Requirements for a treatment planning system for radioimmunotherapy

Non-dehalogenation Mechanisms for Excretion of Radioiodine after Administration of Labeled Antibodies

In patients or mice with cancer the pharmacokinetic behavior of radioiodinated and radiometal chelated antibodies has been observed to be different. Rapid clearance from the tissues and excretion into the urine can occur after injection of radioiodinated antibodies. These observations have been interpreted to reflect in vivo dehalogenation of the antibody. This publication describes a variety of other mechanisms that can underlie these phenomena. These mechanisms include receptor uptake and catabolism of antibody and instability of the labeled antibody due to the labeling conditions. Specifically, the relative masses of chloramine-T and antibody in the iodination reaction mixture, the level of iodination of the antibody, and the amount of antibody administered to the recipient are all factors which can influence the clearance of radioiodinated antibody from the recipient. The final determinant for the different behavior of radioiodinated and In-111 metal chelated antibody relate to the different biologic pathways of indium when compared to iodine

Dosimetry of 175Ytterbium-poly (amidoamine) Therapy for Humans' Organs

Purpose:

This investigation focuses on biodistribution of irradiated dendrimer encapsulated ytterbium-175 (¹⁷⁵Yb) and to estimate the absorbed dose from intravenous injection of PAMAM encapsulated ¹⁷⁵Yb to human organs.

Methods:

A dendrimer compound containing an average of 55 Yb⁺³ ions per dendrimer was prepared and irradiated with neutrons for 2h at 3×10¹¹ n.cm-²s-¹ neutron flux. The resulting mixture was injected into a group of tumor bearing mice and the mice were excised, weighed and counted at certain times to study the biodistribution. The human organs absorbed dose was assessed by MIRD schema and MCNP simulation.

Results:

The specific activity and radiochemical purity of the irradiated nano-composite were 7MBq/mg and >99% respectively. The rapid up take of dendrimer was in liver, lung, and, spleen. MIRD and MCNPX were applied for dose estimation. The human absorbed dose in liver, lung, spleen, kidney and bone that simulated by MCNP are 1.266, 0.8081, 0.8347, 0.03979 and 0.01706 mGy/MBq respectively and these values for MIRD schema are 1.351, 0.73, 1.03, 0.039, and 0.0097 mGy/MBq respectively.

Conclusion:

The results showed that ¹⁷⁵Yb-PAMAM nano-radiopharmaceutical has potential of application for liver and lung tumors.

Treatment of a Patient with b Cell Lymphoma by 1-131 Lym-1 Monoclonal Antibodies

A patient with Richter's syndrome, a malignant lymphomatous transformation of chronic lymphocytic leukemia, had become moribund with rapidly enlarging masses, granulocytopenia and thrombocytopenia despite the use of conventional chemotherapy and radiotherapy. Greater than ten percent of a test dose of I-131 Lym-1, a murine monoclonal antibody produced against Burkitt's African B cell lymphoma, was accumulated by her tumor. The patient was subsequently treated with a series of injections of I-131 Lym-1 with dramatic clinical response, reduction of tumor volume by x-ray computerized tomography and progression of circulating cellular elements toward normality. Her course over the next ten months was not like that to be expected for Richter's syndrome, which has an average survival of four months. This mode of treatment appears promising.

Radioimmunotherapy for Breast Cancer: Treatment of a Patient with I-131 L6 Chimeric Monoclonal Antibody

We report the first treatment of metastatic breast cancer by systemic radioimmunotherapy. The serial therapy doses were chosen based on quantitative imaging data in a treatment planning approach. A terminally ill patient with aggressive, locally advanced breast cancer who had failed radiation treatment and chemotherapy was injected intravenously with radiolabeled I-131 chimeric L6, a human-mouse chimeric IgG1 monoclonal antibody to adenocarcinoma. Initially, an imaging 10 mCi dose of 1-131 chimeric L6 (dose 1) deposited 8.8% of the injected dose in her chest wall tumor at 48 hours. Ten days later the patient was given a 150 mCi I-131 chimeric L6 dose (dose 2) followed three weeks later by a 100 mCi dose (dose 3). Tumor uptake and retention were comparable for doses 1 and 2, and decreased for dose 3. Following dose 3 the patient developed a manageable thrombocytopenia and transient Grade IV granulocytopenia. The tumor was observed to decrease in size with peak tumor regression occurring two weeks after dose 3. This partial response (PR) was achieved by radioimmunotherapy at a time when conventional therapy had been unable to impact the growth of the patient's massive and aggressive tumor.

Imaging in targeted delivery of therapy to cancer

We review the current status of imaging as applied to targeted therapy with particular focus on antibody-based therapeutics. Antibodies have high tumor specificity and can be engineered to optimize delivery to, and retention within, the tumor. Whole antibodies can activate natural immune effector mechanisms and can be conjugated to beta- and alpha-emitting radionuclides, toxins, enzymes, and nanoparticles for enhanced therapeutic effect. Imaging is central to the development of these agents and is used for patient selection, performing dosimetry and assessment of response. gamma- and positron-emitting radionuclides may be used to image the distribution of antibody-targeted therapeutics While some radionuclides such as iodine-131 emit both beta and gamma radiation and are therefore suitable for both imaging and therapy, others are more suited to imaging or therapy alone. Hence for radionuclide therapy of neuroendocrine tumors, patients can be selected for therapy on the basis of gamma-emitting indium-111-octreotide imaging and treated with beta-emitting yttrium-90-octreotate. Positron-emitting radionuclides can give greater sensitivity that gamma-emitters but only a single radionuclide can be imaged at one time and the range of radionuclides is more limited. The multiple options for antibody-based therapeutic molecules, imaging technologies and therapeutic scenarios mean that very large amounts of diverse data are being acquired. This can be most effectively shared and progress accelerated by use of common data standards for imaging, biological, and clinical data.

