MIRD dose estimate report No. 20: radiation absorbed-dose estimates for 111In- and 90Y-ibritumomab tiuxetan

Dosimetry of a Novel 111Indium-Labeled Anti-P-Cadherin Monoclonal Antibody (FF-21101) in Non-Human Primates

P-cadherin is associated with a wide range of tumor types, making it an attractive therapeutic target. FF-21101 is a human-mouse chimeric monoclonal antibody (mAb) directed against human P-cadherin, which has been radioconjugated with indium-111 (111In) utilizing a DOTA chelator. We investigated the biodistribution of FF-21101(111In) in cynomolgus macaques and extrapolated the results to estimate internal radiation doses of 111In- and yttrium-90 (90Y)-FF-21101 for targeted radioimmunotherapy in humans. Whole-body planar and SPECT imaging were performed at 0, 2, 24, 48, 72, 96, and 120 h post-injection, using a dual-head gamma camera. Volumes of interest of identifiable source organs of radioactivity were defined on aligned reference CT and serial SPECT images. Organs with the highest estimated dose values (mSv/MBq) for FF-21101(111In) were the lungs (0.840), spleen (0.816), liver (0.751), kidneys (0.629), and heart wall (0.451); and for FF-21101(90Y) dose values were: lungs (10.49), spleen (8.21), kidneys (5.92), liver (5.46), and heart wall (2.61). FF-21101(111In) exhibits favorable biodistribution in cynomolgus macaques and estimated human dosimetric characteristics. Data obtained in this study were used to support the filing of an investigational new drug application with the FDA for a Phase I clinical trial.

Radioimmunotherapy of Non-Hodgkin B-cell Lymphoma: An update

Systemic radioimmunotherapy (RIT) is arguably the most effective and least toxic anticancer treatment for non-Hodgkin lymphoma (NHL). In treatment-naïve patients with indolent NHL, the efficacy of a single injection of RIT compares with that of multiple cycles of combination chemotherapy. However, 20 years following the approval of the first CD20-targeting radioimmunoconjugates ⁹⁰Y-Ibritumomab-tiuxetan (Zevalin) and ¹³¹I-tositumomab (Bexxar), the number of patients referred for RIT in western countries has dramatically decreased. Notwithstanding this, the development of RIT has continued. Therapeutic targets other than CD20 have been identified, new vector molecules have been produced allowing for faster delivery of RIT to the target, and innovative radionuclides with favorable physical characteristics such as alpha emitters have been more widely available. In this article, we reviewed the current status of RIT in NHL, with particular focus on recent clinical and preclinical developments.

Clinical Applications of Immuno-PET in Lymphoma: A Systematic Review

Simple Summary

A systematic review of the published literature was performed to assess current clinicalapplications of immuno-PET in patients diagnosed with any histological type of lymphoma. The initial search yielded 1407 articles from PubMed/Medline and Scopus databases, but only 2 articles were found to comply with the inclusion criteria and 2 more were found during the cross-reference check. All four articles were deemed of sufficient methodological quality according to the QUADAS-2 assessment and were included in the review. Among the four included articles, three described the use of ⁸⁹Zr-labelled antibodies targeting CD20+ relapsed/refractory B-cell lymphomas and one concerned the use of ⁶⁸Ga-labelled mAb targeting CXCR4 in patients with non-Hodgkin lymphomas. Very limited literature data are currently available on the use of iPET in patients with lymphoma. However, iPET may represent a useful tool to non-invasively visualize the heterogeneous individual immunological environment, thus potentially guiding treatment-planning in lymphoma patients, and hence deserves further exploitation.

Abstract

Objective: Immuno-positron emission tomography (iPET) combines the sensitivity of the PET imaging technique and the targeting specificity of radio-labelled monoclonal antibodies (mAb). Its first clinical applications in humans were described in the late 1990s, and several pathologies have benefitted from this molecular imaging modality since then. Our scope was to assess current clinical applications of immuno-PET in patients with lymphoma. Therefore, a systematic review of the published literature was performed. Methods: PubMed/Medline and Scopus databases were independently searched by two nuclear medicine physicians, to identify studies describing the clinical use of immuno-PET in patients with lymphoma. Methodological quality of the included articles was assessed by using the Quality Assessment of Diagnostic Accuracy Studies criteria. The studies were then analyzed concerning the molecular target of interest. Results: The initial search yielded 1407 articles. After elimination of duplicates, 1339 titles/abstracts were evaluated. Only two articles were found to comply with the inclusion criteria and two more were found during the cross-reference check. Among the four included articles, three described the use of ⁸⁹Zr-labelled antibodies targeting CD20+ relapsed/refractory B-cell lymphomas and one concerned the use of ⁶⁸Ga-labelled mAb targeting CXCR4 in patients with non-Hodgkin lymphomas. Conclusions: Very limited literature data are currently available on the clinical use of iPET in patients with lymphoma. This technique is encountering obstacles in its wider use, possibly because of the need of specific facilities, unfavorable dosimetry, and unclear correlation of immuno-tracer biodistribution with patients’ clinical and tumors’ molecular characteristics. However, iPET may represent a useful tool to non-invasively visualize the heterogenous individual immunological environment, thus potentially guiding treatment-planning in lymphoma patients, and hence deserves further exploitation.

A Phase 1 and Biodistribution Study of ABT-806i, an 111In-Radiolabeled Conjugate of the Tumor-Specific Anti-EGFR Antibody ABT-806

ABT-806 is a tumor-specific antibody targeting the epidermal growth factor receptor (EGFR). This study assessed safety, biodistribution, and pharmacokinetics of ¹¹¹In-radiolabeled ABT-806 (ABT-806i) and effects of repeated doses of ABT-806 on receptor occupancy. Methods: Eligible patients had advanced tumors likely to express EGFR/EGFRvIII; adequate performance status and organ function; and measurable disease by RECIST 1.1. In cohort 1, 6 patients received a bolus administration of ABT-806i and underwent SPECT followed by whole-body planar scans. In cohort 2, 12 patients were imaged similarly as in 1 initially; thereafter, they received 3 doses of unlabeled ABT-806, before another dose of ABT-806i with associated SPECT and whole-body planar scans. At the end of both cohorts, patients who had stable or responding disease were able to enroll into an extension study (M12-326) in which they received unlabeled ABT-806 every 2 wk until disease progression, withdrawal of consent, or intolerable toxicity. Results: No toxicity related to ABT-806i infusion was observed. ABT-806i showed minimal uptake in normal tissues and cleared gradually from blood with a half-life of 6.0 ± 1.5 d. The mean effective dose of ABT-806i was 0.137 mSv/MBq for males and 0.183 mSv/MBq for females. ABT-806i tumor uptake varied and did not correlate with archived tumor EGFR expression. No change in ABT-806i uptake was observed after interval ABT-806 treatment, indicating stable EGFR expression in tumor. The patient with highest tumor uptake of ABT-806i had advanced head and neck cancer and experienced a partial response. Conclusion: ABT-806i allows for real-time imaging of EGFR conformational expression in tumors, has an acceptable radiation dosimetry, and provides important additional information about antigen expression compared with standard approaches using archival tissue. Its role to assist in patient selection for EGFR-based therapeutics and investigate treatment resistance should be further investigated.

