Overview of the First NRG Oncology-National Cancer Institute Workshop on Dosimetry of Systemic Radiopharmaceutical Therapy.

In 2018, the National Cancer Institute and NRG Oncology partnered for the first time to host a joint workshop on systemic radiopharmaceutical therapy (RPT) to specifically address dosimetry issues and strategies for future clinical trials. The workshop focused on current dosimetric approaches for clinical trials, strategies under development that would optimize dose reporting, and future desired or optimized approaches for novel emerging radionuclides and carriers in development. In this article, we review the main approaches that are applied clinically to calculate the absorbed dose. These include absorbed doses calculated over a variety of spatial scales, including whole body, organ, suborgan, and voxel, the last 3 of which are achievable within the MIRD schema (S value) and can be calculated with analytic methods or Monte Carlo methods, the latter in most circumstances. This article will also contrast currently available methods and tools with those used in the past, to propose a pathway whereby dosimetry helps the field by optimizing the biologic effect of the treatment and trial design in the drug approval process to reduce financial and logistical costs. We also briefly discuss the dosimetric equivalent of biomarkers to help bring a precision medicine approach to RPT implementation when merited by evidence collected during early-phase trial investigations. Advances in the methodology and related tools have made dosimetry the optimum biomarker for RPT.

Overcoming Barriers to Radiopharmaceutical Therapy (RPT): An Overview From the NRG-NCI Working Group on Dosimetry of Radiopharmaceutical Therapy.

Radiopharmaceutical therapy (RPT) continues to demonstrate tremendous potential in improving the therapeutic gains in radiation therapy by specifically delivering radiation to tumors that can be well assessed in terms of dosimetry and imaging. Dosimetry in external beam radiation therapy is standard practice. This is not the case, however, in RPT. This NRG (acronym formed from the first letter of the 3 original groups: National Surgical Adjuvant Breast and Bowel Project, the Radiation Therapy Oncology Group, and the Gynecologic Oncology Group)-National Cancer Institute Working Group review describes some of the challenges to improving RPT. The main priorities for advancing the field include (1) developing and adopting best practice guidelines for incorporating patient-specific dosimetry for RPT that can be used at both large clinics with substantial resources and more modest clinics that have limited resources, (2) establishing and improving strategies for introducing new radiopharmaceuticals for clinical investigation, (3) developing approaches to address the radiophobia that is associated with the administration of radioactivity for cancer therapy, and (4) solving the financial and logistical issues of expertise and training in the developing field of RPT.

A STUDY OF A DOSE CONSTRAINT FOR OCCUPATIONALLY EXPOSED WORKERS IN THE US NUCLEAR POWER PLANTS.

One of the most important issues in the nuclear power industry is the implementation of the 2007 Recommendations of the International Commission on Radiological Protection (ICRP) published in ICRP Publication 103. These recommendations include the implementation of the concept of dose constraints for occupationally exposed workers at nuclear power plants (NPPs). When considering these changes from a cost-benefit standpoint, the implementation of dose constraints is still highly controversial. This study analysed annual occupational dose distributions to determine whether a dose constraint is needed for occupationally exposed workers at the US NPPs. Results of the analysis showed that the use of dose constraints had no positive impact on radiation safety of workers at NPPs in the USA. In fact, it appears that the implementation of dose constraints will impose an unnecessary regulatory burden on licensees. Based on these results, implementation of dose constraints is not recommended.

A Novel Method to Extend a Partial-Body CT for the Reconstruction of Dose to Organs beyond the Scan Range.

Epidemiological investigation is an important approach to assessing the risk of late effects after radiotherapy, and organ dosimetry is a crucial part of such analysis. Computed tomography (CT) images, if available, can be a valuable resource for individualizing the dosimetry, because they describe the specific anatomy of the patient. However, CT images acquired for radiation treatment planning purposes cover only a portion of the body near the target volume, whereas for epidemiology, the interest lies in the more distant normal tissues, which may be located outside the scan range. To address this challenge, we developed a novel method, called the Anatomically Predictive Extension (APE), to extend a partial-body CT image stack using images of a computational human phantom matched to the patient based on their height and weight. To test our method, we created five APE phantoms from chest and abdominal images extracted from the chest-abdomen-pelvis (CAP) CT scans of five patients. Organ doses were calculated for simple chest and prostate irradiations that were planned on the reference computational phantom (assumed patient geometry if no CT images are available), APE phantoms (patient-phantom hybrid given a partial-body patient CT) and full patient CAP CT scans (ground truth). The APE phantoms and patient CAP CT scans resulted in nearly identical dosimetry for those organs that were fully included in the partial-body CT used to construct the APE. The calculated doses to these same organs in the reference phantoms differed by up to 20% and 52% for the chest and prostate cases, respectively. For organs outside the scan coverage, the reference phantom showed, on average, dose differences of 31% (chest case) and 41% (prostate case). For the APE phantoms, these values were 26% (chest) and 17% (prostate). The APE method combines patient and phantom images to improve organ dosimetry both inside and outside the scan range. We intend to use the APE method for estimating dose for organs peripheral to the treatment fields; however, this method is quite generalizable with many potential applications.