Monte Carlo treatment planning for molecular targeted radiotherapy within the MINERVA system

The aim of this project is to extend accurate and patient-specific treatment planning to new treatment modalities, such as molecular targeted radiation therapy, incorporating previously crafted and proven Monte Carlo and deterministic computation methods. A flexible software environment is being created that allows planning radiation treatment for these new modalities and combining different forms of radiation treatment with consideration of biological effects. The system uses common input interfaces, medical image sets for definition of patient geometry and dose reporting protocols. Previously, the Idaho National Engineering and Environmental Laboratory (INEEL), Montana State University (MSU) and Lawrence Livermore National Laboratory (LLNL) had accrued experience in the development and application of Monte Carlo based, three-dimensional, computational dosimetry and treatment planning tools for radiotherapy in several specialized areas. In particular, INEEL and MSU have developed computational dosimetry systems for neutron radiotherapy and neutron capture therapy, while LLNL has developed the PEREGRINE computational system for external beam photon-electron therapy. Building on that experience, the INEEL and MSU are developing the MINERVA (modality inclusive environment for radiotherapeutic variable analysis) software system as a general framework for computational dosimetry and treatment planning for a variety of emerging forms of radiotherapy. In collaboration with this development, LLNL has extended its PEREGRINE code to accommodate internal sources for molecular targeted radiotherapy (MTR), and has interfaced it with the plugin architecture of MINERVA. Results from the extended PEREGRINE code have been compared to published data from other codes, and found to be in general agreement (EGS4-2%, MCNP-10%) (Descalle et al 2003 Cancer Biother. Radiopharm. 18 71-9). The code is currently being benchmarked against experimental data. The interpatient variability of the drug pharmacokinetics in MTR can only be properly accounted for by image-based, patient-specific treatment planning, as has been common in external beam radiation therapy for many years. MINERVA offers 3D Monte Carlo-based MTR treatment planning as its first integrated operational capability. The new MINERVA system will ultimately incorporate capabilities for a comprehensive list of radiation therapies. In progress are modules for external beam photon-electron therapy and boron neutron capture therapy (BNCT). Brachytherapy and proton therapy are planned. Through the open application programming interface (API), other groups can add their own modules and share them with the community.

Application of MINERVA Monte Carlo simulations to targeted radionuclide therapy

Recent clinical results have demonstrated the promise of targeted radionuclide therapy for advanced cancer. As the success of this emerging form of radiation therapy grows, accurate treatment planning and radiation dose simulations are likely to become increasingly important. To address this need, we have initiated the development of a new, Monte Carlo transport-based treatment planning system for molecular targeted radiation therapy as part of the MINERVA system. The goal of the MINERVA dose calculation system is to provide 3-D Monte Carlo simulation-based dosimetry for radiation therapy, focusing on experimental and emerging applications. For molecular targeted radionuclide therapy applications, MINERVA calculates patient-specific radiation dose estimates using computed tomography to describe the patient anatomy, combined with a user-defined 3-D radiation source. This paper describes the validation of the 3-D Monte Carlo transport methods to be used in MINERVA for molecular targeted radionuclide dosimetry. It reports comparisons of MINERVA dose simulations with published absorbed fraction data for distributed, monoenergetic photon and electron sources, and for radioisotope photon emission. MINERVA simulations are generally within 2% of EGS4 results and 10% of MCNP results, but differ by up to 40% from the recommendations given in MIRD Pamphlets 3 and 8 for identical medium composition and density. For several representative source and target organs in the abdomen and thorax, specific absorbed fractions calculated with the MINERVA system are generally within 5% of those published in the revised MIRD Pamphlet 5 for 100 keV photons. However, results differ by up to 23% for the adrenal glands, the smallest of our target organs. Finally, we show examples of Monte Carlo simulations in a patient-like geometry for a source of uniform activity located in the kidney.

Quantitative radioimmunoimaging for radioimmunotherapy treatment planning: effect of reduction in data sampling on dosimetric estimates