EANM position paper on article 56 of the Council Directive 2013/59/Euratom (basic safety standards) for nuclear medicine therapy

The EC Directive 2013/59/Euratom states in article 56 that exposures of target volumes in nuclear medicine treatments shall be individually planned and their delivery appropriately verified. The Directive also mentions that medical physics experts should always be appropriately involved in those treatments. Although it is obvious that, in nuclear medicine practice, every nuclear medicine physician and physicist should follow national rules and legislation, the EANM considered it necessary to provide guidance on how to interpret the Directive statements for nuclear medicine treatments.For this purpose, the EANM proposes to distinguish three levels in compliance to the optimization principle in the directive, inspired by the indication of levels in prescribing, recording and reporting of absorbed doses after radiotherapy defined by the International Commission on Radiation Units and Measurements (ICRU): Most nuclear medicine treatments currently applied in Europe are standardized. The minimum requirement for those treatments is ICRU level 1 ("activity-based prescription and patient-averaged dosimetry"), which is defined by administering the activity within 10% of the intended activity, typically according to the package insert or to the respective EANM guidelines, followed by verification of the therapy delivery, if applicable. Non-standardized treatments are essentially those in developmental phase or approved radiopharmaceuticals being used off-label with significantly (> 25% more than in the label) higher activities. These treatments should comply with ICRU level 2 ("activity-based prescription and patient-specific dosimetry"), which implies recording and reporting of the absorbed dose to organs at risk and optionally the absorbed dose to treatment regions. The EANM strongly encourages to foster research that eventually leads to treatment planning according to ICRU level 3 ("dosimetry-guided patient-specific prescription and verification"), whenever possible and relevant. Evidence for superiority of therapy prescription on basis of patient-specific dosimetry has not been obtained. However, the authors believe that a better understanding of therapy dosimetry, i.e. how much and where the energy is delivered, and radiobiology, i.e. radiation-related processes in tissues, are keys to the long-term improvement of our treatments.

Yttrium-90 Anti-CD45 Immunotherapy Followed by Autologous Hematopoietic Cell Transplantation for Relapsed or Refractory Lymphoma

Autologous hematopoietic cell transplantation (AHCT) is a standard of care for several subtypes of high-risk lymphoma, but durable remissions are not achieved in the majority of patients. Intensified conditioning using CD45-targeted antibody-radionuclide conjugate (ARC) preceding AHCT may improve outcomes in lymphoma by permitting the delivery of curative doses of radiation to disease sites while minimizing toxicity. We performed sequential phase I trials of escalating doses of yttrium-90 (⁹⁰Y)-labeled anti-CD45 antibody with or without BEAM (carmustine, etoposide, cytarabine, melphalan) chemotherapy followed by AHCT in adults with relapsed/refractory or high-risk B cell non-Hodgkin lymphoma (NHL), T cell NHL (T-NHL), or Hodgkin lymphoma (HL). Twenty-one patients were enrolled (16 NHL, 4 HL, 1 T-NHL). Nineteen patients received BEAM concurrently. No dose-limiting toxicities were observed; therefore, the maximum tolerated dose is estimated to be ≥34 Gy to the liver. Nonhematologic toxicities and engraftment kinetics were similar to standard myeloablative AHCT. Late myeloid malignancies and 100-day nonrelapse deaths were not observed. At a median follow-up of 5 years, the estimates of progression-free and overall survival of 19 patients were 37% and 68%, respectively. Two patients did not receive BEAM; one had stable disease and the other progressive disease post-transplant. The combination of ⁹⁰Y-anti-CD45 with BEAM and AHCT was feasible and tolerable in patients with relapsed and refractory lymphoma. The use of anti-CD45 ARC as an adjunct to hematopoietic cell transplantation regimens or in combination with novel therapies/immunotherapies should be further explored based on these and other data.

Preclinical Evaluation and Dosimetry of [111In]CHX-DTPA-scFv78-Fc Targeting Endosialin/Tumor Endothelial Marker 1 (TEM1)

PURPOSE

Endosialin/tumor endothelial marker-1 (TEM1) is an attractive theranostic target expressed by the microenvironment of a wide range of tumors, as well as by sarcoma and neuroblastoma cells. We report on the radiolabeling and preclinical evaluation of the scFv78-Fc, a fully human TEM1-targeting antibody fragment cross-reactive with mouse TEM1.

PROCEDURES

The scFv78-Fc was conjugated with the chelator p-SCN-Bn-CHX-A"-DTPA, followed by labeling with indium-111. The number of chelators per molecule was estimated by mass spectrometry. A conventional saturation assay, extrapolated to infinite antigen concentration, was used to determine the immunoreactive fraction of the radioimmunoconjugate. The radiopharmaceutical biodistribution was assessed in immunodeficient mice grafted with Ewing's sarcoma RD-ES and neuroblastoma SK-N-AS human TEM1-positive tumors. The full biodistribution studies were preceded by a dose-escalation experiment based on the simultaneous administration of the radiopharmaceutical with increasing amounts of unlabeled scFv78-Fc. Radiation dosimetry extrapolations to human adults were obtained from mouse biodistribution data according to established methodologies and additional assumptions concerning the impact of the tumor antigenic sink in the cross-species translation.

RESULTS

[111In]CHX-DTPA-scFv78-Fc was obtained with a radiochemical purity > 98 % after 1 h incubation at 42 °C and ultrafiltration. It showed good stability in human serum and > 70 % immunoreactive fraction. Biodistribution data acquired in tumor-bearing mice confirmed fast blood clearance and specific tumor targeting in both xenograft models. The radiopharmaceutical off-target uptake was predominantly abdominal. After a theoretical injection of [111In]CHX-DTPA-scFv78-Fc to the reference person, the organs receiving the highest absorbed dose would be the spleen (0.876 mGy/MBq), the liver (0.570 mGy/MBq) and the kidneys (0.298 mGy/MBq). The total body dose and the effective dose would be 0.058 mGy/MBq and 0.116 mSv/MBq, respectively.