Chemical Design of Both a Glutathione-Sensitive Dimeric Drug Guest and a Glucose-Derived Nanocarrier Host to Achieve Enhanced Osteosarcoma Lung Metastatic Anticancer Selectivity.

Although nanomedicines have been pursued for nearly 20 years, fundamental chemical strategies that seek to optimize both the drug and drug carrier together in a concerted effort remain uncommon yet may be powerful. In this work, two block polymers and one dimeric prodrug molecule were designed to be coassembled into degradable, functional nanocarriers, where the chemistry of each component was defined to accomplish important tasks. The result is a poly(ethylene glycol) (PEG)-protected redox-responsive dimeric paclitaxel (diPTX)-loaded cationic poly(d-glucose carbonate) micelle (diPTX@CPGC). These nanostructures showed tunable sizes and surface charges and displayed controlled PTX drug release profiles in the presence of reducing agents, such as glutathione (GSH) and dithiothreitol (DTT), thereby resulting in significant selectivity for killing cancer cells over healthy cells. Compared to free PTX and diPTX, diPTX@CPGC exhibited improved tumor penetration and significant inhibition of tumor cell growth toward osteosarcoma (OS) lung metastases with minimal side effects both in vitro and in vivo, indicating the promise of diPTX@CPGC as optimized anticancer therapeutic agents for treatment of OS lung metastases.

Rapid synthesis of 125I integrated gold nanoparticles for use in combined neoplasm imaging and targeted radionuclide therapy.

The selective delivery of radionuclides to tissues of interest remains a problematic task during treatment. The lack of tissue specificity for many therapeutics limit their efficacy by putting healthy organs and tissues at risk (e.g., side effects). Therefore, high specificity therapeutic strategies are needed to overcome these risks. The objective of this study was to use a modified citrate reduction technique to synthesize gold nanoparticles (AuNPs) containing 125I in order to combine their unique therapeutic and diagnostic properties. This task was accomplished by varying the insertion time of 125I, which will cause complete aggregation if added too early in the AuNP synthesis process. Even though 125I was utilized in this experiment, studies are underway to see if this approach can be extrapolated to shorter-lived isotopes (e.g., 211At). Characterization of the 125I-AuNPs was carried out using UV-Vis spectrometry and Transmission Electron Microscopy (TEM). The appropriate addition time of 125I was determined to be approximately 50s after the addition of sodium citrate. TEM measured the nanoparticles' diameters to be in the 10-20nm range. The AuNPs were found to be extremely stable, with no observable leaching of radioactivity into the solution. 125I-AuNPs could be beneficial as a contrast agent in CT imaging and therapy since AuNPs enhance the bio-delivery of 125I to neoplasms.

Control of seizures by ketogenic diet-induced modulation of metabolic pathways.

Epilepsy is too complex to be considered as a disease; it is more of a syndrome, characterized by seizures, which can be caused by a diverse array of afflictions. As such, drug interventions that target a single biological pathway will only help the specific individuals where that drug's mechanism of action is relevant to their disorder. Most likely, this will not alleviate all forms of epilepsy nor the potential biological pathways causing the seizures, such as glucose/amino acid transport, mitochondrial dysfunction, or neuronal myelination. Considering our current inability to test every individual effectively for the true causes of their epilepsy and the alarming number of misdiagnoses observed, we propose the use of the ketogenic diet (KD) as an effective and efficient preliminary/long-term treatment. The KD mimics fasting by altering substrate metabolism from carbohydrates to fatty acids and ketone bodies (KBs). Here, we underscore the need to understand the underlying cellular mechanisms governing the KD's modulation of various forms of epilepsy and how a diverse array of metabolites including soluble fibers, specific fatty acids, and functional amino acids (e.g., leucine, D-serine, glycine, arginine metabolites, and N-acetyl-cysteine) may potentially enhance the KD's ability to treat and reverse, not mask, these neurological disorders that lead to epilepsy.

