Quantitation of 211At in small volumes for evaluation of targeted radiotherapy in animal models

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Pergamon

Vol. 22, No. I, pp. 4%54, 1995 Copyright c 1995ElsevierScienceLtd Printedin Great Britain. Al1 rights reserved 0969-805 I /95 $9.50+ 0.00 Med. Biol.

~nantitation of 211At in Small Volumes for Evaluation of Targeted Radiotherapy in Animal Models E. L. JOHNSON, T. G. TURKINGTON, R. J. JASZCZAK, D. R. GILLAND, G. VAIDYANATHAN, K. L. GREER, R. E. COLEMAN and M. R. ZALUTSKY” Duke University Medical Center, Department of Radiology, Durham, NC 27710, USA (Accepted

5 Mq

1994)

We have evaluated SPECT and two planar imaging methods, geometric mean (GM) and buildup factor (BF), for their potential to quantitate in civo 2”At distributions in rat spinal subarachnoid spaces using phantom studies. The use of medium-energy collimators and the small diameter (3 mm) of the subarachnoid spacecomplicate quantitation. Net activities from distributionsin variousbackgroundswere obtained using a large region of interest with background subtraction. Results showed quantitation accuracy within 10% for SPECT and BF in low backgrounds increasing to 25% at higher background levels while GM errors ranged from 20 to 45% We have also obtained imagesof [*“At]astatide

distributions, administeredintrathecally, in rats.

therapeutic agents include 21’At-labeled methylene blue (Link et ai., 1989; Link and Carpenter, 1990), In targeted radiotherapy, carriers with potentially m-[2”At]astato-benzylguanidine (Vaidyanathan and high selectivity for malignant tissue offer the possi- Zalutsky, 1992) and *“At-labeled monoclonal antibility of delivering curative doses of radiation to bodies (Zalutsky et al., 1989). tumor while minimizing exposure to surrounding Because of concerns with toxicity of *“At CXhealthy tissues. Numerous strategies for targeted particles, extensive evaluation of potential theraradiotherapy are under active investigation including peutic agents with regard to pharmacokinetics, monoclonal antibodies, receptor-avid compounds efficacy and toxicity will be required prior to the and metabolic percursors. For certain applications, initiation of clinical trials. Athymic rodents with radionuclides such as 2’2Bi and “‘At are of particular human tumor xenografts are almost exclusively used interest because they decay by the emission of a- for the preclinical assessmentof targeted therapeutic particles. Because of their high decay energy and agents, and studies in rodents would be greatly short path length, a-particles are radiation of high facilitated if the distribution of 21’At could be linear energy transfer and have a radiobiological quantitated by external imaging. Astatine-211 emits effectiveness much greater than p-particles (Hall, polonium K x-rays of 77-92 keV (Lambrecht and 1988). The therapeutic potential of *‘*Bi-labeled anti- Mirzadeh, 1985) that are appropriate for imaging; bodies has been demonstrated in both in vitro and in however, low abundance, high energy gamma-rays vim models (Kozak et al., 1986; Macklis et al., 1988, are a confounding factor. 1992). One potential application is the intrathecal adminAstatine-211 has decay properties that have been istration of 2i’At-labeled radiopharmaceuticals for considered to be nearly ideal for certain targeted the treatment of neoplastic meningitis, a particularly radiotherapeutic applications (Kassis et al., 1986). Its devastating malignancy spread along the surface of half-life of 7.2 h is long enough to perform radio- the spinal cord. This approach follows the previous chemical syntheses and achieve targeting in vim The work of Fuchs et al. (1990) where a rat model of x-particles of 2”At have a mean range of about 65 pm human neoplastic meningitis was developed to assess and a linear energy transfer of 99 keV/pm. Astatine- the effectiveness of intrathecal administration of 211 labeled agents that have been investigated as chemotherapeutic agents. With the goal of imaging 2’1At, we have investigated the ability of single photon emission computed