CONCLUSIONS

[111In]CHX-DTPA-scFv78-Fc binds specifically to endosialin/TEM1 in vitro and in vivo. Dosimetry estimates are in the range of other monoclonal antibodies radiolabeled with indium-111. [111In]CHX-DTPA-scFv78-Fc could be potentially translated into clinic.

Radiolabeled Human Monoclonal Antibody 067-213 has the Potential for Noninvasive Quantification of CD73 Expression

BACKGROUND

CD73 is an ectonucleotidase regulating extracellular adenosine concentration and plays an important role in adenosine-mediated immunosuppressive pathways. The efficacy of CD73-targeted therapy depends on the expression levels of CD73; therefore, monitoring CD73 status in cancer patients would provide helpful information for selection of patients who would benefit from CD73-targeted therapy. Here, we evaluated the ability of 111In-labeled antibody 067-213, which has high affinity for human CD73, to act as a noninvasive imaging probe.

METHODS

Cell binding and competitive inhibition assays for 111In-labeled 067-213 were conducted using MIAPaCa-2 (high CD73 expression) and A431 (low CD73 expression) cells. For in vivo assessments, biodistribution and SPECT/CT studies were conducted in MIAPaCa-2 and A431 tumor-bearing mice. To estimate the absorbed dose in humans, biodistribution and SPECT/CT studies were conducted in healthy rats.

RESULTS

111In-labeled 067-213 bound to MIAPaCa-2 and A431 cells in a CD73-dependent manner and the affinity loss after 111In-labeling was limited. Biodistribution and SPECT/CT studies with 111In-labeled 067-213 in mice showed high uptake in MIAPaCa-2 tumors and lower uptake in A431 tumors. In rats, the probe did not show high uptake in normal organs, including endogenously CD73-expressing organs. The estimated absorbed doses in humans were reasonably low.

CONCLUSIONS

111In-labeled 067-213 showed CD73-expression-dependent tumor uptake and low uptake in normal organs and tissues. Radiolabeled 067-213 holds promise as an imaging probe for noninvasive evaluation of CD73 expression levels in patients. Our data encourage further clinical studies to clarify a role for CD73 monitoring in patients receiving CD73-targeted immune therapy.

Biokinetics of Radiolabeled Monoclonal Antibody BC8: Differences in Biodistribution and Dosimetry Among Hematologic Malignancies

We reviewed ¹¹¹In-DOTA-anti-CD45 antibody (BC8) imaging and bone marrow biopsy measurements to ascertain the biodistribution and biokinetics of the radiolabeled antibody and to investigate differences based on type of hematologic malignancy. Methods: Serial whole-body scintigraphic images (4 time points) were obtained after infusion of the ¹¹¹In-DOTA-BC8 (176-406 MBq) into 52 adult patients with hematologic malignancies (lymphoma, multiple myeloma, acute myeloid leukemia, and myelodysplastic syndrome). Counts were obtained for the regions of interest for spleen, liver, kidneys, testicles (in men), and 2 marrow sites (acetabulum and sacrum), and correction for attenuation and background was made. Bone marrow biopsies were obtained 14-24 h after infusion, and the percentage of administered activity was determined. Absorbed radiation doses were calculated. Results: Initial uptake in liver averaged 32% ± 8.4% (SD) of administered activity (52 patients), which cleared monoexponentially with a biologic half-time of 293 ± 157 h (33 patients) or did not clear (19 patients). Initial uptake in spleen averaged 22% ± 12% and cleared with a biologic half-time of 271 ± 185 h (36 patients) or longer (6 patients). Initial uptake in kidney averaged 2.4% ± 2.0% and cleared with a biologic half-time of 243 ± 144 h (27 patients) or longer (9 patients). Initial uptake in red marrow averaged 23% ± 11% and cleared with a biologic half-time of 215 ± 107 h (43 patients) or longer (5 patients). Whole-body retention half-time averaged 198 ± 75 h. Splenic uptake was higher in the AML/MDS group than in the lymphoma group (P ≤ 0.05) or the multiple myeloma group (P ≤ 0.10). Liver represented the dose-limiting organ. For liver uptake, no significant differences were observed among the 3 malignancy groups. Average calculated radiation absorbed doses per unit of administered activity for a therapy infusion of ⁹⁰Y-DOTA-BC8 were 0.35 ± 0.20 cGy/MBq for red marrow, 0.80 ± 0.24 cGy/MBq for liver, 3.0 ± 1.4 cGy/MBq for spleen, 0.055 ± 0.014 cGy/MBq for total body, 0.21 ± 0.15 cGy/MBq for osteogenic cells, and 0.17 ± 0.15 cGy/MBq for kidneys. Conclusion: ¹¹¹In-DOTA-BC8 had a long retention time in liver, spleen, kidneys, and red marrow, and the highest absorbed doses were in spleen and liver. Few differences were observed by malignancy type. The exception was greater splenic uptake in the leukemia/MDS group than in the lymphoma or multiple myeloma group.

Auger electrons for cancer therapy - a review

Background

Auger electrons (AEs) are very low energy electrons that are emitted by radionuclides that decay by electron capture (e.g. ¹¹¹In, ⁶⁷Ga, ^99mTc, ^195mPt, ¹²⁵I and ¹²³I). This energy is deposited over nanometre-micrometre distances, resulting in high linear energy transfer (LET) that is potent for causing lethal damage in cancer cells. Thus, AE-emitting radiotherapeutic agents have great potential for treatment of cancer. In this review, we describe the radiobiological properties of AEs, their radiation dosimetry, radiolabelling methods, and preclinical and clinical studies that have been performed to investigate AEs for cancer treatment.

