Bystander responses in three-dimensional cultures containing radiolabelled and unlabelled human cells.

Research on the radiation-induced bystander effect has been carried out mainly in 2-D tissue culture systems. This study uses a 3-D model, wherein apparently normal human diploid fibroblasts (AG1522) are grown in a carbon scaffold, to investigate the induction of a G(1) checkpoint in bystander cells present alongside radiolabelled cells. Cultures were simultaneously pulse-labelled with (3)H-deoxycytidine ((3)HdC) to selectively irradiate a minor fraction of cells, and bromodeoxyuridine (BrdU) to identify the radiolabelled cells. After thorough washing of cultures, iododeoxyuridine (IdU) was administered to detect proliferating bystander cells. The cultures were harvested at various times thereafter, and cells were reacted with two monoclonal antibodies specific to IdU/BrdU or BrdU, respectively, stained with propidium iodide, and subjected to multi-parameter flow cytometry. Cell-cycle progression was followed in radiolabelled cells (BrdU(+)) that were chronically irradiated by low energy beta particles emitted by DNA-incorporated (3)H, and in unlabelled bystander cells (BrdU(-)) by a flow cytometry based cumulative labelling index assay. As expected, radiolabelled cells were delayed, in a dose-dependent manner, in G(2) and subsequently G(1). No delay occurred in progression of bystander cells through G(1), when the labelled cells were irradiated at dose rates up to 0.32 Gy h(-1).

Challenges and progress in predicting biological responses to incorporated radioactivity.

Prediction of risks and therapeutic outcome in nuclear medicine largely rely on calculation of the absorbed dose. Absorbed dose specification is complex due to the wide variety of radiations emitted, non-uniform activity distribution, biokinetics, etc. Conventional organ absorbed dose estimates assumed that radioactivity is distributed uniformly throughout the organ. However, there have been dramatic improvements in dosimetry models that reflect the substructure of organs as well as tissue elements within them. These models rely on improved nuclear medicine imaging capabilities that facilitate determination of activity within voxels that represent tissue elements of approximately 0.2-1 cm(3). However, even these improved approaches assume that all cells within the tissue element receive the same dose. The tissue element may be comprised of a variety of cells having different radiosensitivities and different incorporated radioactivity. Furthermore, the extent to which non-uniform distributions of radioactivity within a small tissue element impact the absorbed dose distribution is strongly dependent on the number, type, and energy of the radiations emitted by the radionuclide. It is also necessary to know whether the dose to a given cell arises from radioactive decays within itself (self-dose) or decays in surrounding cells (cross-dose). Cellular response to self-dose can be considerably different than its response to cross-dose from the same radiopharmaceutical. Bystander effects can also play a role in the response. Evidence shows that even under conditions of 'uniform' distribution of radioactivity, a combination of organ dosimetry, voxel dosimetry and dosimetry at the cellular and multicellular levels can be required to predict response.

Modeling radiation dose and effects from internal emitters in nuclear medicine: from the whole body to individual cells.

In nuclear medicine, proper application of radiation protection principles depends on balancing the potential risks of exposure to ionizing radiation against its possible benefits. Average doses to organs, in diagnostic or therapeutic applications, are not always representative of the doses received at the tissue or cellular level. Therefore, understanding of the relationship between the overall biological effect and absorbed dose delivered by the radiopharmaceutical may require study of doses at the organ, tissue, or cell level. In this paper, we review current models for radiation dose assessment, with consideration of the different models and assumptions employed for study at all levels of investigation.

Biologic dosimetry of bone marrow: induction of micronuclei in reticulocytes after exposure to 32P and 90Y.