*Author for correspondence. 45

tomography

(SPECT) and

E. L. JOHNSOK

46

two planar methods, geometric mean (GM) and buildup factor analysis (BF), for quantitating “‘At distributions in a phantom that approximates a rat subarachnoid space. In addition, the feasibility of imaging rats administered [2”At]astatide intrathetally was demonstrated in an initial animal experiment. Only limited information is available concerning imaging 2”At distributions. In a thesis dissertation, Amoedo (1974) attempted to image rodents using planar imaging with pinhole collimation. More recently, our group investigated the ability to image a simple phantom containing 2”At using a SPECT camera fitted with several types of collimators (Turkington et al., 1993). In the current study, the small size of the “spinal canal”, i.e. subarachnoid space, presents further complications and provides a realistic challenge to assess the ability of SPECT, GM and BF approaches for “‘At quantitation.

Materials and Methods Production of “‘At Astatine-211 (half-life 7.21 h) used in this study was produced at the Duke University Medical Center cyclotron facility by the 209Bi (c(,2n) 2”At reaction. Procedures for target preparation, irradiation and extraction of 2”At have been described previously (Zalutsky and Narula, 1988). The activity used in all experiments was in the chemical form [21’At]astatide. Phantom The rat spinal canal phantom, shown in Fig. 1, consisted of a 5 cm o.d. (3 mm wall) x 15 cm long acrylic cylinder “body” with a “spinal canal” insert made from a 9 cm length of a 1 mL polystyrene pipet having a 3 mm i.d. A three-way stopcock was attached to each end of the pipet section resulting in an effective canal length of about 11.5 cm. The spinal canal insert was oriented parallel to the cylinder axis and positioned approx. 1 cm from the outer surface of the cylinder.

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Phantom experiments In animal studies, the radiotherapeutic agent IS administered intrathecally through a catheter into the cerebrospinal fluid and is initially localized within the subarachnoid space. To mimic this situation in phantom studies, uniform distributions of “‘At in scaled “canals” were placed into a water-filled phantom and imaged in various uniform background conditions. A cold background and canal : background concentration ratios of approx. 15 : I ) 20 : I, 50 : I and IO0 : 1 were tested. A canal of non-uniform activity rcpresenting increased uptake tumor was also scanned in a cold water background. This canal was made by inserting a 7 cm length of 1.8 mm o.d. Teflon tubing. sealed at each end, into the canal to create a nonuniform filling volume. The actual canal activity varied between experiments with the 15 : 1 and 100 : 1 phantoms each containing about 15 PCi while canals in all other experiments contained approx. 30 PCi. True activities were determined for each phantom experiment from in-air measurements of the canal. The canal activities used in these experiments are similar to therapeutic doses given in rat studies. Rat experiments Two female athymic rats weighing 225-250 g were obtained from a closed colony maintained in the Duke University Comprehensive Cancer Center Isolation Facility. Approximately 3 weeks prior to the imaging study, subarachnoid catheters were put in place as described in a previous publication (Fuchs et al., 1990). Administration of [“‘Atlastatide (20 /tCi. 40 pL) through the catheter was accomplished via a Hamilton syringe and injector fitted with a 30 gauge needle. A saline flush (20 pL) was used to clear residual 2”At activity from the catheter. Rats were anesthetized prior to each imaging session using a ketamine/xylazine solution (55 mg/mL ketamine, 9 mg/mL xylazine) given at the rate of 1 mL/kg by intraperitoneal injection, placed in a polyvinyl chloride holder and imaged in dorsal recumbancy. All animal experiments were performed in compliance with a protocol established by the Duke University Institutional Animal Care and Use Committee. Imaging system

Fig. 1. Rat spinal canal phantom.

A Trionix Triad triple-camera SPECT imaging system (Trionix Research Laboratory, Twinsburg, OH) was used for data acquisition. Experimental parameters and conditions used in this study were based on those obtained in previous feasibility studies (Turkington et al., 1993). Cameras were equipped with 300 keV medium-energy parallel-hole collimators having a measured resolution of 12 mm FWHM at 12 cm from the collimator surface. All uniformity corrections were based on 99mTc floods. Data were acquired in a 20% photopeak window centered at 80 keV (72-88 keV) and a 35% scatter window

Quantitation of 2”At in small volumes (SC-72 keV) using a 3.56 x 3.56 mm projection size and a 128 x 64 matrix.