Results

AEs are most lethal to cancer cells when emitted near the cell nucleus and especially when incorporated into DNA (e.g. ¹²⁵I-IUdR). AEs cause DNA damage both directly and indirectly via water radiolysis. AEs can also kill targeted cancer cells by damaging the cell membrane, and kill non-targeted cells through a cross-dose or bystander effect. The radiation dosimetry of AEs considers both organ doses and cellular doses. The Medical Internal Radiation Dose (MIRD) schema may be applied. Radiolabelling methods for complexing AE-emitters to biomolecules (antibodies and peptides) and nanoparticles include radioiodination (¹²⁵I and ¹²³I) or radiometal chelation (¹¹¹In, ⁶⁷Ga, ^99mTc). Cancer cells exposed in vitro to AE-emitting radiotherapeutic agents exhibit decreased clonogenic survival correlated at least in part with unrepaired DNA double-strand breaks (DSBs) detected by immunofluorescence for γH2AX, and chromosomal aberrations. Preclinical studies of AE-emitting radiotherapeutic agents have shown strong tumour growth inhibition in vivo in tumour xenograft mouse models. Minimal normal tissue toxicity was found due to the restricted toxicity of AEs mostly on tumour cells targeted by the radiotherapeutic agents. Clinical studies of AEs for cancer treatment have been limited but some encouraging results were obtained in early studies using ¹¹¹In-DTPA-octreotide and ¹²⁵I-IUdR, in which tumour remissions were achieved in several patients at administered amounts that caused low normal tissue toxicity, as well as promising improvements in the survival of glioblastoma patients with ¹²⁵I-mAb 425, with minimal normal tissue toxicity.

Conclusions

Proof-of-principle for AE radiotherapy of cancer has been shown preclinically, and clinically in a limited number of studies. The recent introduction of many biologically-targeted therapies for cancer creates new opportunities to design novel AE-emitting agents for cancer treatment. Pierre Auger did not conceive of the application of AEs for targeted cancer treatment, but this is a tremendously exciting future that we and many other scientists in this field envision.

A new pharmacokinetic model for 90Y-ibritumomab tiuxetan based on 3-dimensional dosimetry

Monoclonal antibodies (mAbs) are key components in several therapies for cancer and inflammatory diseases but current knowledge of their clinical pharmacokinetics and distribution in human tissues remains incomplete. Consequently, optimal dosing and scheduling in clinics are affected. With sequential radiolabeled mAb-based imaging, radiation dosing in tissues/organs can be calculated to provide a better assessment of mAb concentrations in tissues. This is the first pharmacokinetic model of ⁹⁰Y-Ibritumomab tiuxetan (⁹⁰Y-IT) in humans to be described, based on three-dimensional (3D) dosimetry using single-photon emission computed-tomography coupled with computed-tomography. 19 patients with follicular lymphoma were treated initially with ⁹⁰Y-IT in the FIZZ trial. Based on a compartmental approach individualising the vascular compartment within studied organs, this study proposes a reliable pharmacokinetic (PK) five-compartment model replacing the currently used two-compartment model and constitutes a new direction for further research. This model provides exchange constants between the different tissues, Area Under the Curve of ¹¹¹In-IT in blood (AUC) and Mean Residence Time (MRT) that have not been reported so far for IT. Finally, the elimination process appears to occur in a compartment other than the liver or the spleen and suggests the metabolism of mAbs may take place mainly on the vascular compartment level.

Analysis of the influence of 111In on 90Y-bremsstrahlung SPECT based on Monte Carlo simulation

OBJECTIVE

⁹⁰Y-ibritumomab tiuxetan (Zevalin) which is used for the treatment of malignant lymphomas can be used for SPECT imaging based on bremsstrahlung from ⁹⁰Y beta particles. However, gamma rays emitted by ¹¹¹In, which is administered to evaluate the indication for the treatment, contaminate the ⁹⁰Y bremsstrahlung images. Our objective is to investigate the influence of ¹¹¹In on the ⁹⁰Y SPECT images using Monte Carlo simulation.

METHODS

We used an in-house developed simulation code for the Monte Carlo simulation of electrons and photons (MCEP). Two hot spheres with diameters of 40 mm were put in an elliptical phantom. Both spheres ("sphere 1" and "sphere 2") were filled with ⁹⁰Y and ¹¹¹In mixed solutions. The activities of ⁹⁰Y in sphere 1 and sphere 2 were 241 and 394 kBq/mL, respectively, and the ones of ¹¹¹In were 8.14 and 13.3 kBq/mL, respectively. The background activity of ⁹⁰Y was 38.6 kBq/mL, whereas that of ¹¹¹In was 1.30 kBq/mL; moreover, the acquisition time was 30 min. Two energy windows were used: one is 90-190 keV included the ¹¹¹In photopeak; the other is 90-160 keV. To evaluate the quality of the SPECT images, the contrast recovery coefficient (CRC) and the constant noise ratio (CNR) of the SPECT images were derived.

RESULTS

For the energy window between 90 and 160 keV, the ¹¹¹In count was 74 % of the total. In that case, the CRC values were 30.1 and 30.7 % for "sphere 1" and "sphere 2", respectively, whereas the CNR values were 6.8 and 12.1, respectively. For the energy window between 90 and 190 keV, the ¹¹¹In count reached 85 % of the total count. The CRC and CNR values were 38.6 and 40.0 % and 10.6 and 19.4, respectively.

CONCLUSIONS

Our simulation study revealed that the cross talk between ¹¹¹In and ⁹⁰Y in SPECT imaging is rather serious. Even for the energy window excluding the ¹¹¹In photopeak, the count ratio of ⁹⁰Y was less than 30 % of the total. However, the influence of ¹¹¹In on ⁹⁰Y-SPECT imaging cannot be ignored, and the count ratio because of ¹¹¹In is important to estimate the density of ⁹⁰Y.

Radiation Dosimetry Study of [(89)Zr]rituximab Tracer for Clinical Translation of B cell NHL Imaging using Positron Emission Tomography

PURPOSE

We evaluated the dosimetry of [(89)Zr]rituximab, an anti-CD20 immunoPET tracer to image B cell non-Hodgkin's lymphoma (NHL) using a humanized transgenic mouse model that expresses human CD20 transgenic mice (huCD20TM).

PROCEDURES

Rituximab was conjugated to desferrioxamine (Df) for radiolabeling of Zirconium-89. [(89)Zr]rituximab (2.8 ± 0.2 MBq) was tail vein-injected into huCD20T mice. Positron emission tomography (PET)/CT imaging was performed on the two groups of mice (blocking = 2 mg/kg pre-dose of rituximab and non-blocking; n = 5) at eight time points (1, 4, 24, 48, 72, 96, 120, and 168 h) post injection.

RESULTS

The novel [(89)Zr]rituximab PET tracer had good immunoreactivity, was stable in human serum, and was able to specifically target human CD20 in mice. The human equivalents of highest dose (mean ± SD) organs with and without pre-dose are liver (345 ± 284 μSv/MBq) and spleen (1165 ± 149 μSv/MBq), respectively.

