Multicellular dosimetry for beta-emitting radionuclides: autoradiography, thermoluminescent dosimetry and three-dimensional dose calculations

Radiation doses from 161Tb and 177Lu in single tumour cells and micrometastases

Background

Targeted radionuclide therapy (TRT) is gaining importance. For TRT to be also used as adjuvant therapy or for treating minimal residual disease, there is a need to increase the radiation dose to small tumours. The aim of this in silico study was to compare the performances of ¹⁶¹Tb (a medium-energy β⁻ emitter with additional Auger and conversion electron emissions) and ¹⁷⁷Lu for irradiating single tumour cells and micrometastases, with various distributions of the radionuclide.

Methods

We used the Monte Carlo track-structure (MCTS) code CELLDOSE to compute the radiation doses delivered by ¹⁶¹Tb and ¹⁷⁷Lu to single cells (14 μm cell diameter with 10 μm nucleus diameter) and to a tumour cluster consisting of a central cell surrounded by two layers of cells (18 neighbours). We focused the analysis on the absorbed dose to the nucleus of the single tumoral cell and to the nuclei of the cells in the cluster. For both radionuclides, the simulations were run assuming that 1 MeV was released per μm³ (1436 MeV/cell). We considered various distributions of the radionuclides: either at the cell surface, intracytoplasmic or intranuclear.

Results

For the single cell, the dose to the nucleus was substantially higher with ¹⁶¹Tb compared to ¹⁷⁷Lu, regardless of the radionuclide distribution: 5.0 Gy vs. 1.9 Gy in the case of cell surface distribution; 8.3 Gy vs. 3.0 Gy for intracytoplasmic distribution; and 38.6 Gy vs. 10.7 Gy for intranuclear location. With the addition of the neighbouring cells, the radiation doses increased, but remained consistently higher for ¹⁶¹Tb compared to ¹⁷⁷Lu. For example, the dose to the nucleus of the central cell of the cluster was 15.1 Gy for ¹⁶¹Tb and 7.2 Gy for ¹⁷⁷Lu in the case of cell surface distribution of the radionuclide, 17.9 Gy for ¹⁶¹Tb and 8.3 Gy for ¹⁷⁷Lu for intracytoplasmic distribution and 47.8 Gy for ¹⁶¹Tb and 15.7 Gy for ¹⁷⁷Lu in the case of intranuclear location.

Conclusion

¹⁶¹Tb should be a better candidate than ¹⁷⁷Lu for irradiating single tumour cells and micrometastases, regardless of the radionuclide distribution.

Phantom validation of quantitative Y-90 PET/CT-based dosimetry in liver radioembolization

Background

PET/CT has recently been shown to be a viable alternative to traditional post-infusion imaging methods providing good quality images of ⁹⁰Y-laden microspheres after selective internal radiation therapy (SIRT). In the present paper, first we assessed the quantitative accuracy of ⁹⁰Y-PET using an anthropomorphic phantom provided with lungs, liver, spine, and a cylindrical homemade lesion located into the hepatic compartment. Then, we explored the accuracy of different computational approaches on dose calculation, including (I) direct Monte Carlo radiation transport using Raydose, (II) Kernel convolution using Philips Stratos, (III) local deposition algorithm, (IV) Monte Carlo technique (MCNP) considering a uniform activity distribution, and (V) MIRD (Medical Internal Radiation Dose) analytical approach. Finally, calculated absorbed doses were compared with those obtained performing measurements with LiF:Mg,Cu,P TLD chips in a liquid environment.

Results

Our results indicate that despite ⁹⁰Y-PET being likely to provide high-resolution images, the ⁹⁰Y low branch ratio, along with other image-degrading factors, may produce non-uniform activity maps, even in the presence of uniform activity. A systematic underestimation of the recovered activity, both for the tumor insert and for the liver background, was found. This is particularly true if no partial volume correction is applied through recovery coefficients. All dose algorithms performed well, the worst case scenario providing an agreement between absorbed dose evaluations within 20%. Average absorbed doses determined with the local deposition method are in excellent agreement with those obtained using the MIRD and the kernel-convolution dose calculation approach.