47

pixel 70

-

----Hotdin backgmund (20: 1)

Planar methods Posterior and anterior planar images were obtained with the object as close as possible to the collimator surface using only camera 1. The narrow beam attenuation coefficient p = 0.18 cm-’ (Turkington et al., 1993) was used for the planar analysis. For the GM method (Thomas et al., 1976), the imbedded single uniform source model was used. The known size and composition of the phantom were used in lieu of a transmission scan to obtain the attenuation factor. In typical GM measurements with 99mTc, a narrow energy window (usually 5%) is used to minimize the effects of scatter. The broad energy window used in this study (20%) and increased Compton scatter contribution for 80 keV photons of “‘At results in the need for additional scatter compensation in these experiments. A scatter window subtraction was applied to the photopeak projection data based on the Jaszczak method for scatter correction in SPECT (Jaszczak et al., 1984). Values of k = 0.5 for posterior images and k = 1.2 for anterior images were determined from a comparative analysis of spinal canal projection data in air and in the waterfilled phantom. For the BF analysis (Wu and Siegel, 1984), buildup factors were measured for the anterior and posterior images using the spinal canal phantom. An iterative solution was not required in this case because the depth of the canal in the posterior and anterior images of the phantom was known. The average of the anterior and posterior values was used to determine the BF activity. Because of the effects of spatial resolution and small canal size, accurate quantitation of intracanal activity requires a region of interest (ROI) much larger than the canal diameter. This large ROI contains a significant background contribution from activity outside the spinal canal. Because both planar methods require net (above background) count rates, total ROT counts must be corrected for this background. The method used to define ROIs and obtain net count rates in this study was based on a simple approximation to the background. An estimated background count density (counts/pixel) near the canal was used as the lower limit threshold to define the canal ROI. The background was assumed to be constant over the canal ROI. The net count rate was determined by subtracting the constant background from the total ROI counts. Phantom data were used to substantiate this approach. Planar profiles of the uniform background phantom were obtained with canal activity present (20: 1) and without canal activity present (0 : 1). The profiles, normalized for decay, are shown in Fig. 2. Although the background, represented by the cold canal profile, is in general non-linear, the shape is approximately flat near the canal position. The constant background

35

45

55 65 Pixel number

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Fig. 2. Posterior planar phantom data showing hot canal-cold canal image profiles with an estimated constant background. assumption should be valid as long as the ROI does not extend too far from the canal location. In practice, cold canal data are not available and the background level must be estimated from the hot canal image. However, it is not obvious from a comparison of the hot and cold canal profiles how best to estimate the background level given only a hot canal profile. For objectivity and simplicity, the background level in planar images was set at the location of the inflection point (i.e. point where the second derivative changes sign) in the shoulder of the profile. Figure 2 illustrates the placement of the constant background in the 2O:l phantom using the inflection point criterion. Figure 3(a) shows a planar posterior image of the 20: 1 phantom while Fig. 3(b) illustrates the ROI obtained from this image using the background level thresholding technique. SPECT methods SPECT projections were acquired at a 12.5 cm radius of rotation with 120 views at a 3 angular sampling interval. SPECT images were reconstructed

Fig. 3. (a) Posterior planar image of 20: 1 phantom anId (b) by applying background lower level

canal ROI obtained

threshold.