CONCLUSIONS

Dosimetry of the human patient whole-body dose was found to be 145 MBq per annum, and the patient dose-limiting organ will be the liver (with rituximab pre-dose blocking) and spleen for non-blocking. The [(89)Zr]rituximab (t½ = 78.4 h) imaging of B cell NHL patients could permit the observation of targeting lesions in NHL patients over an extended period due to longer half-life as compared to the [(64)Cu] rituximab (t½ = 12.7 h).

Development of an ELISA for the Pharmacokinetic Evaluation of a Murine Anti CD66 Monoclonal Antibody in Human Serum

An enzyme-linked immunosorbent assay (ELISA) was needed to assist in the pharmacokinetic evaluation of the murine antibody conjugate CHX A" DTPA Besilesomab in serum samples in a clinical trial . A search failed to locate a validated assay that quantified murine antibodies in human serum so the purpose of this article was to develop a robust assay, validated against current guidelines. A detailed method for an ELISA to measure a murine antibody in human serum is described. The assay was validated as fit for purpose against the target values of coefficient of variation < 20% and accuracy ± 20%.

A small-scale anatomical dosimetry model of the liver

Radionuclide therapy is a growing and promising approach for treating and prolonging the lives of patients with cancer. For therapies where high activities are administered, the liver can become a dose-limiting organ; often with a complex, non-uniform activity distribution and resulting non-uniform absorbed-dose distribution. This paper therefore presents a small-scale dosimetry model for various source-target combinations within the human liver microarchitecture. Using Monte Carlo simulations, Medical Internal Radiation Dose formalism-compatible specific absorbed fractions were calculated for monoenergetic electrons; photons; alpha particles; and (125)I, (90)Y, (211)At, (99m)Tc, (111)In, (177)Lu, (131)I and (18)F. S values and the ratio of local absorbed dose to the whole-organ average absorbed dose was calculated, enabling a transformation of dosimetry calculations from macro- to microstructure level. For heterogeneous activity distributions, for example uptake in Kupffer cells of radionuclides emitting low-energy electrons ((125)I) or high-LET alpha particles ((211)At) the target absorbed dose for the part of the space of Disse, closest to the source, was more than eight- and five-fold the average absorbed dose to the liver, respectively. With the increasing interest in radionuclide therapy of the liver, the presented model is an applicable tool for small-scale liver dosimetry in order to study detailed dose-effect relationships in the liver.

A preclinical model of CD38-pretargeted radioimmunotherapy for plasma cell malignancies

The vast majority of patients with plasma cell neoplasms die of progressive disease despite high response rates to novel agents. Malignant plasma cells are very radiosensitive, but the potential role of radioimmunotherapy (RIT) in the management of plasmacytomas and multiple myeloma has undergone only limited evaluation. Furthermore, CD38 has not been explored as a RIT target despite its uniform high expression on malignant plasma cells. In this report, both conventional RIT (directly radiolabeled antibody) and streptavidin-biotin pretargeted RIT (PRIT) directed against the CD38 antigen were assessed as approaches to deliver radiation doses sufficient for multiple myeloma cell eradication. PRIT demonstrated biodistributions that were markedly superior to conventional RIT. Tumor-to-blood ratios as high as 638:1 were seen 24 hours after PRIT, whereas ratios never exceeded 1:1 with conventional RIT. (90)Yttrium absorbed dose estimates demonstrated excellent target-to-normal organ ratios (6:1 for the kidney, lung, liver; 10:1 for the whole body). Objective remissions were observed within 7 days in 100% of the mice treated with doses ranging from 800 to 1,200 μCi of anti-CD38 pretargeted (90)Y-DOTA-biotin, including 100% complete remissions (no detectable tumor in treated mice compared with tumors that were 2,982% ± 2,834% of initial tumor volume in control animals) by day 23. Furthermore, 100% of animals bearing NCI-H929 multiple myeloma tumor xenografts treated with 800 μCi of anti-CD38 pretargeted (90)Y-DOTA-biotin achieved long-term myeloma-free survival (>70 days) compared with none (0%) of the control animals.

Patient-specific whole-body attenuation correction maps from a CT system for conjugate-view-based activity quantification: method development and evaluation

For activity quantification based on planar scintillation camera measurements, photon attenuation is an important factor that needs to be corrected for in a patient- and organ-specific manner. One possibility for obtaining attenuation correction maps is to use X-ray CT scout images. Since the intensity of scout images is in relative numbers, their image values need to be multiplied by a factor to become quantitative and thus useful for attenuation correction. The calibration factor can for our current imaging system be obtained from a scanner system file, but is generally not available. For this purpose, a method based on the patient weight has been developed. Results based on 79 patient scout images show that the calibration factor thus determined correlates well with values that, in this case, are independently specified by the system. The accuracy of attenuation correction factors (ACFs) derived from the scout-based attenuation correction maps is evaluated by comparison to ACFs derived from three-dimensional CT studies. For photon energies of 208, 245, and 364 keV, scout-based ACFs are on average 1.2% and 0.5% from the CT-derived values, using the system-based and the weight-based values of the scout-image calibration factor, respectively. The imprecision is somewhat higher for the weight-based method, due to variability in the delineation of the patient contour used as a part of this method. In conclusion, X-ray scouts are found useful for attenuation correction with a satisfactory accuracy obtained, both using the new, weight-based method, and using the previous, system-based method, for determining the required calibration factor.

131I-tositumomab myeloablative radioimmunotherapy for non-Hodgkin's lymphoma: radiation dose to the testes

PURPOSE

To investigate radiation doses to the testes delivered by a radiolabeled anti-CD20 antibody and its effects on male sex hormone levels.

MATERIALS AND METHODS

Testicular uptake and retention of (131)I-tositumomab were measured, and testicular absorbed doses were calculated for 67 male patients (54 ± 11 years of age) with non-Hodgkin's lymphoma who had undergone myeloablative radioimmunotherapy (RIT) using (131)I-tositumomab. Time-activity curves for the major organs, testes, and whole body were generated from planar imaging studies. In a subset of patients, male sex hormones were measured before and 1 year after the therapy.

RESULTS

The absorbed dose to the testes showed considerable variability (range = 4.4-70.2 Gy). Pretherapy levels of total testosterone were below the lower limit of the reference range, and post-therapy evaluation demonstrated further reduction [4.6 ± 1.8 nmol/l (pre-RIT) vs. 3.8 ± 2.9 nmol/l (post-RIT), P<0.05]. Patients receiving higher radiation doses to the testes (≥ 25 Gy) showed a greater reduction [4.7 ± 1.6 nmol/l (pre-RIT) vs. 3.3 ± 2.7 nmol/l (post-RIT), P<0.05] compared with patients receiving lower doses (<25 Gy), who showed no significant change in total testosterone levels.