Microbial influences on hormesis, oncogenesis, and therapy: A review of the literature.

Utilization of environmental stimuli for growth is the main factor contributing to the evolution of prokaryotes and eukaryotes, independently and mutualistically. Epigenetics describes an organism's ability to vary expression of certain genes based on their environmental stimuli. The diverse degree of dose-dependent responses based on their variances in expressed genetic profiles makes it difficult to ascertain whether hormesis or oncogenesis has or is occurring. In the medical field this is shown where survival curves used in determining radiotherapeutic doses have substantial uncertainties, some as large as 50% (Barendsen, 1990). Many in-vitro radiobiological studies have been limited by not taking into consideration the innate presence of microbes in biological systems, which have either grown symbiotically or pathogenically. Present in-vitro studies neglect to take into consideration the varied responses that commensal and opportunistic pathogens will have when exposed to the same stimuli and how such responses could act as stimuli for their macro/microenvironment. As a result many theories such as radiation carcinogenesis explain microscopic events but fail to describe macroscopic events (Cohen, 1995). As such, this review shows how microorganisms have the ability to perturb risks of cancer and enhance hormesis after irradiation. It will also look at bacterial significance in the microenvironment of the tumor before and during treatment. In addition, bacterial systemic communication after irradiation and the host's immune responses to infection could explain many of the phenomena associated with bystander effects. Therefore, the present literature review considers the paradigms of hormesis and oncogenesis in order to find a rationale that ties them all together. This relationship was thus characterized to be the microbiome.

Functional genomics screening utilizing mutant mouse embryonic stem cells identifies novel radiation-response genes.

Elucidating the genetic determinants of radiation response is crucial to optimizing and individualizing radiotherapy for cancer patients. In order to identify genes that are involved in enhanced sensitivity or resistance to radiation, a library of stable mutant murine embryonic stem cells (ESCs), each with a defined mutation, was screened for cell viability and gene expression in response to radiation exposure. We focused on a cancer-relevant subset of over 500 mutant ESC lines. We identified 13 genes; 7 genes that have been previously implicated in radiation response and 6 other genes that have never been implicated in radiation response. After screening, proteomic analysis showed enrichment for genes involved in cellular component disassembly (e.g. Dstn and Pex14) and regulation of growth (e.g. Adnp2, Epc1, and Ing4). Overall, the best targets with the highest potential for sensitizing cancer cells to radiation were Dstn and Map2k6, and the best targets for enhancing resistance to radiation were Iqgap and Vcan. Hence, we provide compelling evidence that screening mutant ESCs is a powerful approach to identify genes that alter radiation response. Ultimately, this knowledge can be used to define genetic variants or therapeutic targets that will enhance clinical therapy.

Dosimetry of a (90)Y-hydroxide liquid brachytherapy treatment approach to canine osteosarcoma using PET/CT.

A new treatment strategy based on direct injections of (90)Y-hydroxide into the tumor bed in dogs with osteosarcoma was studied. Direct injections of the radiopharmaceutical into the tumor bed were made according to a pretreatment plan established using (18)F-FDG images. Using a special drill, cannulas were inserted going through tissue, tumor and bone. Using these cannulas, direct injections of the radiopharmaceutical were made. The in vivo biodistribution of (90)Y-hydroxide and the anatomical tumor bed were imaged using a time-of-flight (TOF) PET/CT scanner. The material properties of the tissues were estimated from corresponding CT numbers using an electron-density calibration. Radiation absorbed dose estimates were calculated using Monte Carlo methods where the biodistribution of the pharmaceutical from PET images was sampled using a collapsing 3-D rejection technique. Dose distributions in the tumor bed and surrounding tissues were calculated, showing significant heterogeneity with multiple hot spots at injection sites. Dose volume histograms showed that approximately 33.9% of bone and tumor and 70.2% of bone marrow and trabecular bone received an absorbed dose over 200Gy; approximately 3.2% of bone and tumor and 31.0% of bone marrow and trabecular bone received a total dose of over 1000Gy.