Finally, absorbed dose assessed with MC codes are in good agreement with those obtained using TLD in liquid solution, thus confirming the soundness of both calculation approaches. This is especially true for Raydose, which provided an absorbed dose value within 3% of the measured dose, well within the stated uncertainties.

Conclusions

Patient-specific dosimetry is possible even in a scenario with low true coincidences and high random fraction, as in ⁹⁰Y–PET imaging, granted that accurate absolute PET calibration is performed and acquisition times are sufficiently long. Despite Monte Carlo calculations seeming to outperform all dose estimation algorithms, our data provide a strong argument for encouraging the use of the local deposition algorithm for routine ⁹⁰Y dosimetry based on PET/CT imaging, due to its simplicity of implementation.

Dose point kernels in liquid water: an intra-comparison between GEANT4-DNA and a variety of Monte Carlo codes

Modeling the radio-induced effects in biological medium still requires accurate physics models to describe the interactions induced by all the charged particles present in the irradiated medium in detail. These interactions include inelastic as well as elastic processes. To check the accuracy of the very low energy models recently implemented into the GEANT4 toolkit for modeling the electron slowing-down in liquid water, the simulation of electron dose point kernels remains the preferential test. In this context, we here report normalized radial dose profiles, for mono-energetic point sources, computed in liquid water by using the very low energy "GEANT4-DNA" physics processes available in the GEANT4 toolkit. In the present study, we report an extensive intra-comparison of profiles obtained by a large selection of existing and well-documented Monte-Carlo codes, namely, EGSnrc, PENELOPE, CPA100, FLUKA and MCNPX.

Radiometal-labeled somatostatin analogs for applications in cancer imaging and therapy

The use of radiolabeled peptides for the diagnosis and therapy of cancer has increased greatly over the last few decades. Skillfully crafted peptide systems, which have high affinity for receptors that are overexpressed in human tumors, offer the potential to improve the characterization, grading, and eventual therapy of human cancer. Robust peptide systems can be labeled with radioactive atoms for imaging purposes using single-photon emission computed tomography and positron emission tomography technologies, or can be labeled with therapeutic nuclides for the efficient killing of tumor cells. This method-based review discusses one such class of receptor-targeted peptides and their radiolabeling with radioactive metals. The somatostatin receptor is upregulated in many types of cancer, and when labeled with a radiometal atom via a bifunctional chelate, can be employed as an agent for the imaging and radiotherapy of cancer. This review will discuss the methods used in the synthesis of the somatostatin peptides, conjugation with bifunctional chelators, and radiolabeling with metal radionuclides. Methods will also be presented for the in vitro and in vivo evaluation of the compounds produced.

Overview of dosimetry for systemic targeted radionuclide therapy (STaRT)

The purposes of systemic targeted radionuclide therapy dosimetry include compiling a database of normal organ radiation-absorbed doses that are carrier- and radionuclide-specific, and assuring that the normal organ radiation doses are within a safe range before therapy. Also of importance is quantitation of radiation delivery to tumors vs. normal tissues to correlate absorbed dose with tumor control. For agents with significant and variable excretion, estimates of individual patient distribution/clearance may be needed to optimize the dose-response relationship.

Nuclear medicine dosimetry

A brief overview is provided of the history of the development of internal dose methods for use in nuclear medicine. Basic methods of internal dosimetry and the systems that have been developed for use in nuclear medicine are described. The development of the MIRD system and the International Radiopharmaceutical Dosimetry Symposium series is outlined. The evolution of models and tools for calculating dose estimates is reviewed. Current efforts in developing more patient-specific methods, particularly for use in therapy calculations, development of small scale and microdosimetry techniques, and of relating internal radiation doses to observed biological effects are described and evaluated.

