for bone-seeking radiopharmaceuticals to deliver radioactivity directly into osteoblastic osteosarcoma lesions. This chapter will review and compare a bone-seeking radiopharmaceutical that emits beta-particles, samarium-153-EDTMP, with an alpha-particle emitter, radium-223. The charged alpha particles from radium-223 have far more mass and energy than beta particles (electrons) from Sm-153-EDTMP. Because radium-223 has less marrow toxicity and more radiobiological effectiveness, especially if inside the bone forming cancer cell than samarium-153-EDTMP, radium-223 may have greater potential to become widely used against osteosarcoma as a targeted therapy. Radium-223 also has more potential to be used with chemotherapy against osteosarcoma and bone metastases. Because osteosarcoma makes bone and radium-223 acts like calcium, this radiopharmaceutical could possibly become a new targeted means to achieve safe and effective reduction of tumor burden as well as facilitate better surgery and/or radiotherapy for difficult to resect large, or metastatic tumors.

Improved dose-volume histogram estimates for radiopharmaceutical therapy by optimizing quantitative SPECT reconstruction parameters

In radiopharmaceutical therapy, an understanding of the dose distribution in normal and target tissues is important for optimizing treatment. Three-dimensional (3D) dosimetry takes into account patient anatomy and the nonuniform uptake of radiopharmaceuticals in tissues. Dose-volume histograms (DVHs) provide a useful summary representation of the 3D dose distribution and have been widely used for external beam treatment planning. Reliable 3D dosimetry requires an accurate 3D radioactivity distribution as the input. However, activity distribution estimates from SPECT are corrupted by noise and partial volume effects (PVEs). In this work, we systematically investigated OS-EM based quantitative SPECT (QSPECT) image reconstruction in terms of its effect on DVHs estimates. A modified 3D NURBS-based Cardiac-Torso (NCAT) phantom that incorporated a non-uniform kidney model and clinically realistic organ activities and biokinetics was used. Projections were generated using a Monte Carlo (MC) simulation; noise effects were studied using 50 noise realizations with clinical count levels. Activity images were reconstructed using QSPECT with compensation for attenuation, scatter and collimator-detector response (CDR). Dose rate distributions were estimated by convolution of the activity image with a voxel S kernel. Cumulative DVHs were calculated from the phantom and QSPECT images and compared both qualitatively and quantitatively. We found that noise, PVEs, and ringing artifacts due to CDR compensation all degraded histogram estimates. Low-pass filtering and early termination of the iterative process were needed to reduce the effects of noise and ringing artifacts on DVHs, but resulted in increased degradations due to PVEs. Large objects with few features, such as the liver, had more accurate histogram estimates and required fewer iterations and more smoothing for optimal results. Smaller objects with fine details, such as the kidneys, required more iterations and less smoothing at early time points post-radiopharmaceutical administration but more smoothing and fewer iterations at later time points when the total organ activity was lower. The results of this study demonstrate the importance of using optimal reconstruction and regularization parameters. Optimal results were obtained with different parameters at each time point, but using a single set of parameters for all time points produced near-optimal dose-volume histograms.

Beyond Palliation: Therapeutic Applications of 153Samarium-EDTMP

Primary and metastatic malignant bone lesions result in significant pain and disability in oncology patients. Targeted bone-seeking radioisotopes including ¹⁵³Samarium ethylene-diamine-tetramethylene-phosphonic acid (¹⁵³Sm-EDTMP) have been shown to effectively palliate bone pain, often when external beam radiotherapy (EBRT) is not feasible. However, recent evidence also suggests ¹⁵³Sm-EDTMP has cytotoxic activity either alone or in combination with chemotherapy or EBRT. ¹⁵³Sm-EDTMP may be useful as anti-neoplastic therapy apart from pain palliation in a variety of malignancies. For prostate cancer patients, several phase I and II clinical trials have shown that combined ¹⁵³Sm-EDTMP and docetaxel-based chemotherapy can result in >50% decrease in prostate-specific antigen with manageable myelosuppression. In hematologic malignancies, ¹⁵³Sm-EDTMP produced clinical responses when combined with bortezomib in multiple myeloma. ¹⁵³Sm-EDTMP also can be used with myeloablative chemotherapy for marrow conditioning prior to stem cell transplant. In osteosarcoma, ¹⁵³Sm-EDTMP infusion delivers radiation to multiple unresectable lesions simultaneously and provides local cytotoxicity without soft tissue damage that can be combined with chemotherapy or radiation. Prior to routine incorporation of ¹⁵³Sm-EDTMP into therapeutic regimens, we must learn how to ensure optimal delivery to tumors, determine which patients are likely to benefit, improve our ability to assess clinical response in bone lesions and further evaluate the efficacy ¹⁵³Sm-EDTMP in combination with chemotherapy, radiation and novel targeted agents.