UNLABELLED: Bone marrow is the dose-limiting organ in targeted radionuclide therapy. Hence, determination of the absorbed dose to bone marrow from incorporated radionuclides is a critical element in treatment planning. This study investigated the potential of the micronucleus assay in peripheral blood reticulocytes (MnRETs) as an in vivo biologic dosimeter for bone marrow. METHODS: After intravenous administration of 32P-orthophosphate or 90Y-citrate in Swiss Webster mice, DNA damage induced in bone marrow erythroblastoid cells was measured by subsequent scoring of MnRETs in peripheral blood. The response to exponentially decreasing dose rates was calibrated by irradiating animals with external 137Cs-gamma-rays. The gamma-ray dose rate was decreased exponentially, with the dose-rate decrease half-time corresponding to the effective clearance half-time (Te) of the radioactivity from the femoral bone (Te = 64 h for 90Y-citrate and Te = 255 h for 32P-orthophosphate). RESULTS: The maximum MnRETs frequency occurred on the second and third day after injection of 90Y-citrate and 32P-orthophosphate, respectively. The same pattern was observed for exponentially decreasing dose rates of 137Cs-gamma-rays. For each type of exposure, the maximum MnRETs frequency increased in a dose-dependent manner. Using the calibrated dosimeter, the initial dose rates to the marrow per unit of injected activity were 0.0020 cGy/h/kBq and 0.0026 cGy/h/kBq for 32P-orthophosphate and 90Y-citrate, respectively. CONCLUSION: Micronuclei in peripheral blood reticulocytes can be used as a noninvasive biologic dosimeter for measuring absorbed dose rate and absorbed dose to bone marrow from incorporated radionuclides.

Free radical-initiated and gap junction-mediated bystander effect due to nonuniform distribution of incorporated radioactivity in a three-dimensional tissue culture model.

To investigate the biological effects of nonuniform distribution of radioactivity in mammalian cells, we have developed a novel three-dimensional tissue culture model. Chinese hamster V79 cells were labeled with tritiated thymidine and mixed with unlabeled cells, and multicellular clusters (approximately 1.6 mm in diameter) were formed by gentle centrifugation. The short-range beta particles emitted by (3)H impart only self-irradiation of labeled cells without significant cross-irradiation of unlabeled bystander cells. The clusters were assembled in the absence or presence of 10% dimethyl sulfoxide (DMSO) and/or 100 microM lindane. DMSO is a hydroxyl radical scavenger, whereas lindane is an inhibitor of gap junctional intercellular communication. The clusters were maintained at 10.5 degrees C for 72 h to allow (3)H decays to accumulate and then dismantled, and the cells were plated for colony formation. When 100% of the cells were labeled, the surviving fraction was exponentially dependent on the mean level of radioactivity per labeled cell. A two-component exponential response was observed when either 50 or 10% of the cells were labeled. Though both DMSO and lindane significantly protected the unlabeled or bystander cells when 50 or 10% of the cells were labeled, the effect of lindane was greater than that of DMSO. In both cases, the combined treatment (DMSO + lindane) elicited maximum protection of the bystander cells. These results suggest that the bystander effects caused by nonuniform distributions of radioactivity are affected by the fraction of cells that are labeled. Furthermore, at least a part of these bystander effects are initiated by free radicals and are likely to be mediated by gap junctional intercellular communication.

Radiation protection by cysteamine against the lethal effects of intracellularly localized Auger electron, alpha- and beta-particle emitting radionuclides.

The mechanisms by which DNA-incorporated radionuclides impart lethal damage to mammalian cells were investigated by examining the capacity of cysteamine (MEA) to protect against lethal damage to V79 cells caused by unbound tritium (3H2O), DNA-incorporated 131/125I-iododeoxyuridine (IdU) and the alpha-particle emitter 210Po-citrate. Radiolabeled cells were maintained at 10.5 degrees C for 72 h in the absence or presence of MEA (0.65-2.6 mM) and the surviving fraction was determined. Protection against lethal damage caused by 3H2O, 131IdU or 125IdU and 210Po-citrate depended on the concentration of MEA with maximum protection at 1.3-1.9 mM. The dose modification factors obtained at 1.3 mM for the radiochemicals were 2.5 +/- 0.3, 1.8 +/- 0.2, 1.7 +/- 0.1 and 1.4 +/- 0.1, respectively. MEA provides more protection against indirect than direct effects of ionizing radiation, and indirect effects play a role in the radiotoxicity of Auger electron emitters incorporated into the DNA of mammalian cells.

Marrow toxicity of 33P-versus 32P-orthophosphate: implications for therapy of bone pain and bone metastases.