Spatial accuracy of 3D reconstructed radioluminographs of serial tissue sections and resultant absorbed dose estimates

Many agents using tumour-associated characteristics are deposited heterogeneously within tumour tissue. Consequently, tumour heterogeneity should be addressed when obtaining information on tumour biology or relating absorbed radiation dose to biological effect. We present a technique that enables radioluminographs of serial tumour sections to be reconstructed using automated computerized techniques, resulting in a three-dimensional map of the dose-rate distribution of a radiolabelled antibody. The purpose of this study is to assess the reconstruction accuracy. Furthermore, we estimate the potential error resulting from registration misalignment, for a range of beta-emitting radionuclides. We compare the actual dose-rate distribution with that obtained from the same activity distribution but with manually defined translational and rotational shifts. As expected, the error produced with the short-range 14C is much larger than that for the longer range 90Y; similarly values for the medium range 131I are between the two. Thus, the impact of registration inaccuracies is greater for short-range sources.

A mouse model for calculating the absorbed beta-particle dose from (131)I- and (90)Y-labeled immunoconjugates, including a method for dealing with heterogeneity in kidney and tumor

Flynn, A. A., Green, A. J., Pedley, R. B., Boxer, G. M., Boden, R. and Begent, R. H. J. A Mouse Model for Calculating the Absorbed Beta-Particle Dose from (131)I- and (90)Y-Labeled Immunoconjugates, Including a Method for Dealing with Heterogeneity in Kidney and Tumor. Radiat. Res. 156, 28-35 (2001). Conventional internal radiation dosimetry methods assume that the beta-particle energy is absorbed uniformly and completely in the source organ and that the radioactivity is distributed uniformly in the source. However, in mice, a considerable proportion of the beta-particle energy can escape the source organ, resulting in large cross-organ doses. Furthermore, the distribution of radioactivity is generally heterogeneous in kidney and tumor. Therefore, a model was developed to account for cross-organ doses and for the effects of heterogeneity in kidney and tumor in mice for two of the most important radionuclides used in therapy, (131)I and (90)Y. Most mouse organs were modeled as single-compartment ellipsoids or cylinders, while heterogeneity in kidney and in tumor was addressed by using two compartments to represent the cortex and the medulla and viable and necrotic cells, respectively. The dimensions of these models were taken from previous studies, with the exception of kidney and tumor, which were defined using radioluminography and mosaics of high-power microscopy images. The absorbed fractions in each compartment were calculated using beta-particle point dose kernels. The self-organ dose was significantly higher for (131)I compared to (90)Y in all compartments, but a considerable amount of beta-particle energy was shown to escape the source organ for both radionuclides, with as much as 85% and 36% escaping the marrow for (90)Y and (131)I, respectively. The cortex was found to occupy a greater proportion of the total kidney volume than the medulla, and consequently the self-dose was higher in the cortex. In addition, the thickness of the viable shell in the tumor increased with tumor size, as did the self-dose fractions in both necrotic and viable areas. This dosimetry model improves dose estimates in mice and gives a conceptual basis for considering dosimetry in humans.

131I-Lym-1 in mice implanted with human Burkitt's lymphoma (Raji) tumors: loss of tumor specificity due to radiolysis

Preliminary evaluations of 125I-labeled Lym-1, an anti-lymphoma mouse IgG2a monoclonal antibody, demonstrated favorable tumor uptake in mice bearing human Burkitt's lymphoma (Raji) tumors. In this study, the pharmacokinetics of 125I- and 131I-Lym-1, and the dosimetry, efficacy, and toxicity of 131I-Lym-1 in Raji-tumored mice were evaluated.

METHODS

Lym-1 was radioiodinated by the chloramine-T method and analyzed for monomeric fraction and immunoreactivity (antigen cell binding, relative to unmodified Lym-1). Nude mice bearing Raji tumors (20-500 mm3) received 1.5 MBq (40 microCi) 125I-Lym-1, or 1.5, 7.4, 14.8, or 18.5 MBq (40, 200, 400, or 500 microCi) 131I-Lym-1. Pharmacokinetic data (total body and blood clearance and biodistribution) were used to estimate radiation dosimetry. Mini-thermoluminescent dosimetry (TLD) was also used to measure radiation dosimetry directly for 7 days after injection of 131I-Lym-1. Tumor size, survival, body weight, and blood counts were monitored for 60 days to evaluate therapeutic efficacy and toxicity of 131I-Lym-1.