Quantitative radioimmunoimaging (serial anterior/posterior imaging and blood sampling) is useful for radioimmunotherapy treatment planning, but can be quite time consuming. To predict whether accurate radiation absorbed dose estimates can be maintained with a reduction in data sampling, 12 patients undergoing indium-111/yttrium-90 anti-CD20 monoclonal therapy for whom absorbed doses were estimated based on eight data samples (acquired at 0, 2, 4, 24, 48, 72, 96, and 144 h, respectively), were retrospectively reanalyzed using only five samples (0, 4, 24, 72, and 144 h, respectively). Calculated residence times (in h) and absorbed doses (in cGy), for the whole body, kidneys, liver, lungs, spleen, and red marrow were compared with the original values based on the eight samples using Student's paired t-test. Linear regression and Bland-Altman analysis of the two data sample groups was also performed. The mean residence times in the five- and eight-data samples groups were essentially the same (17.7 +/- 26.6 h [range, 0.3-79.0 h] versus 17.6 +/- 26.6 h [range, 0.3-79.5 h]; p = 0.72), as were the mean absorbed doses (336 +/- 411 cGy [range, 38-2434 cGy] versus 325 +/- 381 cGy [range, 39-2246 cGy]; p = 0.24). Also, the linear regressions were excellent (residence time y = 1.00x + 0.09 h [r = 0.99]; absorbed dose y = 1.06x - 7.74 cGy [r = 0.98]). Additionally, Bland-Altman analysis revealed no significant sample bias in residence time (0.03 +/- 0.68 h, 0.9% +/- 10.0) or absorbed dose (11 +/- 76 cGy, 1.0% +/- 9.3). These results demonstrate that reduced data sampling in quantitative radioimmunoimaging can be achieved without significantly altering radiation absorbed dose estimates, but with a significant savings in imaging, blood sampling, and processing time.

Treatment planning for molecular targeted radionuclide therapy

Molecular targeted radionuclide therapy promises to expand the usefulness of radiation to successfully treat widespread cancer. The unique properties of radioactive tags make it possible to plan treatments by predicting the radiation absorbed dose to both tumors and normal organs, using a pre-treatment test dose of radiopharmaceutical. This requires a combination of quantitative, high-resolution, radiation-detection hardware and computerized dose-estimation software, and would ideally include biological dose-response data in order to translate radiation absorbed dose into biological effects. Data derived from conventional (external beam) radiation therapy suggests that accurate assessment of the radiation absorbed dose in dose-limiting normal organs could substantially improve the observed clinical response for current agents used in a myeloablative regimen, enabling higher levels of tumor control at lower tumor-to-normal tissue therapeutic indices. Treatment planning based on current radiation detection and simulations technology is sufficient to impact on clinical response. The incorporation of new imaging methods, combined with patient-specific radiation transport simulations, promises to provide unprecedented levels of resolution and quantitative accuracy, which are likely to increase the impact of treatment planning in targeted radionuclide therapy.

Choosing an optimal radioimmunotherapy dose for clinical response

Clinical trials have documented the single-agent efficacy of radioimmunotherapy (RIT) in lymphoma, and several combination therapy studies are now in progress. RIT agents are currently becoming generally available for clinical use in lymphoma therapy. Solid tumors, which are notoriously less responsive to any single agent, have demonstrated clinically useful responses, albeit temporary, and multimodality studies have been instituted. However, a sincere debate continues regarding the basic parameters to be used to define appropriate therapeutic dosing when using this modality in clinical cancer care. It is a good time to reevaluate relevant dose response information from preclinical and clinical RIT. Preclinical studies have demonstrated abundant evidence of dose response in tumor and normal tissue in homogenous model systems; however, substantive variation occurs between the dose responses of tumors with low and variable (or shed) antigen expression, as well as between histologically different tumor models. Clinical studies of various heavily pretreated patient populations given several very different RIT pharmaceuticals have led to disparate conclusions regarding patient dosing methods and dosimetric predictions of toxicity and efficacy. Single-study data on previously untreated lymphoma patients with similar histology has demonstrated a correlation of imaging dosimetry with toxicity and tumor response. High-dose therapy with bone marrow support has also demonstrated a high tumor response rate and nonmarrow normal organ toxicities that correlate with the calculated dose to those organs from imaging. In iodine-131 ((131)I)--anti-CD20 studies, (131)I was demonstrated to have variable excretion, and estimated total-body radiation dose from tracer study proved a predictive surrogate for marrow toxicity. Yttrium-90 ((90)Y)--anti-CD20, which has little (90)Y excretion from the body, demonstrated the injected dose per body weight to be more predictive of marrow toxicity than indium-111 ((111)In) tracer dosimetry methods in heavily pretreated patients, and showed maximal safety with standard mCi/kg therapy dosing. Variations in clinical RIT choices, dosing methods, and dosimetry methods emphasize the need to review the relevant information to date. Future clinical trial designs, the sophistication of dosimetry, treatment planning, and clinical treatment decisions should all be focused on achieving the best benefit-risk relationship for each patient.

Radiation dosimetry for radionuclide therapy in a nonmyeloablative strategy

Radionuclide therapy extends the usefulness of radiation from localized disease of multifocal disease by combining radionuclides with disease-seeking drugs, such as antibodies or custom-designed synthetic agents. Like conventional radiotherapy, the effectiveness of targeted radionuclides is ultimately limited by the amount of undesired radiation given to a critical, dose-limiting normal tissue, most often the bone marrow. Because radionuclide therapy relies on biological delivery of radiation, its optimization and characterization are necessarily different than for conventional radiation therapy. However, the principals of radiobiology and of absorbed radiation dose remain important for predicting radiation effects. Fortunately, most radionuclides emit gamma rays that allow the measurement of isotope concentrations in both tumor and normal tissues in the body. By administering a small "test dose" of the intended therapeutic drug, the clinician can predict the radiation dose distribution in the patient. This can serve as a basis to predict therapy effectiveness, optimize drug selection, and select the appropriate drug dose, in order to provide the safest, most effective treatment for each patient. Although treatment planning for individual patients based upon tracer radiation dosimetry is an attractive concept and opportunity, practical considerations may dictate simpler solutions under some circumstances. There is agreement that radiation dosimetry (radiation absorbed dose distribution, cGy) should be utilized to establish the safety of a specific radionuclide drug during drug development, but it is less generally accepted that absorbed radiation dose should be used to determine the dose of radionuclide (radioactivity, GBq) to be administered to a specific patient (i.e., radiation dose-based therapy). However, radiation dosimetry can always be utilized as a tool for developing drugs, assessing clinical results, and establishing the safety of a specific radionuclide drug. Bone marrow dosimetry continues to be a "work in progress." Blood-derived and/or body-derived marrow dosimetry may be acceptable under specific conditions but clearly do not account for marrow and skeletal targeting of radionuclide. Marrow dosimetry can be expected to improve significantly but no method for marrow dosimetry seems likely to account for decreased bone marrow reserve.