UNLABELLED: Several bone-seeking radiopharmaceuticals, such as 32P-orthophosphate, 89Sr-chloride, 186Re-1,1 hydroxyethylidene diphosphonate (HEDP), and 153Sm-ethylene diamine tetramethylene phosphonic acid (EDTMP), have been used to treat bone pain. The major limiting factor with this modality is bone marrow toxicity, which arises from the penetrating nature of the high-energy beta particles emitted by the radionuclides. It has been hypothesized that marrow toxicity can be reduced while maintaining therapeutic efficacy by using radionuclides that emit short-range beta particles or conversion electrons. In view of the significant clinical experience with 32P-orthophosphate, and the similarity in pain relief afforded by 32P-orthophosphate and 89Sr-chloride, this hypothesis is examined in this study using 32P- and 33P-orthophosphate in a mouse femur model. METHODS: Survival of granulocyte macrophage colony-forming cells (GM-CFCs) in femoral marrow was used as a biologic dosimeter for bone marrow. 32P- and 33P-orthophosphate were administered intravenously, and GM-CFC survival was determined as a function of time after injection and, at the nadir, as a function of injected activity. The kinetics of radioactivity in the marrow, muscle, and femoral bone were also determined. The biologic dosimeter was calibrated by assessing GM-CFC survival at its nadir after chronic irradiation of Swiss Webster mice with exponentially decreasing dose rates of gamma rays (relative biologic effectiveness equivalent to that of beta particles) from a low-dose rate 137Cs irradiator. Dose-rate decrease half-times (Td) (time required for 137Cs gamma ray dose rate to decrease by one half) of 62, 255, and 425 h and infinity were used to simulate the dose rate patterns delivered by the radiopharmaceuticals as dictated by their effective clearance half-times from the mouse femurs. These data were used to experimentally determine the mean absorbed dose to the femoral marrow per unit injected activity. Finally, a theoretical dosimetry model of the mouse femur was developed, and the absorbed doses to the femoral marrow, bone, and endosteum were calculated using the EGS4 Monte Carlo code. RESULTS: When the animals were irradiated with exponentially decreasing dose rates of 137Cs gamma rays, initial dose rates required to achieve 37% survival were 1.9, 0.98, 0.88, and 0.79 cGy/h for dose rate decrease half-times of 62, 255, and 425 h and infinity, respectively. The D37 values were 144 +/- 15, 132 +/- 12, 129 +/- 3, and 133 +/- 10 cGy, respectively, compared with a value of 103 cGy for acute irradiation. When 32P and 33P were administered, the injected activities required to achieve 37% survival were 313 and 2,820 kBq, respectively. Theoretical dosimetry calculations show that 33P offers a 3- to 6-fold therapeutic advantage over 32P, depending on the source and target regions assumed. CONCLUSION: The low-energy beta-particle emitter 33P appears to offer a substantial dosimetric advantage over energetic beta-particle emitters (e.g., 32p, 89Sr, 186Re) for irradiating bone and minimizing marrow toxicity. This suggests that low-energy beta or conversion electron emitters may offer a substantial advantage for alleviation of bone pain as well as for specifically irradiating metastatic disease in bone.

Protection by DMSO against cell death caused by intracellularly localized iodine-125, iodine-131 and polonium-210.

The mechanisms by which DNA-incorporated radionuclides impart lethal damage to mammalian cells were investigated by examining the capacity of dimethyl sulfoxide (DMSO) to protect against lethal damage to Chinese hamster V79 cells caused by unbound tritium ((3)H(2)O), DNA-incorporated (125)I- and (131)I-iododeoxyuridine ((125)IdU, (131)IdU), and cytoplasmically localized (210)Po citrate. The radionuclides (3)H and (131)I emit low- and medium-energy beta particles, respectively, (125)I is a prolific Auger electron emitter, and (210)Po emits 5.3 MeV alpha particles. Cells were radiolabeled and maintained at 10.5 degrees C for 72 h in the presence of different concentrations of DMSO (5-12.5% v/v), and the surviving fraction compared to that of unlabeled controls was determined. DMSO afforded no protection against the lethal effects of the high-LET alpha particles emitted by (210)Po. Protection against lethal damage caused by unbound (3)H, (131)IdU and (125)IdU depended on the concentration of DMSO in the culture medium. Ten percent DMSO provided maximum protection in all cases. The dose modification factors obtained at 10% DMSO for (3)H(2)O, (131)IdU, (125)IdU and (210)Po citrate were 2.9 +/- 0.01, 2.3 +/- 0.5, 2.6 +/- 0.2 and 0.95 +/- 0.07, respectively. These results indicate that the toxicity of Auger electron and beta-particle emitters incorporated into the DNA of mammalian cells is largely radical-mediated and is therefore indirect in nature. This is also the case for the low-energy beta particles emitted by (3)H(2)O. In contrast, alpha particles impart lethal damage largely by direct effects. Finally, calculations of cellular absorbed doses indicate that beta-particle emitters are substantially more toxic when incorporated into the DNA of mammalian cells than when they are localized extracellularly.