Preliminary production of 211At at the Texas A&M University Cyclotron Institute.

A feasibility study for the production of the alpha particle-emitting radionuclide At was performed at the Texas A&M University Cyclotron Institute as part of the Interdisciplinary Radioisotope Production and Radiochemistry Program. The mission of this program centers upon the production of radionuclides for use in diagnostic and therapeutic nuclear medicine with the primary focus on development of novel therapeutic strategies. As a first step in establishing this program, two goals were outlined: (i) verify production of At and compare results to published data, and (ii) evaluate shielding and radiological safety issues for large-scale implementation using an external target. The radionuclide At was produced via the Bi (α, 2n) At reaction using the K500 cyclotron. Two experiments were conducted, using beam energies of 27.8 MeV and 25.3 MeV, respectively. The resulting yields for At were found to be 36.0 MBq µA h and 12.4 MBq µA h, respectively, which fall within the range of published yield data. Strategies for increasing absolute yield and production efficiency were also evaluated, which focused chiefly on using a new target designed for use with the K150 cyclotron, which will enable the use of a higher beam current. Finally, neutron and gamma dose rates during production were evaluated by using the Monte Carlo code MCNPX. It was determined that a simple structure consisting of 4-in thick borated polyethylene will reduce the neutron dose rate within the cyclotron production vault by approximately a factor of 2, thereby decreasing activation of equipment.

Applicability test of methods for discrimination between external and internal contamination using an ORTEC whole body counter.

Whole-body counters (WBCs) are generally used for monitoring internal radioactive contamination of radiation workers in nuclear power plants. However, it has been found that external contamination is occasionally counted as internal contamination. The previous study was conducted to provide guidance on measuring the exact internal radioactivity using a Canberra WBC. However, there is strong need to verify the application of the previous study to whole-body counting using a different type of WBC, ORTEC StandFAST II. Thus, in this study, several experiments were conducted focusing on the discrimination between external and internal contamination. Finally, it was found that counts from the front and back are still effective to distinguish external contamination from internal contamination for whole-body counting.

Radiological safety concerns for the accelerator production of diagnostic and therapeutic radionuclides in a university setting.

Accelerator production of radionuclides for diagnostic and therapeutic research at a university has many advantages. Radionuclides not commonly available through commercial suppliers may be readily produced for innovative research applications. Loss of material due to decay in transit is minimized, and product lead times may be significantly reduced. Furthermore, graduate students and research assistants have the opportunity to gain considerable hands-on experience during the production, extraction, and processing operations. However, the benefits of implementing accelerator production into an existing radiological protection program must be balanced against increased safety procedures and maintenance of as-low-as-reasonably-achievable work practices. This article outlines the basics for radioactive material production and corresponding issues in radiological protection associated with the production, use, and disposal on a college campus.

MIRD Pamphlet No. 22 (abridged): radiobiology and dosimetry of alpha-particle emitters for targeted radionuclide therapy.

The potential of alpha-particle emitters to treat cancer has been recognized since the early 1900s. Advances in the targeted delivery of radionuclides and radionuclide conjugation chemistry, and the increased availability of alpha-emitters appropriate for clinical use, have recently led to patient trials of radiopharmaceuticals labeled with alpha-particle emitters. Although alpha-emitters have been studied for many decades, their current use in humans for targeted therapy is an important milestone. The objective of this work is to review those aspects of the field that are pertinent to targeted alpha-particle emitter therapy and to provide guidance and recommendations for human alpha-particle emitter dosimetry.

Feasibility of whole-body functional mouse imaging using helical pinhole SPECT.

PURPOSE: Detailed in vivo whole-body biodistributions of radiolabeled tracers may characterize the longitudinal progression of disease, and changes with therapeutic interventions. Small-animal imaging in mice is particularly attractive due to the wide array of well characterized genetically and surgically created models of disease. Single Photon Emission Computed Tomography (SPECT) imaging using pinhole collimation provides high resolution and sensitivity, but conventional methods using circular acquisitions result in severe image truncation and incomplete sampling of data, which prevent the accurate determination of whole-body radiotracer biodistributions. This study describes the feasibility of helical acquisition paths to mitigate these effects. PROCEDURES: Helical paths of pinhole apertures were implemented using an external robotic stage aligned with the axis of rotation (AOR) of the scanner. Phantom and mouse scans were performed using helical paths and either circular or bi-circular orbits at the same radius of rotation (ROR). The bi-circular orbits consisted of two 360-degree scans separated by an axial shift to increase the axial field of view (FOV) and to improve the complete-sampling properties. RESULTS: Reconstructions of phantoms and mice acquired with helical paths show good image quality and are visually free of both truncation and axial-blurring artifacts. Circular orbits yielded reconstructions with both artifacts and a limited effective FOV. The bi-circular scans enlarged the axial FOV, but still suffered from truncation and sampling artifacts. CONCLUSIONS: Helical paths can provide complete sampling data and large effective FOV, yielding 3D full-body in vivo biodistributions while still maintaining a small distance from the aperture to the object for good sensitivity and resolution.