E. L. JOHNSON CI (11

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using filtered backprojection with ramp filter. Reconmultiplicative structions incorporated uniform Chang attenuation compensation (Chang, 1978) using /I = 0.18 cm ’ and dual-window Jaszczak scatter subtraction (Jaszczak et al., 1984) with k =0.X. As illustrated in Fig. 4, spatial resolution and the small canal diameter are also complicating factors in SPECT quantitation. In the cold canal profile (0: I). spillover from activity outside the canal into the canal region gives the appearance of a continuous, nonzero background level through the canal. Because these images contain compensation for scatter, this apparent background results mainly from spatial resolution effects. As a result, the hot canal profile (20: 1) can be considered as a separate intracanal activity profile superimposed on a non-zero background, a situation analogous to the planar acquisition. This resemblance to the planar study suggests that the background threshold ROILbackground subtraction approach used for planar imaging may be applicable for SPECT imaging under these conditions. The ROI technique described above for planar imaging has been adopted for use in the SPECT imaging measurements with the following modification: the estimated background level is obtained by finding the average count density in a small ROI contained in the uniform region outside the canal. A constant background estimated in this way is shown in Fig. 4. SPECT images of the 20: 1 phantom and the background threshold ROI are shown in Fig. 5(a) and (b), respectively. In SPECT imaging, the systematic error associated with this approach is reflected by the area that lies between the true background profile and the assumed constant level as illustrated in Fig. 4. The relative magnitude of this error is dependent upon the canal: background ratio. Profiles for the 15: 1 and 100: 1 phantoms shown in Fig. 6 qualitatively demonstrate this dependence. This deficit error is due, at least in part, to the small volume displaced in the uniform background by the spinal canal. If the canal size is known or can be reasonably estimated, an

additive correction to the canal activity can be easily generated that approximates this deficit. The correction is obtained from the product of the background activity density (/~Ci/cm’ ) and the volume of the canal. This correction should result in a significant reduction in magnitude of the systematic error in SPECT quantitation for high background cases.

Results Phantom experiments

25

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Fig. 4. SPECT phantom data showing hot canal-cold canal image profiles with an estimated constant background.

Reconstructed SPECT images and corresponding profiles for the 20: 1 and 100: I phantom configurations are shown in Fig. 7. The images in Fig. 7(a) and (c) are sums of the 33 transaxial slices (11.7 cm thickness) which intersect the spinal canal for the 20: 1 and 100: I phantoms, respectively. Planar posterior images and profiles for the 20: 1 and 100: 1 phantoms are shown in Fig. 8. Results of the activity quantification for each of the three methods evaluated in phantom studies are summarized in Table 1. For canal : background ratios > 50: 1, systematic errors are within 10% for SPECT and BF but are as large as 31% for GM. Errors increase to 23,25 and 45% for SPECT, BF and GM, respectively, for the 15 : 1 phantom configuration. All methods were within 10% for the no background

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Fig. 6. SPECT profiles of (a) 15: 1 phantom and (b) 100: t phantom with constant background approximation. This figure illustrates the effect of background on the magnitude of the systematic error incurred using the constant background assumption. cases including the increased uptake tumor region. Corrected SPECT activities based on the known canal size (3 mm diameter) and estimated background level are given in Table 2. Systematic errors are reduced to < 10% in all cases for the corrected SPECT activities. Also shown in Table 2 are corrected SPECT activities assuming canal diameters of 2 and 4mm. Rut experiments Figure 9 shows posterior planar rat images obtained at 1 min, 1 h and 3 h after intrathecal administration of [2’*At]astatide. The subarachnoid space is clearly defined in the image obtained I min after administration of [“‘Atlastatide. After 1 h, the subarachnoid space is still seen, but contrast is significantly reduced because of the increased background. Two other structures, presumably the thyroid and stomach. are visible in this image. In the image

Fig. 7. Reconstructed SPECT phantom images and profiles. (a) Sum of 33 slices containing the spinal canal, (b) typical profile for the 20: 1 phantom, (c) and (d) spinal canal and profile for 100: I phantom. Fig. 7

Phantom configuration IS I 20 I so:1 1OO:l

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Discussion The: development of radiotherapeutlc agents UC11 as ?I,At-labeled monoclonal antibodies wouldd bc fat :ilit ated by a knowledge of the time-depcr Idlent

Fig. 8. Posterior planar images and profiles. (a) and (b) 20: 1 phantom: (c) and (d) 100: I phantom.