Considerations in the selection of radiopharmaceuticals for palliation of bone pain from metastatic osseous lesions.

UNLABELLED: Bone pain is a common complication for terminal patients with bone metastases from prostate, lung, breast, and other malignancies. A multidisciplinary approach in treating bone pain is generally required, 1 which includes a combination of analgesic drug therapy, radiation therapy, hormonal therapy, and chemotherapy. Over the years, treatment of bone pain using bone-seeking radiopharmaceuticals has been explored extensively. Pharmaceuticals labeled with energetic 1-particle emitters such as 32p, 89Sr, 153Sm, and 186Re, in addition to the low-energy electron emitter 117mSn, have been studied for this purpose. Bone-marrow toxicity as a consequence of chronic irradiation by the energetic , particles is a general problem associated with this form of treatment. It is therefore desirable to identify radiochemicals that minimize the dose to the bone marrow and at the same time deliver therapeutic doses to the bone. METHODS: New S values (mean absorbed dose per unit cumulated activity) for target regions of human bone and marrow were used to ascertain the capacity of various radiochemicals to deliver a high bone dose while minimizing the marrow dose. The relative dosimetric advantage of a given radiopharmaceutical compared with a reference radiochemical was quantitated as a dosimetric relative advantage factor (RAF). Several radionuclides that emit energetic 1 particles (32p, 89Sr, 153Sm, 186Re, and 177Lu) and radionuclides that emit low-energy electrons or beta particles (169Er, 117mSn, and 33p) were evaluated. For these calculations, ratios of the cumulated activity in the bone relative to cumulated activity in the marrow alpha equal to 10 and 100 were used. RESULTS: When the radiopharmaceutical was assumed to be uniformly distributed in the endosteum and alpha was taken as 100 for both the reference and test radiochemicals, the RAF values compared with the reference radionuclide 32p were 1.0, 1.2, 1.4, 1.6, 1.7, 1.9, and 2.0 for 89Sr, 186Re, 153Sm, 177Lu, 169Er, 117mSn, and 33P, respectively. In contrast, when the radiopharmaceutical is assumed to be uniformly distributed in the bone volume, the RAF values for these 7 radionuclides were 1.1, 1.5, 2.4, 3.2, 4.5, 5.1, and 6.5, respectively. CONCLUSION: These results suggest that low-energy electron emitters such as 117mSn and 33P are more likely to deliver a therapeutic dose to the bone while sparing the bone marrow than are energetic beta emitters such as 32p and 89Sr. Therefore, radiochemicals tagged with low-energy electron or beta emitters are the radiopharmaceuticals of choice for treatment of painful metastatic disease in bone.

S values for radionuclides localized within the skeleton.

UNLABELLED: Calculations of radiation absorbed dose to the active marrow are important to radionuclide therapies such as radioimmunotherapy and bone pain palliation. In diagnostic nuclear medicine, calculations of the effective dose for radiopharmaceutical procedures also require the assessment of radiation dose to the skeletal endosteum. We have previously reported the development of 2 3-dimensional electron transport models for assessing absorbed fractions to both marrow and endosteum in trabecular and cortical bone, respectively. Here, we extend these calculations to the assignment of radionuclide S values. METHODS: Data published in International Commission on Radiological Protection Publication 70 were used to develop tables of masses for total marrow space, active and inactive marrow, endosteum, and bone matrix within 22 skeletal sites in the adult. Using our site-specific tissue masses, along with electron absorbed fractions given by our 3-dimensional transport models, radionuclide S values (electron and beta particle components only) were subsequently calculated using the MIRD schema for 32P, 33P, 89Sr, 90Sr, 90Y, 117mSn, 153Sm, 169Er, 177Lu, and 186Re. Specific consideration was given to the trabecular active marrow as both a source and a target region. RESULTS: Site-specific radionuclide S values are reported for 22 skeletal sites, for 9 source-target tissue combinations within trabecular bone, and for 6 source-target tissue combinations within cortical bone. Skeletal-averaged S values are also provided. CONCLUSION: A fully documented model is presented for the adult for use in radionuclide dosimetry of the skeleton. The model is based on both the latest international recommendations for skeletal tissue masses and results from three-dimensional electron transport calculations within the skeleton. Comparisons are additionally made against the radionuclide S values published in MIRD Pamphlet No. 11 and those calculated using the MIRDOSE2 and MIRDOSE3 computer codes. Differences in these datasets vary with the source-target combination considered and may be attributed to 1 of 3 causes: (a) assumptions on reference target masses, (b) transport models used to assign absorbed fractions, and (c) implicit assumptions made in considering the trabecular active marrow as both a source and a target tissue.