Role of radiation dosimetry in radioimmunotherapy planning and treatment dosing

Cancer-seeking antibodies (Abs) carrying radionuclides can be powerful drugs for delivering radiotherapy to cancer. As with all radiotherapy, undesired radiation dose to critical organs is the limiting factor. It has been proposed that optimization of radioimmunotherapy (RIT), that is, maximization of therapeutic efficacy and minimization of normal tissue toxicity, depends on a foreknowledge of the radiation dose distributions to be expected. The necessary data can be acquired by established tracer techniques, in individual patients, using quantitative radionuclide imaging. Object-oriented software systems for estimating internal emitter radiation doses to the tissues of individual patients (patient-specific radiation dosimetry), using computer modules, are available for RIT, as well as for other radionuclide therapies. There is general agreement that radiation dosimetry (radiation absorbed dose distribution, cGy) should be utilized to establish the safety of RIT with a specific radiolabeled Ab in the early stages (i.e. phase I or II) of drug evaluation. However, it is less well established that radiation dose should be used to determine the radionuclide dose (amount of radioactivity, GBq) to be administered to a specific patient (i.e. radiation dose-based therapy). Although treatment planning for individual patients based upon tracer radiation dosimetry is an attractive concept and opportunity, particularly for multimodality RIT with intent to cure, practical considerations may dictate simpler solutions under some circumstances.

Radiation dosimetry in nuclear medicine

Radionuclides are used in nuclear medicine in a variety of diagnostic and therapeutic procedures. A knowledge of the radiation dose received by different organs in the body is essential to an evaluation of the risks and benefits of any procedure. In this paper, current methods for internal dosimetry are reviewed, as they are applied in nuclear medicine. Particularly, the Medical Internal Radiation Dose (MIRD) system for dosimetry is explained, and many of its published resources discussed. Available models representing individuals of different age and gender, including those representing the pregnant woman are described; current trends in establishing models for individual patients are also evaluated. The proper design of kinetic studies for establishing radiation doses for radiopharmaceuticals is discussed. An overview of how to use information obtained in a dosimetry study, including that of the effective dose equivalent (ICRP 30) and effective dose (ICRP 60), is given. Current trends and issues in internal dosimetry, including the calculation of patient-specific doses and in the use of small scale and microdosimetry techniques, are also reviewed.

Use of the fast Hartley transform for three-dimensional dose calculation in radionuclide therapy

Effective radioimmunotherapy may depend on a priori knowledge of the radiation absorbed dose distribution obtained by trace imaging activities administered to a patient before treatment. A new, fast, and effective treatment planning approach is developed to deal with a heterogeneous activity distribution. Calculation of the three-dimensional absorbed dose distribution requires convolution of a cumulated activity distribution matrix with a point-source kernel; both are represented by large matrices (64 x 64 x 64). To reduce the computation time required for these calculations, an implementation of convolution using three-dimensional (3-D) fast Hartley transform (FHT) is realized. Using the 3-D FHT convolution, absorbed dose calculation time was reduced over 1000 times. With this system, fast and accurate absorbed dose calculations are possible in radioimmunotherapy. This approach was validated in simple geometries and then was used to calculate the absorbed dose distribution for a patient's tumor and a bone marrow sample.

Treatment planning for radio-immunotherapy

To foster the success of clinical trials in radio-immunotherapy (RIT), one needs to determine (i) the quantity and spatial distribution of the administered radionuclide carrier in the patient over time, (ii) the absorbed dose in the tumour sites and critical organs based on this distribution and (iii) the volume of tumour mass(es) and normal organs from computerized tomography or magnetic resonance imaging and appropriately correlated with nuclear medicine imaging techniques (such as planar, single-photon emission computerized tomography or positron-emission tomography). Treatment planning for RIT has become an important tool in predicting the relative benefit of therapy based on individualized dosimetry as derived from diagnostic, pre-therapy administration of the radiolabelled antibody. This allows the investigator to pre-select those patients who have 'favorable' dosimetry characteristics (high time-averaged target: non-target ratios) so that the chances for treatment success may be more accurately quantified before placing the patient at risk for treatment-related organ toxicities. The future prospects for RIT treatment planning may yield a more accurate correlation of response and critical organ toxicity with computed absorbed dose, and the compilation of dose-volume histogram information for tumour(s) and normal organ(s) such that computing tumour control probabilities and normal tissue complication probabilities becomes possible for heterogeneous distributions of the radiolabelled antibody. Additionally, radiobiological consequences of depositing absorbed doses from exponentially decaying sources must be factored into the interpretation when trying to compute the effects of standard external beam isodose display patterns combined with those associated with RIT.