Intracerebral infusion of an EGFR-targeted toxin in recurrent malignant brain tumors.

The purpose of this study is to determine the maximum tolerated dose (MTD), dose-limiting toxicity (DLT), and intracerebral distribution of a recombinant toxin (TP-38) targeting the epidermal growth factor receptor in patients with recurrent malignant brain tumors using the intracerebral infusion technique of convection-enhanced delivery (CED). Twenty patients were enrolled and stratified for dose escalation by the presence of residual tumor from 25 to 100 ng/ml in a 40-ml infusion volume. In the last eight patients, coinfusion of (123)I-albumin was performed to monitor distribution within the brain. The MTD was not reached in this study. Dose escalation was stopped at 100 ng/ml due to inconsistent drug delivery as evidenced by imaging the coinfused (123)I-albumin. Two DLTs were seen, and both were neurologic. Median survival after TP-38 was 28 weeks (95% confidence interval, 26.5-102.8). Of 15 patients treated with residual disease, two (13.3%) demonstrated radiographic responses, including one patient with glioblastoma multiforme who had a nearly complete response and remains alive >260 weeks after therapy. Coinfusion of (123)I-albumin demonstrated that high concentrations of the infusate could be delivered >4 cm from the catheter tip. However, only 3 of 16 (19%) catheters produced intraparenchymal infusate distribution, while the majority leaked infusate into the cerebrospinal fluid spaces. Intracerebral CED of TP-38 was well tolerated and produced some durable radiographic responses at doses

A pilot study: 131I-antitenascin monoclonal antibody 81c6 to deliver a 44-Gy resection cavity boost.

The purpose of this study was to determine the feasibility and assess the efficacy and toxicity, among newly diagnosed malignant glioma patients, of administering (131)I-labeled murine antitenascin monoclonal antibody 81C6 ((131)I-81C6) into a surgically created resection cavity (SCRC) to achieve a patient-specific, 44-Gy boost to the 2-cm SCRC margin. A radioactivity dose of (131)I-81C6 calculated to achieve a 44-Gy boost to the SCRC was administered, followed by conventional external beam radiotherapy (XRT) and chemotherapy. Twenty-one patients were enrolled in the study: 16 with glioblastoma multiforme (GBM) and 5 with anaplastic astrocytoma. Twenty patients received the targeted 44-Gy boost (+/-10%) to the SCRC. Attributable toxicity was mild and limited to reversible grade 3 neutropenia or thrombocytopenia (n = 3; 14%), CNS wound infections (n = 3; 14%), and headache (n = 2; 10%). With a median follow-up of 151 weeks, median overall survival times for all patients and those with GBM are 96.6 and 90.6 weeks, respectively; 87% of GBM patients are alive at 1 year. It is feasible to consistently achieve a 44-Gy boost dose to the SCRC margin with patient-specific dosing of (131)I-81C6. Our study regimen ((131)I-81C6 + XRT + temozolomide) was well tolerated and had encouraging survival. To determine if selection of good-prognosis patients affects outcome associated with this approach, the U.S. Food and Drug Administration has approved a trial randomizing newly diagnosed GBM patients to either our study regimen or standard XRT plus temozolomide.

Clinical experience with alpha-particle emitting 211At: treatment of recurrent brain tumor patients with 211At-labeled chimeric antitenascin monoclonal antibody 81C6.