Marrow-sparing effects of 117mSn(4+)diethylenetriaminepentaacetic acid for radionuclide therapy of bone cancer.

UNLABELLED: Several bone-seeking radionuclides (32P, 89Sr, 186Re, and 153Sm) have been used to treat bone pain. The limiting factor in this modality is marrow toxicity. Our hypothesis is that marrow toxicity can be reduced while maintaining therapeutic efficacy using radionuclides that emit short-range beta particles or conversion electrons (CEs). A recent study on 47 patients using the short-range CE emitter 117mSn(4+)diethylenetriaminepentaacetic acid (117mSn(4+)DTPA) supports this hypothesis. The hypothesis is now tested using 117mSn(4+)DTPA in a mouse femur model. METHODS: The survival of granulocyte-macrophage colony-forming cells (GM-CFCs) in femoral marrow is used as a biologic dosimeter for bone marrow. The dosimeter is calibrated by irradiating mice with exponentially decreasing dose rates of 137Cs gamma-rays with a dose-rate decrease half-time, Td, equal to the effective clearance half-time of 117mSn(4+)DTPA from the femur (222 h). When Td = 222 h, the mean absorbed dose required to achieve a survival fraction of 37% is 151 cGy. After calibration, 117mSn(4+)DTPA is administered and GM-CFC survival is determined as a function of injected activity. These data are used to experimentally determine the mean absorbed dose to the femoral marrow per unit injected activity. The kinetics of radioactivity in the marrow, muscle, and femoral bone are also determined. Finally, a theoretic dosimetry model of the mouse femur is used, and the absorbed doses to the femoral marrow and bone are calculated. RESULTS: The experimental mean absorbed dose to the femoral marrow per unit injected activity of 117mSn(4+)DTPA is 0.043 cGy/kBq. The theoretic mean absorbed dose to the femoral bone per unit injected activity is 1.07 cGy/kBq. If these data are compared with those obtained previously for 32P-orthophosphate, the radiochemical 117mSn(4+)DTPA yields up to an 8-fold therapeutic advantage over the energetic beta emitter 32P. CONCLUSION: The CE emitter 117mSn offers a large dosimetric advantage over energetic beta-particle emitters for alleviating bone pain, and possibly for other therapeutic applications, while minimizing marrow toxicity.

The MIRD perspective 1999. Medical Internal Radiation Dose Committee.

The MIRD schema is a general approach for medical internal radiation dosimetry. Although the schema has traditionally been used for organ dosimetry, it is also applicable to dosimetry at the suborgan, voxel, multicellular and cellular levels. The MIRD pamphlets that follow in this issue and in coming issues, as well as the recent monograph on cellular dosimetry, demonstrate the flexibility of this approach. Furthermore, these pamphlets provide new tools for radionuclide dosimetry applications, including the dynamic bladder model, S values for small structures within the brain (i.e., suborgan dosimetry), voxel S values for constructing three-dimensional dose distributions and dose-volume histograms and techniques for acquiring quantitative distribution and pharmacokinetic data.

MIRD pamphlet No. 17: the dosimetry of nonuniform activity distributions--radionuclide S values at the voxel level. Medical Internal Radiation Dose Committee.