obtained after 3 h, the subarachnoid space is no longer clearly distinguished and activity in the thyroid region has become more obvious. Count densities in the 7 h scan were too low to obtain useful images. SPECT images of the rat at I h post-injection are shown in Fig. IO. A section of the central spine (slice thickness 3.6 cm) is shown in Fig. IO(a) while Fig. IO(b) and (c) each contain segments (slice thickness 1.I cm) of the thyroid and stomach regions, respectively. Relative quantitation of intracanal activity during the 3 h observation period was determined using BF analysis and is summarized in Table 3. The intrathecal residence half-time for astatide in tumor-free rats was estimated at about 1 h. Table 1. Phantom spinal canal activity given as ratio of measured to known activity Method

Phantom configuration 15:l” 20:1t 50:lf 1OO:l’

No background?

of meawrement

Geometric “lea”

Buildup factor

SPECT

1.45 f 0.12 1.30 1.24 1.31 kO.05

1.25io.12 I .23 I .05 I .08 F 0.05 1.01

0.77 f 0.04 0.86 0.90 0.90 +_0.02 1.02

0.97

0.92

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background7 0.96 *Mean f SD (n = 6); tmca” (n = 3)

Fig :. 9. Posterior planar rat images obtained at (a) I mi n, (b) lh ar Id (c) 3 h following intrathecal administratic Bn of [*“At]astatide.

Quantitation of “‘At in small volumes

51

spread malignancies such as neoplastic meningitis, ovarian cancer and cystic gliomas (Roeske and Chen, 1991). Unfortunately, the surface nature of these tumors makes it difficult to isolate malignant tissue for counting at necropsy, making serial imaging of particular value for determining tracer pharmacokinetics. A phantom designed to mimic a rat spinal canal was selected because of its relative simplicity and applicability to determine the ability to quantitatc ‘“At activity levels in small volumes. In addition, we are using rats with neoplastic meningitis to evaluate the therapeutic potential of intrathecally administered 2”At-labeled monoclonal antibodies (Zalutsky et al., 1991) and the results have been highly encouraging. Methods for determining 2”At distribution kinetics in this model thus would be of great value in optimizing this therapeutic approach as well as other approaches involving the use of this nuclide in rodent models. The complex decay scheme of “‘At creates problems for nuclear medicine imaging. The emission of 570, 688 and 898 keV gamma-rays, although they occur in less than 1% abundance, increase background in the projection images due to septal penetration. This problem is analogous to that of imaging I?3I in the presence of 1241impurities (Gilland et al., 1991; King et al., 1986; Macey et al., 1986; Polak et al., 1984; Coleman et al., 1983). Other difficulties that occur in imaging *“At include attenuation of relatively low energy polonium x-rays, proximity of these emissions to lead x-rays and the relatively low activity levels (generally less than 50 PCi) of ‘“At that are typically used in animals. Quantitation

Fig. 10. SPECT rat images obtained 1 hour following intral thecal administration of [2”At]astatide. (a) Central region (slice thickness 3.6 cm), (b) thyroid region (slice t&k ness I .I cm) and (c) stomach region (slice thickness 1.1cm). upta ke of “‘At in tumor and normal tissues. Because of t1ie short range of its a-particles, “‘At is particularly appealing for the treatment of compartmentally Table 3. Estimated spinal canal activity for rat 2 as determined by BF and expressed as a fraction of activity measured at the I min scan time Time of scan (min) I IO 20 60 180

Canal activitv I .oo 0.95 0.74 0.57 0.32

methods

smaller than For objects with dimensions 1.O FWHM of the system resolution, activity quantitation using correction mechanisms which account for imaging system response (i.e. recovery coefficients) is subject to large uncertainty (Hoffman et al., 1979). For quantitation of 2”At activity in rat spinal canals (typically 25% of FWHM resolution), we have circumvented these problems by using a large ROI and adopting a simple background approximation-subtraction approach. The accuracy of this method depends on the canal: background concentration ratio and on how well the assumed constant background approximates the true background profile. The effect of background concentration on the accuracy of this approximation in SPECT imaging is illustrated qualitatively in Fig. 6. The systematic error relative to the total activity is significantly less for the 100: 1 configuration than for the 15: 1 case. The results, given in Table I, illustrate this trend in increasing error and suggest a reasonable limit for method to of this approximate application canal : background ratios > 20 : 1. As shown in Fig. 4, estimating the background level from a ROI in the