RESULTS

At the time of injection, the mean quality assurance (QA) values for 125I-Lym-1 were 100% monomer and 100% relative immunoreactivity; the corresponding values for 131I-Lym-1 were 73% and 66%, indicating that radiolysis had occurred during the interval between radiolabeling and injection. 125I-Lym-1 exhibited high and sustained concentration in tumors relative to normal organs, whereas 131I-Lym-1 did not. Assuming identical pharmacokinetic behavior to 125I-Lym-1, 131I-Lym-1 would deliver radiation doses of 3.45, 0.83, 1.03, 0.34, and 0.56 Gy per MBq injected (12.8, 3.1, 3.8, 1.3, and 2.1 rad/microCi), to tumor, liver, lungs, total body, and marrow, respectively. When the actual pharmacokinetic data for 131I-Lym-1 (1.5 MBq) were used to estimate dosimetry, corresponding values of 0.51, 0.72, 0.49, 0.31, and 0.41 Gy/MBq (1.9, 2.7, 1.8, 1.1, and 1.5 rad/microCi) were obtained. Similar values were obtained for mice receiving 7.4 or 14.8 MBq of 131I-Lym-1. Similarly, TLD data indicated little preferential radiation dosimetry to tumor. Response rates (cure + CR + PR) for mice receiving 0, 7.4, 14.8, and 18.5 MBq of 131I-Lym-1 were 8%, 7%, 21%, and 45%, respectively. The LD50/30 dose of 131I-Lym-1 was 12.7 MBq (343 microCi).

CONCLUSIONS

125I-Lym-1 exhibited high and sustained concentration in Raji tumors in mice, indicating excellent therapeutic potential for 131I-Lym-1. However, in vitro QA results for 131I-Lym-1 indicated that radiolysis had occurred, and 131I-Lym-1 demonstrated little accumulation in tumor, or preferential radiation dosimetry to tumor in the same model.

A comparison of image registration techniques for the correlation of radiolabelled antibody distribution with tumour morphology

Image registration is a powerful tool for correlating functional images with images of anatomical structure. This facilitates more accurate quantitation of regional radiopharmaceutical uptake. Similarly, registration of images of radiolabelled antibody distribution, in tissue sections, with the equivalent histological images allows the comparison and measurement of radiopharmaceutical distribution with morphological structure. The images used were obtained by storage phosphor plate technology, for the radiopharmaceutical distribution, and by digitization of the stained histological sections. Here we compare four fully automatic registration techniques and one manual technique in terms of their spatial accuracy. We have found that there was no difference in accuracy between cross-correlation, minimization of variance and mutual information. These techniques were more accurate than principal axes and the manual technique. However, minimization of variance and mutual information were more time-consuming than the other methods. Consequently, cross-correlation is the method of choice for automatic registration of large numbers of these image pairs.

Estimation of organ cumulated activities and absorbed doses on intakes of several 11C labelled radiopharmaceuticals from external measurement with thermoluminescent dosimeters