The availability of quantitative three-dimensional in vivo data on radionuclide distributions within the body makes it possible to calculate the corresponding nonuniform distribution of radiation absorbed dose in body organs and tissues. This pamphlet emphasizes the utility of the MIRD schema for such calculations through the use of radionuclide S values defined at the voxel level. The use of both dose point-kernels and Monte Carlo simulation methods is also discussed. PET and SPECT imaging can provide quantitative activity data in voxels of several millimeters on edge. For smaller voxel sizes, accurate data cannot be obtained using present imaging technology. For submillimeter dimensions, autoradiographic methods may be used when tissues are obtained through biopsy or autopsy. Sample S value tabulations for five radionuclides within cubical voxels of 3 mm and 6 mm on edge are given in the appendices to this pamphlet. These S values may be used to construct three-dimensional dose profiles for nonuniform distributions of radioactivity encountered in therapeutic and diagnostic nuclear medicine. Data are also tabulated for 131I in 0.1-mm voxels for use in autoradiography. Two examples illustrating the use of voxel S values are given, followed by a discussion of the use of three-dimensional dose distributions in understanding and predicting biologic response.

MIRD pamphlet no. 16: Techniques for quantitative radiopharmaceutical biodistribution data acquisition and analysis for use in human radiation dose estimates.

This report describes recommended techniques for radiopharmaceutical biodistribution data acquisition and analysis in human subjects to estimate radiation absorbed dose using the Medical Internal Radiation Dose (MIRD) schema. The document has been prepared in a format to address two audiences: individuals with a primary interest in designing clinical trials who are not experts in dosimetry and individuals with extensive experience with dosimetry-based protocols and calculational methodology. For the first group, the general concepts involved in biodistribution data acquisition are presented, with guidance provided for the number of measurements (data points) required. For those with expertise in dosimetry, highlighted sections, examples and appendices have been included to provide calculational details, as well as references, for the techniques involved. This document is intended also to serve as a guide for the investigator in choosing the appropriate methodologies when acquiring and preparing product data for review by national regulatory agencies. The emphasis is on planar imaging techniques commonly available in most nuclear medicine departments and laboratories. The measurement of the biodistribution of radiopharmaceuticals is an important aspect in calculating absorbed dose from internally deposited radionuclides. Three phases are presented: data collection, data analysis and data processing. In the first phase, data collection, the identification of source regions, the determination of their appropriate temporal sampling and the acquisition of data are discussed. In the second phase, quantitative measurement techniques involving imaging by planar scintillation camera, SPECT and PET for the calculation of activity in source regions as a function of time are discussed. In addition, nonimaging measurement techniques, including external radiation monitoring, tissue-sample counting (blood and biopsy) and excreta counting are also considered. The third phase, data processing, involves curve-fitting techniques to integrate the source time-activity curves (determining the area under these curves). For some applications, compartmental modeling procedures may be used. Last, appendices are included that provide a table of symbols and definitions, a checklist for study protocol design, example formats for quantitative imaging protocols, temporal sampling error analysis techniques and selected calculational examples. The utilization of the presented approach should aid in the standardization of protocol design for collecting kinetic data and in the calculation of absorbed dose estimates.

Radioprotection against lethal damage caused by chronic irradiation with radionuclides in vitro.

To examine the capacity of chemical protectors to mitigate damage caused by chronic irradiation by incorporated radionuclides in vitro, cells must be maintained in the presence of the protector during the course of the irradiation. Such long exposures to chemical protectors at concentrations high enough to afford protection usually results in extreme chemotoxicity. To overcome this problem, experimental conditions were developed to allow Chinese hamster V79 cells to be maintained in 5% DMSO for prolonged periods (up to 72 h) with no observable chemotoxicity. Under these conditions, the capacity of DMSO to protect against damage to V79 cells caused by unbound 32P and 3H2O and DNA-incorporated (131)IdU, [3H]dThd and 125IdU was examined. The dose modification factors for 32P, 3H2O, (131)IdU, [3H]dThd and 125IdU were 2.6+/-0.5, 2.3+/-0.3, 1.0+/-0.1, 1.16+/-0.07 and 1.07+/-0.02, respectively. These results show that 5% DMSO is capable of protecting cultured V79 cells against lethal damage caused by beta particles emitted by unbound 32P and 3H2O, whereas little or no protection is afforded against damage caused by beta particles emitted by DNA-incorporated (131)I and 3H or low-energy Auger electrons emitted by DNA-incorporated 125I.

Biological dosimetry of bone marrow for incorporated yttrium-90.