CONCLUSION

The testicular radiation absorbed dose varied highly among individual patients. Patients receiving higher doses to the testes were more likely to show post-RIT suppression of testosterone levels.

Personalized image-based radiation dosimetry for routine clinical use in peptide receptor radionuclide therapy: pretherapy experience

Patient-specific dose calculations are not routinely performed for targeted radionuclide therapy procedures, partly because they are time consuming and challenging to perform. However, it is becoming widely recognized that a personalized dosimetry approach can help plan treatment and improve understanding of the dose-response relationship. In this chapter, we review the procedures and essential elements of an accurate internal dose calculation and propose a simplified approach that is aimed to be practical for use in a busy nuclear medicine department.

Managing lymphoma with non-FDG radiotracers: current clinical and preclinical applications

Nuclear medicine imaging modalities such as positron emission tomography (PET) and single-photon emission computed tomography (SPECT) have played a prominent role in lymphoma management. PET with [(18)F]Fluoro-2-deoxy-D-glucose (FDG) is the most commonly used tool for lymphoma imaging. However, FDG-PET has several limitations that give the false positive or false negative diagnosis of lymphoma. Therefore, development of new radiotracers with higher sensitivity, specificity, and different uptake mechanism is in great demand in the management of lymphoma. This paper reviews non-FDG radiopharmaceuticals that have been applied for PET and SPECT imaging in patients with different types of lymphoma, with attention to diagnosis, staging, therapy response assessment, and surveillance for disease relapse. In addition, we introduce three radiolabeled anti-CD20 antibodies for radioimmunotherapy, which is another important arm for lymphoma treatment and management. Finally, the relatively promising radiotracers that are currently under preclinical development are also discussed in this paper.

Evaluation and comparison of human absorbed dose of (90)Y-DOTA-Cetuximab in various age groups based on distribution data in rats

The organ radiation-absorbed doses have been evaluated for humans in six age groups and both genders based on animal data. After intravenous administration of ⁹⁰Y-DOTA-Cetuximab to five groups of rats, they were sacrificed at exact time intervals (2, 24, 48, 72, and 96 h) and the percentage of injected dose per gram of each organ was calculated by direct counting from rat data. By using the formulation that Medical Internal Radiation Dose suggests, radiation-absorbed doses for all organs were calculated and extrapolated from rat to human. The total body absorbed dose for all groups was >22 mGy due to pure β-emission of the applied radiopharmaceutical. The effective dose resulting from an intravenously injected activity of 100 MBq is 56.7 mSv for a 60-kg female adult and 60.3 mSv for a 73-kg male adult. The results demonstrated the usefulness of this method for estimation of β-absorbed dose in humans.

Dosimetric evaluation of nanotargeted (188)Re-liposome with the MIRDOSE3 and OLINDA/EXM programs

OBJECTIVE

The OLINDA/EXM computer code was created as a replacement for the widely used MIRDOSE3 code for radiation dosimetry in nuclear medicine. A dosimetric analysis with these codes was performed to evaluate nanoliposomes as carriers of radionuclides ((188)Re-liposomes) in colon carcinoma-bearing mice.

METHODS

Pharmacokinetic data for (188)Re-N, N-bis (2-mercaptoethyl)-N',N'-diethylethylenediamine ((188)Re-BMEDA) and (188)Re-liposome were obtained for estimation of absorbed doses in normal organs. Radiation dose estimates for normal tissues were calculated using the MIRDOSE3 and OLINDA/EXM programs for a colon carcinoma solid tumor mouse model.

RESULTS

Mean absorbed doses derived from(188)Re-BMEDA and (188)Re-liposome in normal tissues were generally similar as calculated by MIRDOSE3 and OLINDA/EXM programs. One notable exception to this was red marrow, wherein MIRDOSE3 resulted in higher absorbed doses than OLINDA/EXM (1.53- and 1.60-fold for (188)Re-BMEDA and (188)Re-liposome, respectively).

CONCLUSIONS

MIRDOSE3 and OLINDA have very similar residence times and organ doses. Bone marrow doses were estimated by designating cortical bone rather than bone marrow as a source organ. The bone marrow doses calculated by MIRDOSE3 are higher than those by OLINDA. If the bone marrow is designated as a source organ, the doses estimated by MIRDOSE3 and OLINDA programs will be very similar.

Biodistribution, radiation dosimetry and scouting of 90Y-ibritumomab tiuxetan therapy in patients with relapsed B-cell non-Hodgkin's lymphoma using 89Zr-ibritumomab tiuxetan and PET

Purpose

Positron emission tomography (PET) with ⁸⁹Zr-ibritumomab tiuxetan can be used to monitor biodistribution of ⁹⁰Y-ibritumomab tiuxetan as shown in mice. The aim of this study was to assess biodistribution and radiation dosimetry of ⁹⁰Y-ibritumomab tiuxetan in humans on the basis of ⁸⁹Zr-ibritumomab tiuxetan imaging, to evaluate whether co-injection of a therapeutic amount of ⁹⁰Y-ibritumomab tiuxetan influences biodistribution of ⁸⁹Zr-ibritumomab tiuxetan and whether pre-therapy scout scans with ⁸⁹Zr-ibritumomab tiuxetan can be used to predict biodistribution of ⁹⁰Y-ibritumomab tiuxetan and the dose-limiting organ during therapy.

Methods

Seven patients with relapsed B-cell non-Hodgkin’s lymphoma scheduled for autologous stem cell transplantation underwent PET scans at 1, 72 and 144 h after injection of ~70 MBq ⁸⁹Zr-ibritumomab tiuxetan and again 2 weeks later after co-injection of 15 MBq/kg or 30 MBq/kg ⁹⁰Y-ibritumomab tiuxetan. Volumes of interest were drawn over liver, kidneys, lungs, spleen and tumours. Ibritumomab tiuxetan organ absorbed doses were calculated using OLINDA. Red marrow dosimetry was based on blood samples. Absorbed doses to tumours were calculated using exponential fits to the measured data.

Results

The highest ⁹⁰Y absorbed dose was observed in liver (3.2 ± 1.8 mGy/MBq) and spleen (2.9 ± 0.7 mGy/MBq) followed by kidneys and lungs. The red marrow dose was 0.52 ± 0.04 mGy/MBq, and the effective dose was 0.87 ± 0.14 mSv/MBq. Tumour absorbed doses ranged from 8.6 to 28.6 mGy/MBq. Correlation between predicted pre-therapy and therapy organ absorbed doses as based on ⁸⁹Zr-ibritumomab tiuxetan images was high (Pearson correlation coefficient r = 0.97). No significant difference between pre-therapy and therapy tumour absorbed doses was found, but correlation was lower (r = 0.75).