We have developed a method for obtaining the cumulated activities in organs from radionuclides, which are injected into the patient in nuclear medicine procedures, by external exposure measurement with thermoluminescent dosimeters (TLDs) which are attached to the patient's body surface close to source organs to obtain information on body-surface doses. As the surface dose is connected to the cumulated activities in source organs through radiation transmission in the human body which can be estimated with the aid of a mathematical phantom, the organ cumulated activities can be obtained by the inverse transform method. The accuracy of this method was investigated by using a water phantom in which several gamma-ray volume sources of known activity were placed to simulate source organs. We then estimated by external measurements the organ cumulated activities and absorbed doses in subjects to whom the radiopharmaceuticals 11C-labelled Doxepin, 11C-labelled YM09151-2 and 11C-labelled Benzotropin were administered in clinical nuclear medicine procedures. The cumulated activities in the brain obtained with TLDs for Doxepin and YM09151-2 are 63.6 +/- 6.2 and 32.1 +/- 12.0 kBq h MBq-1 respectively, which are compared with the respective values of 33.3 +/- 9.9 and 23.9 +/- 6.2 kBq h MBq-1 with direct PET (positron emission tomography) measurements. The agreement between the two methods is within a factor of two. The effective doses of Doxepin, YM09151-2 and Benzotropin are determined as 6.92 x 10(-3), 7.08 x 10(-3) and 7.65 x 10(-3) mSv MBq-1 respectively with the TLD method. This method has great advantages, in that cumulated activities in several organs can be obtained easily with a single procedure, and the measurements of body surface doses are performed simultaneously with the nuclear medicine procedure, as TLDs are too small to interfere with other medical measurements.

Absorbed dose distribution of the auger emitters 67GA and 125I and the beta-emitters 67CU, 90Y, 131I, and 186RE as a function of tumor size, uptake, and intracellular distribution

PURPOSE

The influence of tumor volume, uptake of radioactive compounds in cells of tumors and normal tissues, and characteristics of the emitted ionizing particles on the efficacy of systemic radiation were studied.

METHODS AND MATERIALS

The influence of these variables was assessed using a point kernel approach combined with a distance histogram technique. Simulation calculations were performed to assess dose distributions for three tumor sizes (phi = 200 microns, 2 mm, or 2 cm) and six radionuclides: 67Ga, 125I, 67Cu, 90Y, 131I, and 186Re.

RESULTS

The energy deposition patterns depended on the relation of the tumor size and range of the emitted particles. Selective uptake was especially important in cases where the range was short compared to the dimension of the tumor.

CONCLUSION

To attain a high dose for treatment of micrometastases, the use of Auger and conversion electron emitters (67Ga and 125I) or beta-emitters with emission spectra including low energetic electrons (67Cu and 131I) was recommended. The results demonstrated the complementary nature of selectivity of energy deposition and crossfire. This implied that for tumor cells or areas with reduced uptake, crossfire from radioactivity in surrounding cells or areas with selective uptake would be provided by intermediate (conversion electrons) or long-range (beta-particles) emissions.

Prediction of tumor response to experimental radioimmunotherapy with 90Y in nude mice

PURPOSE

To identify those factors that predict variability in tumor response to 90Y-radioimmunotherapy based on measurement of incorporated activity and physical dimensions of individual tumors and to apply the concept of effective dose to radioimmunotherapy.

METHODS AND MATERIALS

Human colon carcinoma xenografts growing in nude mice were treated with anti-CEA antibodies labeled with 90Y directly or through a bispecific antibody/labeled hapten system. Tumor response was measured as the delay in growth to eight times the treatment volume. Noninvasive activity (based on bremsstrahlung radiation) and dimension measurements were made in these animals at several times after label injection. The following parameters were compared for their ability to predict individual tumor response: (a) injected activity, (b) injected activity times a factor based on average uptake as a function of volume, (c) in vivo activity per volume measured in each animal at a single time, (d) the integral over time of in vivo activity per volume in each animal, and (e) the minimum dose for each animal in a uniformly active ellipsoid whose total activity and dimensions varied over time the same as the tumor.

RESULTS AND CONCLUSION

After correcting for differences in injected activity, two parameters account for much of the variability in tumor response. One of these is the general trend of larger tumors to take up less activity per volume. Additional variability can be accounted for by the in vivo activity per volume measurements. The minimum dose as introduced here is likely to be useful in estimating the biologically effective dose delivered by each treatment.