A radioimmunoimaging and MIRD dosimetry treatment planning program for radioimmunotherapy

A treatment planning program for radioimmunotherapy employing quantitative Anger camera imaging and the MIRD formalism has been designed and implemented on a clinical nuclear medicine computer. Radionuclide residence times are calculated from linear, mono- and bi-exponential, and cubic spline fits to regional activity versus time curves, and radiation-absorbed dose estimates for all target organs for 131I, 67Cu, and 58 other radionuclides can be calculated. This software has been successfully applied to radioimmunotherapy of B-cell malignancies and breast adenocarcinomas.

Validation of a dose-point kernel convolution technique for internal dosimetry

The objective of this study was to validate a dose-point kernel convolution technique that provides a three-dimensional (3D) distribution of absorbed dose from a 3D distribution of the radionuclide 131I. A dose-point kernel for the penetrating radiations was calculated by a Monte Carlo simulation and cast in a 3D rectangular matrix. This matrix was convolved with the 3D activity map furnished by quantitative single-photon-emission computed tomography (SPECT) to provide a 3D distribution of absorbed dose. The convolution calculation was performed using a 3D fast Fourier transform (FFT) technique, which takes less than 40 s for a 128 x 128 x 16 matrix on an Intel 486 DX2 (66 MHz) personal computer. The calculated photon absorbed dose was compared with values measured by thermoluminescent dosimeters (TLDS) inserted along the diameter of a 22 cm diameter annular source of 131I. The mean and standard deviation of the percentage difference between the measurements and the calculations were equal to -1% and 3.6%, respectively. This convolution method was also used to calculate the 3D dose distribution in an Alderson abdominal phantom containing a liver, a spleen, and a spherical tumour volume loaded with various concentrations of 131I. By averaging the dose calculated throughout the liver, spleen, and tumour the dose-point kernel approach was compared with values derived using the MIRD formalism, and found to agree to better than 15%.

Single-dose intraperitoneal radioimmunotherapy with the murine monoclonal antibody I-131 MOv18: clinical results in patients with minimal residual disease of ovarian cancer

Sixteen of 19 enrolled patients with minimal residual disease of ovarian cancer (macroscopic disease < 5 mm or positive blind biopsies and/or positive peritoneal washing), demonstrated by surgical second-look, underwent intraperitoneal radioimmunotherapy (RIT) with the radiolabelled monoclonal antibody I-131 MOv18 (mean dose 14 mg of MOv18 with 3700 GBq of I-131) 30-40 days after the second-look procedure. Clinical follow-up and/or third-look evaluation performed 90 days after RIT showed complete response (CR) in 5 patients, no change (NC) in 6 patients and progressive disease (PD) in 5 patients. Follow-up study showed long-term maintained CR in 1 patient (34 months) and relapses in the other 4 patients after a mean disease-free period of 10.5 months. 5 NC patients showed clinical or instrumental progression after a mean disease-free period of 13 months. The toxicity of RIT was negligible. Only 1 patient showed mild and transient bone marrow suppression (platelet count nadir 52,000 mm3 after 30 days). HAMA production was demonstrated in 94% (15/16) of patients. In conclusion, RIT appears to be a very promising therapeutic approach to treat minimal residual disease of ovarian cancer.

Radio-immunotherapy dosimetry with special emphasis on SPECT quantification and extracorporeal immuno-adsorption

Results from therapeutic trials with radiolabelled monoclonal antibodies are difficult to compare, because of lack of accurate macroscopic and microscopic dosimetry for both tumours and normal tissues. Requirements for such a dosimetry are covered in the paper. Accurate in vivo dosimetric measurement techniques for verification of calculated absorbed doses are also needed to verify treatment planning. In the review, important topics related to dosimetry in therapeutic trials in RIT are covered, such as, absorbed-dose calculations and activity-quantification techniques for planar imaging and SPECT. The latter is particularly discussed, including a summary of different correction techniques. Absorbed-dose calculations and treatment-planning techniques are also discussed. Possible ways of enhancing the therapeutic ratio are reviewed, especially the novel technique with extracorporeal immuno-adsorption. The review could form the basis of the development of future treatment-planning protocols and for dosimetry calculations in radio-immunotherapy, considering some of the most important parameters for approaching an accurate in vivo dosimetry.

Intralesional radioimmunotherapy of malignant gliomas. An effective treatment in recurrent tumors

BACKGROUND

Intralesional radioimmunotherapy (RAIT) may improve the management of malignant gliomas whose prognosis is, at present, very poor. Current treatment modalities (e.g., surgery, radiotherapy, and chemotherapy) may prolong survival by a few months but cannot prevent tumor recurrence.

METHODS

Following one or more surgical operations, radiotherapy, and chemotherapy, 24 patients with recurrent malignant gliomas (23 brain and 1 spinal cord) underwent RAIT with 2 murine monoclonal antibodies (MoAb), BC-2 and BC-4, raised against tenascin (TN). This antigen is expressed in large amounts in the stroma of glial tumors but not normal brain tissue. The isotope used was iodine-131 (131I). The radiolabelled antibodies were injected directly into the tumor by means of a removable catheter or an indwelling catheter placed in the site of disease at the time of craniotomy. The patients were admitted to the protocol if histochemical analysis of their tumors demonstrated the presence of TN in high abundance. Biodistribution and dosimetry of an intralesional tracer dose (1 mg MoAb and 37 MBq 131I) were studied. RAIT was performed by the administration of escalating doses of radioiodine, ranging from 15 mCi to 57 mCi. In many cases, RAIT was was repeated two, three, or four times (on 8, 3 and 4 patients, respectively).