Conclusion

Biodistribution of ⁸⁹Zr-ibritumomab tiuxetan is not influenced by simultaneous therapy with ⁹⁰Y-ibritumomab tiuxetan, and ⁸⁹Zr-ibritumomab tiuxetan scout scans can thus be used to predict biodistribution and dose-limiting organ during therapy. Absorbed doses to spleen were lower than those previously estimated using ¹¹¹In-ibritumomab tiuxetan. The dose-limiting organ in patients undergoing stem cell transplantation is the liver.

Biokinetics and dosimetry of commonly used radiopharmaceuticals in diagnostic nuclear medicine - a review

PURPOSE

The impact on patients' health of radiopharmaceuticals in nuclear medicine diagnostics has not until now been evaluated systematically in a European context. Therefore, as part of the EU-funded Project PEDDOSE.NET ( www.peddose.net ), we review and summarize the current knowledge on biokinetics and dosimetry of commonly used diagnostic radiopharmaceuticals.

METHODS

A detailed literature search on published biokinetic and dosimetric data was performed mostly via PubMed ( www.ncbi.nlm.nih.gov/pubmed ). In principle the criteria for inclusion of data followed the EANM Dosimetry Committee guidance document on good clinical reporting.

RESULTS

Data on dosimetry and biokinetics can be difficult to find, are scattered in various journals and, especially in paediatric nuclear medicine, are very scarce. The data collection and calculation methods vary with respect to the time-points, bladder voiding, dose assessment after the last data point and the way the effective dose was calculated. In many studies the number of subjects included for obtaining biokinetic and dosimetry data was fewer than ten, and some of the biokinetic data were acquired more than 20 years ago.

CONCLUSION

It would be of interest to generate new data on biokinetics and dosimetry in diagnostic nuclear medicine using state-of-the-art equipment and more uniform dosimetry protocols. For easier public access to dosimetry data for diagnostic radiopharmaceuticals, a database containing these data should be created and maintained.

A small-scale anatomic model for testicular radiation dosimetry for radionuclides localized in the human testes

The testis is a radiosensitive tissue. It contains a large number of lobules, which in turn are composed of convoluted seminiferous tubules. The epithelium inside each tubule consists of a complex mosaic of supporting cells and germ cells of different sizes and degrees of maturation. These cells are known to have diverse sensitivity to radiation, those with the highest sensitivity being the spermatogonia, which form part of the basal cell layer, and those with the lowest sensitivity being the mature sperm cells closest to the lumen of the tubule. For many years, the internal dosimetry community has discussed the need for improvements to bring about more detailed, cell-level testicular dosimetry. This paper presents a small-scale dosimetry model for calculation of S factors for several different source-target configurations within the testicular tissue.

METHODS

A model of the testis was designed in which the lobules were approximated by a cross-section of seminiferous tubules arranged in a hexagonal pattern, with interstitial tissue between them. The seminiferous tubules were divided into concentric layers representing spermatogenic development in the seminiferous epithelium. S factors were calculated for electrons, photons, α-particles, and for (18)F, (90)Y, (99m)Tc, (111)In, (125)I, (131)I, (177)Lu, and (211)At using Monte Carlo simulations.

RESULTS

For electrons with low energies the range was small, compared with the diameter of the seminiferous tubules, resulting in high energy deposition close to the source, whereas for higher electron energies more uniform energy deposition was seen, as expected. The same trend was seen for low-energy photons, whose mean free paths are small, compared with the diameter of the seminiferous tubules, resulting in high energy deposition close to the source, whereas for higher photon energies the location of the activity in the testis is less important.

CONCLUSION

The model presented in this paper is a simplification of the organized chaos that constitutes the structure of the actual testis. However, it provides a relevant, small-scale anatomic model to help us understand the significance of the heterogeneity of radioactivity in this important radiosensitive tissue.

Biodistribution and dosimetry of In-111/Y-90-HuCC49ΔCh2 (IDEC-159) in patients with metastatic colorectal adenocarcinoma

BACKGROUND

CC49, an antibody (mAb) reactive to tumor-associated glycoprotein (TAG-72), has been extensively studied for radioimmunotherapy for colon cancer. Myelotoxicity has been dose-limiting because of prolonged circulation time in the plasma, and human anti-mouse antibody responses were observed in the majority of patients. A CH2 domain deleted and humanized CC49 (HuCC49ΔCh2) was developed to ameliorate these problems. This study reports biodistribution and dosimetry of (111)In/(90)Y-HuCC49ΔCh2 (IDEC-159).

MATERIALS AND METHODS

Five (5) patients with colon cancer were enrolled. Each patient received intravenous administration of 185 MBq (111)In-HuCC49ΔCh2, followed by sequential gamma camera imaging, and blood counting. Uptakes and clearance half-lives for organs and tumors were quantified from images. Absorbed doses for (90)Y-HuCC49ΔCh2 were derived from (111)In-HuCC49ΔCh2 kinetic data.

RESULTS

Compared to reported (111)In/(90)Y-CC49 data in the literature, median blood circulation T(1/2β) was less at 38 (31-43) hours for (90)Y-HuCC49ΔCh2, than 50 hours for (90)Y-CC49. Median tumor-to-marrow absorbed dose ratio was 18 for (90)Y-HuCC49ΔCh2, and 9.53 for (90)Y-CC49. Median tumor-to-liver absorbed dose ratio was 3.14 for (90)Y-HuCC49ΔCh2, and 1.0 for (90)Y-CC49. Median tumor-to-spleen absorbed dose was 3.19 for (90)Y-HuCC49ΔCh2, and 1.07 for (90)Y-CC49.

CONCLUSIONS

A humanized and CH2 domain-deleted CC49 antibody radiolabeled with (111)In/(90)Y showed improved tumor-to-normal dose ratios over those reported from studies with intact CC49.

Biodistribution and clearance of small molecule hapten chelates for pretargeted radioimmunotherapy

PURPOSE

The favorable pharmacokinetics and clinical safety profile of metal-chelated 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) suggests that it might be an ideal hapten for pretargeted radioimmunotherapy. In an effort to minimize hapten retention in normal tissues and determine the effect of various chemical adducts on in vivo properties, a series of DOTA-based derivatives were evaluated.