Direct measurement of intratumor dose-rate distributions in experimental xenografts treated with 90Y-labeled radioimmunotherapy

PURPOSE

To measure, quantify, and evaluate the planar dose-rate distribution for human tumor xenografts implanted into mice that are treated with 90Y-labeled monoclonal antibodies or bispecific antibodies and 90Y-labeled haptens.

METHODS AND MATERIALS

Twenty-five LS174T human colon carcinoma tumors grown subcutaneously in nude mice were treated with 90Y by either directly labeled ZCE025 or bispecific ECA001-DBX antibody systems. A simple, quick technique using GAF radiochromic medium determined the dose-rate distribution in a plane passing through the tumor center. The dose-rate distribution is generated from exposure to activity situated in one-half of the tumor (0.045 to 0.83 g).

RESULTS

Planar dose-rate distributions were obtained from the tumor xenografts. Planar dose-rate histograms were computed along with the coefficients of variance and skewness of the distributions. The observed dose-rate distributions were quantitatively compared to those calculated for a uniformly distributed activity in a half-ellipsoid of the same volume and approximate shape as the tumor half. The observed dose-rate distributions were usually broader with a more positive coefficient of skewness than the dose-rate distributions calculated from the uniformly active half-ellipsoids. For 90Y, tumor shape plays an important role in determining the minimum tumor dose. For these tumors, the tumor minimum dose-rate is always observed along the edge, usually where the edge curvature is most convex. Larger tumors tended to have broader dose-rate distributions and more positive coefficients of skewness. Exceptions to this trend were associated with dose-rate maxima displaced from the central regions due to activity heterogeneity or tumor size greatly exceeding the range of emission. Calculations for dose rate from the conventional Medical Internal Radiation Dose (MIRD) formulation exceeded the average and minimum dose rate derived from radiochromic media. The coefficient of skewness became more positive for increasing time between injection and tumor excision, consistent with the activity evolving into a more uniform activity distribution.

CONCLUSION

Using radiochromic media to measure the spatial dose-rate distribution is a valuable method for comparing the dose-rate heterogeneity among experimental tumor xenografts in animals treated with radiolabeled antibodies. Tumor size (relative to the particle range) and changes in activity distribution radiolabeled antibodies. Tumor size (relative to the particle range) and changes in activity distribution affect the dose-rate distribution that are reflected by changes in the coefficients of skewness and variation of the dose-rate area histogram. The increase in coefficients of variation and skewness with tumor size and time results from the size of the 90Y beta particle penetration range that either exceeds or is comparable to the tumor dimensions. The minimum dose rate is more dependent, relative to the average and the maximum dose rates, on the curvature of the tumor surface.

Calculated and TLD-based absorbed dose estimates for I-131-labeled 3F8 monoclonal antibody in a human neuroblastoma xenograft nude mouse model

Preclinical evaluation of the therapeutic potential of radiolabeled antibodies is commonly performed in a xenografted nude mouse model. To assess therapeutic efficacy it is important to estimate the absorbed dose to the tumor and normal tissues of the nude mouse. The current study was designed to accurately measure radiation does to human neuroblastoma xenografts and normal organs in nude mice treated with I-131-labeled 3F8 monoclonal antibody (MoAb) against disialoganglioside GD2 antigen. Absorbed dose estimates were obtained using two different approaches: (1) measurement with teflon-imbedded CaSO4:Dy mini-thermoluminescent dosimeters (TLDs) and (2) calculations using mouse S-factors. The calculated total dose to tumor one week after i.v. injection of the 50 microCi I-131-3F8 MoAb was 604 cGy. The corresponding decay corrected and not corrected TLD measurements were 109 +/- 9 and 48.7 +/- 3.4 cGy respectively. The calculated to TLD-derived dose ratios for tumor ranged from 6.1 at 24 h to 5.5 at 1 week. The light output fading rate was found to depend upon the tissue type within which the TLDs were implanted. The decay rate in tumor, muscle, subcutaneous tissue and in vitro, were 9.5, 5.0, 3.7 and 0.67% per day, respectively. We have demonstrated that the type of tissue in which the TLD was implanted strongly influenced the in vivo decay of light output. Even with decay correction, a significant discrepancy was observed between MIRD-based calculated and CaSO4:Dy mini-TLD measured absorbed doses. Batch dependence, pH of the tumor or other variables associated with TLDs which are not as yet well known may account for this discrepancy.