RESULTS

Pharmacokinetic data resulted, on average, as follows: the 24-hour tumor/background ratio was 16.6; the percentage of injected dose concentrated per gram of tumor at 24 hours was 2.4%; and the effective half-life of the MoAb at the tumor was 74.5 hours. The mean radiation dose to the tumor was 36.48 cGy per MBq of 131I injected. Both systemic and brain toxicities were absent, while human anti-mouse antibody production after MoAb administration occurred in only a few cases. At present, 17 patients are assessable, with a median survival time of 16 months. Objective responses consisted of 5 tumor stabilizations (median time, 9 months), 3 partial remissions (11 months), and 3 complete remissions (15 months).

Quantitative single photon emission tomography: verification for sources in an elliptical water phantom

Accurate absorbed dose calculations are important for a proper dose planning in internal radionuclide therapy. The activity distribution must be measured and the target volume defined. This can be done with single photon emission tomography (SPET) if proper attenuation and scatter correction are employed. This study investigated the calculation of the activity and the volume of different spherical sources. These two parameters are essential for a proper dose calculation. The scatter and attenuation correction method is based on spatially variant scatter functions and density maps. The volume calculation method is based on obtaining a threshold from a grey-level histogram. Both point sources and spheres of different diameters containing technetium-99m were placed in different locations in an elliptical water phantom and imaged by SPET. The activity and the volume of the spheres were calculated from the SPET images and compared with known activities. Results show a quantification of activity within 10% for most of the sources. Important influences on the quantification are (a) the presence of artefacts due to improper reconstruction and (b) the finite spatial resolution which affects the total number of counts within the determined volume.

State-of-the-art of external photon beam radiation treatment planning. Photon Treatment Planning Collaborative Working Group

A virtual revolution in computer capability has occurred in the last few years, largely based on rapidly decreasing costs and increasing reliability of digital memory and mass-storage capability. These developments have now made it possible to consider the application of both computer and display technologies to a much broader range of problems in radiation therapy, including planning of treatment, dose computation, and treatment verification. Several methods of three-dimensional dose computations in heterogeneous media capable of 3% accuracy are likely to be available, but significant work still remains, particularly for high energy x-rays where electron transport, and possibly pair production, need to be considered. Innovative display and planning techniques, as well as plan evaluation schemes, are emerging and show great promise for the future. No doubt these advances will lead to substantially improved treatment planning systems in the next few years. However, it must be emphasized that for many of these applications a tremendous software and hardware development effort is required.

Quantitative measurement of uptake of radiolabeled monoclonal antibody by means of planar data

Data obtained from planar images were used to measure the uptake of monoclonal antibody in organs and tumors. Background counts included in the region of interest were eliminated, and the attenuation of the photons emitted by the radionuclide through the intervening tissues was compensated for. The background counts and the intensity of the attenuation were determined from the results of phantom studies and numerical integration with a personal computer. The quantitative uptake of 111In labeled anti-melanoma Monoclonal Antibody (ZME-018) in a melanoma lesion, the liver, and the bone marrow of a patient was measured by the planar method which we developed.

Treatment of metastatic colorectal cancer by means of specific monoclonal antibodies conjugated with iodine-131: a phase II study

Eighteen consecutive patients with advanced and/or metastatic colorectal carcinoma have been treated with intraperitoneal administration of radiolabelled (iodine-131) monoclonal antibodies raised against different antigens associated to these kinds of tumours: anti-CEA FO23C5, anti-CEA BW494/32, anti-TAG B72.3, AUA1. The doses of isotope ranged between 21 and 150 mCi (777-5550 MBq) which delivered a radiation dose to the target tumour from 768 to 4628 cGy. Thirteen patients were previously treated with conventional regimens which consisted of chemotherapy (5-fluoracil with or without other anti-neoplastic drugs) both in adjuvant or palliative setting. Three patients are considered non-evaluable owing to concomitant chemotherapy in 2 and lack of objective parameters in 1. Out of 15 evaluable patients 2 achieved complete remission and 2 partial remission with a response rate of 26.6%. Three stable and 8 with progressive disease have also been registered. The toxicity was negligible consisting of hematologic WHO grade 1 in 7 patients, grade 2 in 1 patient and grade 3 in 1 patient, as well as hepatic WHO grade 1 in 8 and grade 2 in 2 patients. The authors conclude that this innovative way of treatment for advanced colorectal carcinoma seems to offer promising therapy; from these data, therefore, a new trial is justified employing radiolabelled MoAbs in well selected patients with metastatic or locally advanced colorectal carcinoma.

CT-SPECT fusion for analysis of radiolabeled antibodies: applications in gastrointestinal and lung carcinoma

Fusing or image registration improves the information obtained by correlating images from various imaging modalities. We "fused" radiolabeled antibody SPECT with CT in patients with colorectal or lung cancer. We identified corresponding landmarks on cross-sectional images and used standard graphics algorithms for untilting to match planes of reconstruction and for two-dimensional warping or transformation of images or regions of interest. Fusing localizes activity on SPECT to specific anatomic structures and decreases SPECT false positives and CT false negatives.