3D dosimetry in patients with early breast cancer undergoing Intraoperative Avidination for Radionuclide Therapy (IART) combined with external beam radiation therapy

PURPOSE

Intraoperative Avidination for Radionuclide Therapy (IART) is a novel targeted radionuclide therapy recently used in patients with early breast cancer. It is a radionuclide approach with (90)Y-biotin combined with external beam radiotherapy (EBRT) to release a boost of radiation in the tumour bed. Two previous clinical trials using dosimetry based on the calculation of mean absorbed dose values with the hypothesis of uniform activity distribution (MIRD 16 method) assessed the feasibility and safety of IART. In the present retrospective study, a voxel dosimetry analysis was performed to investigate heterogeneity in distribution of the absorbed dose. The aim of this work was to compare dosimetric and radiobiological evaluations derived from average absorbed dose vs. voxel absorbed dose approaches.

METHODS

We evaluated 14 patients who were injected with avidin into the tumour bed after conservative surgery and 1 day later received an intravenous injection of 3.7 GBq of (90)Y-biotin (together with 185 MBq (111)In-biotin for imaging). Sequential images were used to estimate the absorbed dose in the target region according to the standard dosimetry method (SDM) and the voxel dosimetry method (VDM). The biologically effective dose (BED) distribution was also evaluated. Dose/volume and BED volume histograms were generated to derive equivalent uniform BED (EUBED) and equivalent uniform dose (EUD) values.

RESULTS

No "cold spots" were highlighted by voxel dosimetry. The median absorbed-dose in the target region was 20 Gy (range 15-27 Gy) by SDM, and the median EUD was 20.4 Gy (range 16.5-29.4 Gy) by the VDM; SDM and VDM estimates differed by about 6 %. The EUD/mean voxel absorbed dose ratio was >0.9 in all patients, indicative of acceptable uniformity in the target. The median BED and EUBED values were 21.8 Gy (range 15.9-29.3 Gy) and 22.8 Gy (range 17.3-31.8 Gy), respectively.

CONCLUSION

VDM highlighted the absence of significant heterogeneity in absorbed dose in the target. The EUD/mean absorbed dose ratio indicated a biological efficacy comparable to that of uniform distribution of absorbed dose. The VDM is recommended for improving accuracy, taking into account actual activity distribution in the target region. The radiobiological model applied allowed us to compare the effects of IART® with those of EBRT and to match the two irradiation modalities.

Tumor dosimetry and response for 153Sm-ethylenediamine tetramethylene phosphonic acid therapy of high-risk osteosarcoma

(153)Sm-ethylenediamine tetramethylene phosphonic acid ((153)Sm-EDTMP) therapy for osteosarcoma is being investigated. In this study, we analyzed the influence of (153)Sm-EDTMP administered activity (AA), osteosarcoma tumor density, mass, and the shape of the tumor on absorbed dose (AD). We also studied the biologic implication of the nonuniform tumor AD distribution using radiobiologic modeling and examined the relationship between tumor AD and response.

METHODS

Nineteen tumors in 6 patients with recurrent, refractory osteosarcoma enrolled in a phase I or II clinical trial of (153)Sm-EDTMP were analyzed using the 3-dimensional radiobiologic dosimetry (3D-RD) software package. Patients received a low dose of (153)Sm-EDTMP (37.0-51.8 MBq/kg), followed on hematologic recovery by a second, high dose (222 MBq/kg). Treatment response was evaluated using either CT or MRI after each therapy. SPECT/CT of the tumor regions were obtained at 4 and 48 h or 72 h after (153)Sm-EDTMP therapy for 3D-RD analysis. Mean tumor AD was also calculated using the OLINDA/EXM unit-density sphere model and was compared with the 3D-RD estimates.