PROCEDURES

Biodistribution and whole-body clearance were evaluated for (177)Lu-labeled DOTA, DOTA-biotin, a di-DOTA peptide, and DOTA-aminobenzene in normal CD1 mice. Kidney, liver, and bone marrow doses were estimated using standard Medical Internal Radiation Dose methodology.

RESULTS

All haptens demonstrated similar low tissue and whole-body retention, with 2-4% of the injected dose remaining in mice 4 h postinjection. The kidney is predicted to be dose limiting for all (177)Lu-labeled haptens tested with an estimated kidney dose of approximately 0.1 mGy/MBq.

CONCLUSIONS

We present here a group of DOTA-based haptens that exhibit rapid clearance and exceptionally low whole-body retention 4 h postinjection. Aminobenzene, tyrosine-lysine, and biotin groups have minimal effects on the blood clearance and biodistribution of (177)Lu-DOTA.

Predicting hematologic toxicity in patients undergoing radioimmunotherapy with 90Y-ibritumomab tiuxetan or 131I-tositumomab

This study aimed at identifying clinical factors for predicting hematologic toxicity after radioimmunotherapy with (90)Y-ibritumomab tiuxetan or (131)I-tositumomab in clinical practice.

METHODS

Hematologic data were available from 14 non-Hodgkin lymphoma patients treated with (90)Y-ibritumomab tiuxetan and 18 who received (131)I-tositumomab. The percentage baseline at nadir and 4 wk post nadir and the time to nadir were selected as the toxicity indicators for both platelets and neutrophils. Multiple linear regression analysis was performed to identify significant predictors (P < 0.05) of each indicator.

RESULTS

For both platelets and neutrophils, pooled and separate analyses of (90)Y-ibritumomab tiuxetan and (131)I-tositumomab data yielded the time elapsed since the last chemotherapy as the only significant predictor of the percentage baseline at nadir. The extent of bone marrow involvement was not a significant factor in this study, possibly because of the short time elapsed since the last chemotherapy of the 7 patients with bone marrow involvement. Because both treatments were designed to deliver a comparable bone marrow dose, this factor also was not significant. None of the 14 factors considered was predictive of the time to nadir. The R(2) value for the model predicting percentage baseline at nadir was 0.60 for platelets and 0.40 for neutrophils. This model predicted the platelet and neutrophil toxicity grade to within ±1 for 28 and 30 of the 32 patients, respectively. For the 7 patients predicted with grade I thrombocytopenia, 6 of whom had actual grade I-II, dosing might be increased to improve treatment efficacy.

CONCLUSION

The elapsed time since the last chemotherapy can be used to predict hematologic toxicity and customize the current dosing method in radioimmunotherapy.

Nanotargeted radionuclides for cancer nuclear imaging and internal radiotherapy

Current progress in nanomedicine has exploited the possibility of designing tumor-targeted nanocarriers being able to deliver radionuclide payloads in a site or molecular selective manner to improve the efficacy and safety of cancer imaging and therapy. Radionuclides of auger electron-, α-, β-, and γ-radiation emitters have been surface-bioconjugated or after-loaded in nanoparticles to improve the efficacy and reduce the toxicity of cancer imaging and therapy in preclinical and clinical studies. This article provides a brief overview of current status of applications, advantages, problems, up-to-date research and development, and future prospects of nanotargeted radionuclides in cancer nuclear imaging and radiotherapy. Passive and active nanotargeting delivery of radionuclides with illustrating examples for tumor imaging and therapy are reviewed and summarized. Research on combing different modes of selective delivery of radionuclides through nanocarriers targeted delivery for tumor imaging and therapy offers the new possibility of large increases in cancer diagnostic efficacy and therapeutic index. However, further efforts and challenges in preclinical and clinical efficacy and toxicity studies are required to translate those advanced technologies to the clinical applications for cancer patients.

Impact of rituximab treatment on (90)Y-ibritumomab dosimetry for patients with non-Hodgkin lymphoma

To determine whether the therapeutic effect of (90)Y-ibritumomab might be enhanced by a full course of rituximab followed by single dose of (90)Y-ibritumomab, the trial included pre- and post-rituximab treatment imaging with (111)In-ibritumomab and blood pharmacokinetics. Comparison of the pre- and post-rituximab imaging and blood data allowed for the assessment of impact of rituximab on (90)Y-ibritumomab dosimetry.

METHODS

Seventeen patients with relapsed B cell non-Hodgkin lymphoma first received 250 mg/m(2) of rituximab plus 185 MBq of (111)In-ibritumomab for initial dosimetry evaluation. In weeks 2-4, patients received 3 weekly 375 mg/m(2) doses of rituximab. In week 6, patients received a 250 mg/m(2) dose of rituximab plus 185 MBq of (111)In-ibritumomab for a second dosimetry evaluation. Five sequential, whole-body gamma-camera images were acquired after the (111)In-ibritumomab injection. Calculated radiation doses were based on patient-specific organ masses. For each patient, paired comparison of radiation doses before and after rituximab treatment was performed. Paired comparison of residence times for spleen and tumor was also performed.

RESULTS

Before rituximab treatment, the median radiation dose (mGy/MBq) was 0.48 (range, 0.24-0.86) for total body, 3.7 (range, 2.1-11.6) for liver, 6.1 (range, 1.8-17.8) for spleen, 3.3 (range, 2.0-4.7) for kidneys, 2.4 (range, 1.3-3.7) for heart wall, 1.1 (range, 0.4-2.3) for lungs, 0.79 (range, 0.32-1.22) for marrow from blood, and 18.1 (range, 4.7-98.9) for tumor. Paired comparisons were performed in 16 patients only because human antimurine antibody developed in 1 patient. The median change was 0.007 mGy/MBq for body, -0.14 mGy/MBq for liver, -0.31 mGy/MBq for kidneys, 0.38 mGy/MBq for heart wall, -0.17 mGy/MBq for lungs, and 0.046 mGy/MBq for marrow from blood. The median change in residence time was -0.92 h for spleen and -0.24 h for tumor. The changes were statistically insignificant for total body, liver, kidneys, lungs, and marrow from blood. The median residence times, or mGy/MBq if there were no volume changes, decreased 24% for spleen (P = 0.0005) and 28% for tumor (P = 0.005). The median radiation dose to heart wall increased 16%, which was statistically significant (P = 0.002).

CONCLUSION

Changes in (90)Y-ibritumomab dosimetry after 4 wk of rituximab treatment were not significant for most organs, except for the heart wall. The reduction of spleen and tumor residence times is more likely to be due to the therapeutic effects of rituximab.