Calculating radiation absorbed dose for pheochromocytoma tumors in 131-I MIBG therapy

A protocol for calculating radiation absorbed dose to pheochromocytoma tumors during treatment with 131I-labeled metaiodobenzylguanidine (MIBG) is described. The technique calls for (a) obtaining tumor volumes from Computed Tomography and/or Magnetic Resonance Imaging, (b) computing energy absorbed by assuming complete beta-particle absorption and a standard shape for gamma-ray absorption and (c) scaling from tracer to therapy dose rate by the ratio of administered activities. Also a 131I time-activity curve is obtained from planar, Anger-camera, conjugate-view images of the tumor and a known-strength source, both over a series of days. In addition, to correct for any systematic errors in the calculated uptakes, a larger activity of 123I MIBG is administered separately and quantitative Single Photon Emission Computed Tomography (SPECT) is undertaken. A known-strength source also undergoes SPECT to calibrate the tomograms. Correction for Compton scattering is accomplished by the dual-energy-window technique. The subtraction fraction was found to be 0.7 for the 1/2" crystal camera and the mean reduction in tumor counts for seven tumors from Compton correction was 0.76. The normalization factor needed to bring the conjugate-view activities into agreement with the SPECT values ranged from 0.74 to 1.06. A test study on an anthropomorphic phantom indicated that the error in resultant activities might be estimated as +/- 13%. Application of the protocol led to the calculation of real, or potential (when decision was finally made to not administer therapy) radiation absorbed dose to seven tumors in three patients from an administration of about 8 GBq of MIBG. For two metastatic tumors in a 19-year old patient who did not have her primary cancer resected, the calculated radiation absorbed dose was 170 and 180 Gy. For the four metastatic deposits evaluated in two older patients, both of whom had their primary tumor surgically removed, the values ranged from 18 to 31 Gy.

Intraperitoneal radioimmunotherapy for ovarian cancer

Twenty-eight patients with assessable residual ovarian cancer after cytoreductive surgery and chemotherapy received intraperitoneal I-131 labelled monoclonal antibodies. There was no response in eight patients with tumour nodules greater than 2 cm, a partial response in two of the 15 patients with tumour nodules less than 2 cm, and a complete response in three of the other five patients with positive peritoneal washings. A further six patients received Y-90 labelled monoclonal antibodies for residual ovarian cancer. There was no response in one patient with nodules greater than 2 cm, and a partial response in one of the other five patients with tumour nodules less than 2 cm. The non-specific radiation dose in the peritoneal cavity from the infused isotope was measured by lithium fluoride thermoluminescent dosimetry (TLD). The radiation dose received by the peritoneal serosa was less than 500 cGy and was not sufficient to account for the observed tumour response. Significant bone marrow suppression was observed with I-131 activities greater than 120 mCi and with Y-90 activities greater than 13 mCi. The haemopoietic bone marrow is the dose-limiting organ in patients receiving radioimmunotherapy.

Intraperitoneal radioimmunotherapy for ovarian cancer: pharmacokinetics, toxicity, and efficacy of I-131 labeled monoclonal antibodies

Thirty-six patients with ovarian cancer were treated with intraperitoneal I-131 labeled monoclonal antibodies to tumor associated antigens. The activity of I-131 administered was increased from 20 mCi to 158 mCi and the pharmacokinetics and toxicity evaluated. Five patients who had developed HAMA (Human Antimouse Antibodies) were retreated, and the pharmacokinetics and toxicity of the first and second treatment compared. Patients receiving their first therapy (HAMA negative), had a maximum of 25% (range 19.8-39.8%) of the injected activity in their circulation. This was accompanied by severe marrow suppression at I-131 activities over 120 mCi. The 5 HAMA positive patients had only 5% injected activity in the systemic circulation (range 3.8-6%), with rapid urinary excretion and neglible marrow suppression. In 31 patients with assessable disease there were no responses in 8 patients with gross disease (nodules greater than 2 cms), partial responses in 2 out of 15 patients with nodules less than 2 cms, and complete responses in 3 out of 6 patients with microscopic disease. The non specific radiation dose to the peritoneal cavity was estimated to be less than 500 cGy by lithium fluoride TLD, and could not be expected to account for the responses seen.

Quantitative SPECT of uptake of monoclonal antibodies

Absolute quantitation of the distribution of radiolabeled antibodies is important to the efficient conduct of research with these agents and their ultimate use for imaging and treatment, but is formidable because of the unrestricted nature of their distribution within the patient. Planar imaging methods have been developed and provide an adequate approximation of the distribution of radionuclide for many purposes, particularly when there is considerable specificity of targeting. This is not currently the case for antibodies and is unlikely in the future. Single photon emission computed tomography (SPECT) provides potential for greater accuracy because it reduces problems caused by superimposition of tissues and non-target contributions to target counts. SPECT measurement of radionuclide content requires: (1) accurate determination of camera sensitivity; (2) accurate determination of the number of counts in a defined region of interest; (3) correction for attenuation; (4) correction for scatter and septal penetration; (5) accurate measurement of the administered dose; (6) adequate statistics; and (7) accurate definition of tissue mass or volume. The major impediment to each of these requirements is scatter of many types. The magnitude of this problem can be diminished by improvements in tomographic camera design, computer algorithms, and methodological approaches.