UNLABELLED: alpha-Particle-emitting radionuclides, such as (211)At, with a 7.2-h half-life, may be optimally suited for the molecularly targeted radiotherapy of strategically sensitive tumor sites, such as those in the central nervous system. Because of the much shorter range and more potent cytotoxicity of alpha-particles than of beta-particles, (211)At-labeled agents may be ideal for the eradication of tumor cells remaining after surgical debulking of malignant brain tumors. The main goal of this study was to investigate the feasibility and safety of this approach in patients with recurrent malignant brain tumors. METHODS: Chimeric antitenascin monoclonal antibody 81C6 (ch81C6) (10 mg) was labeled with 71-347 MBq of (211)At by use of N-succinimidyl 3-[(211)At]astatobenzoate. Eighteen patients were treated with (211)At-labeled ch81C6 ((211)At-ch81C6) administered into a surgically created resection cavity (SCRC) and then with salvage chemotherapy. Serial gamma-camera imaging and blood sampling over 24 h were performed. RESULTS: A total of 96.7% +/- 3.6% (mean +/- SD) of (211)At decays occurred in the SCRC, and the mean blood-pool percentage injected dose was < or = 0.3. No patient experienced dose-limiting toxicity, and the maximum tolerated dose was not identified. Six patients experienced grade 2 neurotoxicity within 6 wk of (211)At-ch81C6 administration; this neurotoxicity resolved fully in all but 1 patient. No toxicities of grade 3 or higher were attributable to the treatment. No patient required repeat surgery for radionecrosis. The median survival times for all patients, those with glioblastoma multiforme, and those with anaplastic astrocytoma or oligodendroglioma were 54, 52, and 116 wk, respectively. CONCLUSION: This study provides proof of concept for regional targeted radiotherapy with (211)At-labeled molecules in oncology. Specifically, the regional administration of (211)At-ch81C6 is feasible, safe, and associated with a promising antitumor benefit in patients with malignant central nervous system tumors.

Tumor resection cavity administered iodine-131-labeled antitenascin 81C6 radioimmunotherapy in patients with malignant glioma: neuropathology aspects.

INTRODUCTION: The neurohistological findings in patients treated with targeted beta emitters such as (131)I are poorly described. We report a histopathologic analysis from patients treated with combined external beam therapy and a brachytherapy consisting of a (131)I-labeled monoclonal antibody (mAb) injected into surgically created resection cavities during brain tumor resections. METHODS: Directed tissue samples of the cavity walls were obtained because of suspected tumor recurrence from 28 patients. Samples and clinical follow-up were evaluated on all patients (Group A) based on total radiation dose received and a subset of these (n=18; Group B, proximal therapy subset) who had received external beam therapy within

Cell therapy in murine atherosclerosis: in vivo imaging with high-resolution helical SPECT.

PURPOSE: To determine the feasibility of in vivo localization and quantification of indium 111 (111In)-oxine-labeled bone marrow (BM) with high-resolution whole-body helical single photon emission computed tomography (SPECT) in an established murine model of atherosclerosis and vascular repair. MATERIALS AND METHODS: The institutional animal care and use committee approved this study. BM from young B6 Rosa 26 Lac Z+/+ mice was radiolabeled with 111In-oxine. On days 1, 4, and 7 after administration of radiolabeled cells, five C57/BL6 apolipoprotein E-deficient mice and five wild-type (WT) control mice were imaged with whole-body high-resolution helical SPECT. Quantification with SPECT was compared with ex vivo analysis by means of gamma counting. Autoradiography and beta-galactosidase staining were used to verify donor cell biodistribution. Linear regression was used to assess the correlation between continuous variables. Two-tailed Student t test was used to compare values between groups, and paired two-tailed t test was used to assess changes within subjects at different time points. RESULTS: SPECT image contrast was high, with clear visualization of BM, liver, and spleen 7 days after administration of radiolabeled cells. SPECT revealed that 42% and 58% more activity was localized to the aorta and BM (P<.05 for both), respectively, in apolipoprotein E-deficient mice versus WT mice. Furthermore, 28% and 27% less activity was localized to the liver and spleen (P<.05 for both), respectively, in apolipoprotein E-deficient mice versus WT mice. SPECT and organ gamma counts showed good quantitative correlation (r=0.9). beta-Galactosidase staining and microautoradiography of recipient aortas showed donor cell localization to the intima of visible atherosclerotic plaque but not to unaffected regions of the vessel wall. CONCLUSION: High-resolution in vivo helical pinhole SPECT can be used to monitor and quantify early biodistribution of 111In-oxine-labeled BM in a murine model of progenitor cell therapy for atherosclerosis.