UNLABELLED: The biological response of bone marrow to incorporated radionuclides depends on several factors such as absorbed dose, dose rate, proliferation and marrow reserve. The determination of the dose rate and absorbed dose to bone marrow from incorporated radionuclides is complex. This research used survival of granulocyte-macrophage colony-forming cells (GM-CFCs) as a biological dosimeter to determine experimentally the dose rate and dose to bone marrow after administration of 90Y-citrate. METHODS: The radiochemical 90Y-citrate was administered intravenously to Swiss Webster mice. Biokinetics studies indicated that the injected 90Y quickly localized in the femurs (0.8% ID/femur) and cleared with an effective half-time of 62 hr. Subsequently, GM-CFC survival was determined as a function of femur uptake and injected activity. Finally, to calibrate GM-CFC survival as a biological dosimeter, mice were irradiated with external 137Cs gamma rays at dose rates that decreased exponentially with a half-time of 62 hr. RESULTS: Femur uptake was linearly proportional to injected activity. The survival of GM-CFCs was exponentially dependent on both the initial 90Y femur activity and the initial dose rate from external 137Cs gamma rays with 5.1 kBq/femur and 1.9 cGy/hr, respectively, required to achieve 37% survival. Thus, 90Y-citrate delivers a dose rate of 0.37 cGy/hr to the femoral marrow per kBq of femur activity and the dose rate decreased with an effective half-time of 62 hr. CONCLUSION: Survival of GM-CFCs can serve as a biological dosimeter to experimentally determine the dose rate kinetics in bone marrow.

Proliferation and the advantage of longer-lived radionuclides in radioimmunotherapy.

In our previous study we used the linear-quadratic model [J. Nucl. Med. 35, 1861 (1994)] to confirm our initial finding, based on the time-dose-fractionation model [J. Nucl. Med. 34, 1801 (1993)], that longer-lived radionuclides (e.g., 32P, 91Y) can offer a substantial therapeutic advantage over the shorter-lived radionuclides presently used in radioimmunotherapy (e.g., 90Y). The original calculations using the linear-quadratic (LQ) model did not account for proliferation of the tumor and critical bone marrow tissues. It has been suggested that inclusion of a proliferation term in the LQ model can have a substantial impact on the biologically effective dose (BED). With this in mind, we have reexamined the therapeutic efficacy of longer versus short-lived radionuclides using the LQ model replete with proliferation terms for tumor and bone marrow. Relative advantage factors (RAF), which quantify the overall therapeutic advantage of a long-lived compared to short-lived radionuclide, were calculated accordingly. While the extrapolated initial dose rate required to achieve a given BED can be affected by the inclusion of proliferation terms for both the tumor and marrow, the relative advantage factors for the longer-lived radionuclides were not significantly affected. Longer-lived radionuclides such as (114m)In and 91Y are about three times more therapeutically effective than the shorter-lived 90Y which is currently used in RIT. In other words, for a given therapeutic effect in the tumor, a longer-lived radionuclide can result in a lower deleterious effect to the bone marrow than a short-lived radionuclide. Given that bone marrow is generally considered to be the dose-limiting organ, these results have important implications for radioimmunotherapy.

Design and performance characteristics of an experimental cesium-137 irradiator to simulate internal radionuclide dose rate patterns.

UNLABELLED: When radionuclides are administered internally, the biological effect can depend on the total absorbed dose and the rate at which it is delivered. A 137Cs irradiator was designed to deliver dose-rate patterns that simulate those encountered in radionuclide therapy. METHODS: An 18-Ci 137Cs irradiator was fitted with a computer-controlled mercury attenuator that facilitated changes in dose rates as desired. The absorbed dose and dose rates were calibrated with MOSFET dosimeters customized for low dose-rates. RESULTS: Initial dose rates ranging from 0.01-30 cGy/hr can be delivered depending on the location of the cage in the irradiator and the thickness of the mercury in the attenuator system. To demonstrate the irradiator system's capability to deliver dose-rate patterns encountered in radionuclide therapy, a simulation was performed where the dose rate initially increased exponentially followed by an exponential decrease in the dose rate. CONCLUSION: The irradiator system is well-suited to expose small animals to any dose-rate pattern, thereby facilitating calibration of biological dosimeters (e.g., cell survival, chromosome aberrations), which can be used to measure the absorbed dose to a target tissue after administration of radionuclides.