The spatial accuracy of cellular dose estimates obtained from 3D reconstructed serial tissue autoradiographs

In order to better predict and understand the effects of radiopharmaceuticals used for therapy, it is necessary to determine more accurately the radiation absorbed dose to cells in tissue. Using thin-section autoradiography, the spatial distribution of sources relative to the cells can be obtained from a single section with micrometre resolution. By collecting and analysing serial sections, the 3D microscopic distribution of radionuclide relative to the cellular histology, and therefore the dose rate distribution, can be established. In this paper, a method of 3D reconstruction of serial sections is proposed, and measurements are reported of (i) the accuracy and reproducibility of quantitative autoradiography and (ii) the spatial precision with which tissue features from one section can be related to adjacent sections. Uncertainties in the activity determination for the specimen result from activity losses during tissue processing (4-11%), and the variation of grain count per unit activity between batches of serial sections (6-25%). Correlation of the section activity to grain count densities showed deviations ranging from 6-34%. The spatial alignment uncertainties were assessed using nylon fibre fiduciary markers incorporated into the tissue block, and compared to those for alignment based on internal tissue landmarks. The standard deviation for the variation in nylon fibre fiduciary alignment was measured to be 41 microns cm-1, compared to 69 microns cm-1 when internal tissue histology landmarks were used. In addition, tissue shrinkage during histological processing of up to 10% was observed. The implications of these measured activity and spatial distribution uncertainties upon the estimate of cellular dose rate distribution depends upon the range of the radiation emissions. For long-range beta particles, uncertainties in both the activity and spatial distribution translate linearly to the uncertainty in dose rate of < 15%. For short-range emitters (< 100 microns), such as alpha particle sources, the magnitude of the uncertainty in serial section alignment is comparable with the particle track length. Under these circumstances, dosimetric errors are introduced in proportion to the serial section alignment inaccuracy.

Radionuclide targeting and dosimetry at the microscopic level: the role of microautoradiography

The understanding of localisation mechanisms and microdosimetry of diagnostic and therapeutic radiopharmaceuticals depends on knowledge of their biodistribution at the microscopic level (cellular and subcellular) in the target tissues. Various methods have been advanced for obtaining information about this microdistribution: subcellular fractionation, secondary ion mass spectrometry imaging, microprobe elemental analysis in the electron microscope, and microautoradiography. This review compares these approaches, and discusses in detail the methodology of microautoradiography (the most generally useful approach) with imaging and therapy radionuclides. Literature examples of applications of microautoradiography in nuclear medicine are reviewed, and the future potential contribution of the techniques is assessed.

Autoradiography-based, three-dimensional calculation of dose rate for murine, human-tumor xenografts

A Fast Fourier Transform method for calculating the three-dimensional dose rate distribution for murine, human-tumor xenografts is outlined. The required input includes evenly-spaced activity slices which span the tumor. Numerical values in these slices are determined by quantitative 125I autoradiography. For the absorbed dose-rate calculation, we assume the activity from both 131I- and 90Y-labeled radiopharmaceuticals would be distributed as is measured with the 125I label. Two example cases are presented: an ovarian-carcinoma xenograft with an IgG 2ak monoclonal antibody and a neuroblastoma xenograft with meta-iodobenzylguanidine (MIBG). Considering all the volume elements in a tumor, we show, by comparison of histograms and also relative standard deviations, that the measured 125I activity and the calculated 131I dose-rate distributions, are similarly non-uniform and that they are more non-uniform than the calculated 90Y dose-rate distribution. However, the maximum-to-minimum ratio, another measure of non-uniformity, decreases by roughly an order of magnitude from one distribution to the next in the order given above.