RESULTS

On average, a 5-fold increase in the AA led to a 4-fold increase in the mean tumor AD over the high- versus low-dose-treated patients. The range of mean tumor AD and equivalent uniform dose (EUD) for low-dose therapy were 1.48-14.6 and 0.98-3.90 Gy, respectively. Corresponding values for high-dose therapy were 2.93-59.3 and 1.89-12.3 Gy, respectively. Mean tumor AD estimates obtained from OLINDA/EXM were within 5% of the mean AD values obtained using 3D-RD. On an individual tumor basis, both mean AD and EUD were positively related to percentage tumor volume reduction (P = 0.031 and 0.023, respectively).

CONCLUSION

The variations in tumor density, mass, and shape seen in these tumors did not affect the mean tumor AD estimation significantly. The tumor EUD was approximately 2- and 3-fold lower than the mean AD for low- and high-dose therapy, respectively. A dose-response relationship was observed for transient tumor volume shrinkage.

A model of cellular dosimetry for macroscopic tumors in radiopharmaceutical therapy

PURPOSE

In the radiopharmaceutical therapy approach to the fight against cancer, in particular when it comes to translating laboratory results to the clinical setting, modeling has served as an invaluable tool for guidance and for understanding the processes operating at the cellular level and how these relate to macroscopic observables. Tumor control probability (TCP) is the dosimetric end point quantity of choice which relates to experimental and clinical data: it requires knowledge of individual cellular absorbed doses since it depends on the assessment of the treatment's ability to kill each and every cell. Macroscopic tumors, seen in both clinical and experimental studies, contain too many cells to be modeled individually in Monte Carlo simulation; yet, in particular for low ratios of decays to cells, a cell-based model that does not smooth away statistical considerations associated with low activity is a necessity. The authors present here an adaptation of the simple sphere-based model from which cellular level dosimetry for macroscopic tumors and their end point quantities, such as TCP, may be extrapolated more reliably.

METHODS

Ten homogenous spheres representing tumors of different sizes were constructed in GEANT4. The radionuclide 131I was randomly allowed to decay for each model size and for seven different ratios of number of decays to number of cells, N(r): 1000, 500, 200, 100, 50, 20, and 10 decays per cell. The deposited energy was collected in radial bins and divided by the bin mass to obtain the average bin absorbed dose. To simulate a cellular model, the number of cells present in each bin was calculated and an absorbed dose attributed to each cell equal to the bin average absorbed dose with a randomly determined adjustment based on a Gaussian probability distribution with a width equal to the statistical uncertainty consistent with the ratio of decays to cells, i.e., equal to Nr-1/2. From dose volume histograms the surviving fraction of cells, equivalent uniform dose (EUD), and TCP for the different scenarios were calculated. Comparably sized spherical models containing individual spherical cells (15 microm diameter) in hexagonal lattices were constructed, and Monte Carlo simulations were executed for all the same previous scenarios. The dosimetric quantities were calculated and compared to the adjusted simple sphere model results. The model was then applied to the Bortezomib-induced enzyme-targeted radiotherapy (BETR) strategy of targeting Epstein-Barr virus (EBV)-expressing cancers.

RESULTS

The TCP values were comparable to within 2% between the adjusted simple sphere and full cellular models. Additionally, models were generated for a nonuniform distribution of activity, and results were compared between the adjusted spherical and cellular models with similar comparability. The TCP values from the experimental macroscopic tumor results were consistent with the experimental observations for BETR-treated 1 g EBV-expressing lymphoma tumors in mice.

CONCLUSIONS

The adjusted spherical model presented here provides more accurate TCP values than simple spheres, on par with full cellular Monte Carlo simulations while maintaining the simplicity of the simple sphere model. This model provides a basis for complementing and understanding laboratory and clinical results pertaining to radiopharmaceutical therapy.