A practical SPECT technique for quantitation of drug delivery to human tumors and organ absorbed radiation dose

A practical quantitative single photon emission computed tomographic (SPECT) technique based on an empirical threshold analysis permits accurate measurements in humans of drug delivery and absorbed radiation dose. The limits of the method have been explored using a wide range of phantom volumes, concentrations, and target-to-nontarget ratios. A threshold of 43% was found to give the best results using volumes of 30 to 3,800 cc. An excellent correlation (r = .99 with a standard error of estimate [SEE] of 41 cc) was found between SPECT measured volumes and actual phantom volumes. A similarly high correlation (r = .98, SEE = 260 counts/voxel) was found in 77 measurements of concentrations between 0.01 and 3.6 microCi/cc. There was a direct relationship between the target-to-nontarget ratio of phantoms and the accuracy of volume measurements. The technique has been validated by an excellent in vivo/in vitro correlation of uptake in human tumors. The tumor cumulative concentration and tumor-to-blood ratio were used for assessment of drug delivery. In vivo quantitative measurements of the pharmacokinetics of technetium-99m (99mTc) glucoheptonate, cobalt-57 (57Co) bleomycin and platinum-195m (195mPt) cisplatin in human tumors in vivo indicates that, in contrast with tumor models in animals, there is a marked variability in drug delivery even in tumors with the same histology. Future development of labeled drugs should make it possible to use quantitative SPECT for predicting tumor response to therapy and for tailoring chemotherapy for the individual patient under treatment. SPECT quantitation of organ concentration was used for Medical Internal Radiation Dose committee (MIRD) calculations of organ absorbed radiation dose from 99mTc-labeled RBCs. Excellent in vivo/in vitro correlations were obtained between SPECT measured concentrations of blood radioactivity in the heart and in vitro measurements of blood samples. The possibilities and limitations of this technique are discussed and its use for in vivo measurement in humans of absorbed radiation dose from radiopharmaceuticals is suggested.

Quantitative SPECT in radiation dosimetry

Accurate and precise radiation dosimetry is critical for the successful therapeutic application of systemically administered radionuclides, including, of course, radionuclides in the form of radiolabeled antibody. This requires determination, based on discrete serial measurements, of the time-dependent concentrations and/or total amounts of radioactivity in situ in order to calculate source region cumulated activities. Based on extensive studies (with clinically realistic numbers of counts and accuracies of the order of 10%) in simple geometric phantoms, in complex anthropomorphic phantoms, in animal models, and in humans, quantitative rotating scintillation camera-based single-photon emission computed tomography (SPECT) now appears to be a practical approach to such measurements. The basis of the quantitative imaging capability of a three-dimensional imaging modality such as SPECT is the elimination in the reconstructed image of counts emanating from activity surrounding the source region. Subject to considerations such as the reconstruction algorithm, attenuation and scatter corrections, and, most importantly, statistical uncertainty, the counts in a pixel in a reconstructed image are therefore directly proportional to the actual counts emanating from the corresponding voxel in situ. Among intrinsic, pre-processing, and post-processing attenuation corrections, post-processing algorithms, the most widely used approach in current commercial SPECT systems, have proven adequate in uniformly attenuating parts of the body (eg, abdomen, pelvis), subject to accurate delineation of the body contour. Although a number of sophisticated scatter correction methods have been developed, the lack of explicit scatter correction has, in practice, not been a major impediment to reasonably accurate quantitative SPECT imaging, despite scattered radiation representing up to 50% of the counts in a large source region (eg, liver). Because of its mathematical propagation in the image reconstruction process, statistical uncertainty (ie, "noise") in SPECT is far greater than would be expected if it were distributed according to Poisson statistics, as in planar imaging. The low "single slice" sensitivity of rotating scintillation camera-based SPECT is therefore the principal limitation of practical quantitative SPECT. Accordingly, absolute quantitation of count-limited clinical images has been accomplished using a judiciously selected "non-ramp" filter function. In summary, reasonable quantitative SPECT imaging is now feasible clinically, even without sophisticated scatter corrections, at least in uniformly attenuating parts of the body.(ABSTRACT TRUNCATED AT 400 WORDS)

Dose planning with SPECT

In nuclear medicine imaging, single photon emission computed tomography (SPECT) is used with increasing frequency for studies of different organs. A new approach is to use quantitative SPECT for dose planning in radionuclide therapy. Important parameters to estimate dose planning are described here.

Immunochemical aspects of monoclonal antibodies important for radiopharmaceutical development

Successful development of radiopharmaceuticals from monoclonal antibodies will require the control of several immunochemical aspects of the antibody molecules. A proposed set of methods is presented here for evaluating these immunochemical parameters. This approach consists of monitoring each monoclonal antibody harvest by selective affinity chromatography to determine the presence of detectable alterations in molecular homogeneity. The products are then evaluated by radioimmunoassay technique standardized for total immunoglobulin immunoreactivity. These assays are utilized to detect variation in the immunoreactivity secondary to changes in the hybridoma cell lines, and to measure any detectable denaturation secondary to purification, fragment production and radiolabeling. HPLC molecular sieving chromatography is presented as a practical and informative monitor of molecular stability of these radiopharmaceuticals in vitro and in vivo.