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uniform region near the canal appears to introduce a negative bias in the measured net activity. Underestimation of the canal activity is reflected in the SPECT results. However, for canal : background ratios > 50: I, the errors are < 10%. For SPECT imaging however, an assumed canal size can be used to compensate for the negative bias introduced by the constant background approximation. The corrected SPECT activities using the known phantom canal diameter do indeed show improvements compared to the uncorrected values, and systematic errors exhibit much less variation with respect to background concentration, although a 5-10% negative bias remains. The location of the canal near the edge of the phantom and the drop-off of the true background near this edge is the likely source of most of the remaining bias. For the range of background levels tested, the magnitude of the correction was not strongly dependent on deviations from the true canal size. In the I5 : I case, a f 33% change in canal diameter from the known value resulted in only a k 6% change in corrected activity. The differences in corrected values are even less at lower backgrounds. We have found little variation in spinal canal diameter among age-matched rats, and the assumption of an average diameter should not lead to significant errors if an activity correction is used in SPECT. Inclusion of the canal size correction should allow extension of the method to accommodate any canal: background ratio for SPECT. Figure 2 shows differences between the true background shape and the constant approximation are more pronounced for the planar methods than for SPECT. Also apparent in Fig. 2 is the lack of an obvious reference from which an appropriate background level can be estimated. The method of setting the background level at the inflection point was used because it was a reasonable estimate of the background near the canal and it could be applied in an objective manner. Figure 2 also suggests that a positive bias may be introduced in the activity measurement using this choice for background estimation. The phantom results do show a tendency of the planar methods to overestimate the true canal activity. A correction could also be applied in the planar case but it is not clear how the correction should be related to the background level chosen, In low background conditions (2 50: l), errors for BF using this approximate method are within acceptable limits (i 10%). The use of a uniform background, while easy to implement in phantom studies, does not provide realistic experimental conditions existing in live animal experiments. In later scans, extracanal activity more selectively accumulates in particular organs producing a non-uniform background. The actual uptake pattern is determined by the chemical form and stability of the 2”At-1abe1ed compound. At these later times, local backgrounds may exceed the 20: I canal : background ratio limitation of the approxi-

c’t d.

mate method used resulting in considerable incrcasc\ in systematic error in these regions. The results obtained for uniform backgrounds indicate errors observed for SPECT and BF are noi significantly different over the range of background levels tested. Both methods produced accurate MImates of intracanal activities in low to modcratc background concentrations. However, uniform background conditions do not present the most stringent test for these methods. SPECT imaging may offer an advantage in the case of non-uniform background since the tomographic nature of SPECT imaging allows better separation of adjacent structures than is possible with planar. Other advantages of SPECT imaging include facilitated implementation of the background threshold ROI technique used and application to a wider range of background conditions when using the canal size correction. SPECT imaging does require longer scan times (3 30 min). and it is difficult to obtain accurate body contours for attcnuation compensation, especially in these low count conditions. In planar imaging, the superposition of activity from overlying tissues is more difficult to correct and problems with obtaining good background level estimates have been discussed. Planar imaging does offer the advantage of fast scans (225 min), and it may be possible to minimize the effects of background interference by acquiring left-right views instead of posterior- anterior conjugates. Rat imaging Two rats were imaged following direct administration of [2”At]astatide through an indwelling catheter into the cerebrospinal fluid. The rat differs from our phantom in at least two respects. First, the cerebrospinal fluid in the rat surrounds the spinal cord, a structure not expected to contain radioactivity. Thus, unlike the phantom, intrathecally administered activity is actually annular. Second, the anatomy of the spinal column is irregular, particularly with regard to bony structures. Early images in these animals indicated distribution of “‘At in a linear region whose length, about 10 cm, was comparable to the length of spinal canals determined at necropsy in age-matched animals. Activity cleared from this region with a half-time of about 1 h. Very little is known about the pharmacokinetics of compounds administered into the cerebrospinal fluid of rats. which is not unexpected considering the methodological difficulties involved in serial cerebrospinal fluid sampling. A number of studies have been performed in the dog using smaller molecular weight chemotherapeutic agents administered intrathecally. The clearance half-times reported range from 30 to 155 min (Levin et al.. 1983, 1984, 1985). The relevance of these studies to the clearance of a halide from the intrathecal compartment of the rat is certainly limited; nonetheless, it is worth noting that they are at least in qualitative agreement.

Quantitation of 2”At in small volumes Later images indicate intense and focal uptake of *“At in the neck. Astatide is known to be taken avidly in the thyroid (Garg et al., 1990), although to a lesser extent than iodide and it is presumed that thyroid accumulation is responsible for this focus of activity. More diffuse accumulation of “‘At was observed in the abdomen, particularly in the 3-h images. We speculate that this may be related to stomach uptake based on previous studies in mice injected intravenously with [“‘Atlastatide that indicated high levels of activity in the stomach (Garg et ul., 1990). We have demonstrated that reasonable quantitation accuracy of intracanal 21’At distributions is possible using standard gamma camera planar or SPECT imaging methods. The background threshold ROI-background subtraction approach used in this study has produced satisfactory results in phantoms within the constraints imposed by the small canal size and imaging requirements for *“At. We have also presented images of subarachnoid distributions of [‘“Atlastatide in rats. These results suggest that it may be possible to use external imaging techniques to quantitate ‘“At distributions in rats given intrathecal doses of *“At-labeled radiotherapeutic agents. Studies using I23I-labeled antibody fragments indicate a half-time for cerebrospinal fluid clearance of about 10 h (Zalutsky et al., 1994). If similar pharmacokinetits are obtained with *“At, then relatively high targetto-background ratios should remain for many hours after administration and yield a reasonable degree of quantitative accuracy. Using these methods, we hope to investigate variables such as antibody dose, molecular size and tumor burden on the pharmacokinetits of “‘At-labeled antibodies. Hopefully, these imaging approaches will be applicable to the preclinical evaluation of other 2”At-labeled compounds of potential applicability as radiotherapeutic agents. publication was supported by grant DE89ER60894 awarded by the Department of Energy, and in part by grants CA33541 and CA42324 awarded by the National Cancer Institute and NS20023 awarded by the National Institute of Neurologic Diseases and Stroke. Its contents are the sole responsibility of the authors and do not necessarily reflect the official views of the Department of Energy, the National Cancer Institute or the National Institute of Neurologic Diseases and Stroke. The assistance of Dr Gary Archer, Department of Pathology, Duke University Medical Center, with the rat experiments is greatly appreciated. Acknowledgements-This

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of intrathecal 4-hydroperoxy-cyclophosphamide in a nude rat model of human neoplastic meningitis. Cancer Res. 50, 1954. Garg P. K., Harrison C. L. and Zalutsky M. R. (1990) Comparative tissue distribution of the alpha emitter “‘At and “‘I as labels of a monoclonal antibody and F(ab’), fragment. Cancer Res. 50, 3514. Gilland D. R.. Jaszczak R. J.. Greer K. L. and Coleman R. E. (1991) Quantitative SPECT reconstruction of I- I23 data. J. Nucl. Med. 32, 527. Hall E. J. (1988) Radiobiology for the Radiologist, 3rd edn. Lippincott, Philadelphia. Hoffman E. J., Huang S. C. and Phelps M. E. (1979) Quantitation in positron emission computed tomography: 1. effect of object size. J. Comput. Assisf. Tomogr. 3, 299. Jaszczak R. J., Greer K. L., Floyd C. E., Harris C. C. and Coleman R. E. (1984) Improved SPECT quantification using compensation for scattered photons. J. NW/. Med. 25, 893. Kassis A. I., Harris C. R., Adelstein S. J., Ruth T. J., Lambrecht R. and Wolf A. P. (1986) The in vitro radiobiology of astatine-21 I decay. Radiut. Res. 105, 27. Kozak R. W., Atcher R. W., Gansow 0. A., Friedman A. M., Hines J. J. and Waldmann T. A. (1986) Bismuth-212-labeled anti-Tat monoclonal antibody: a-particle-emitting radionuclides as modalities for radioimmunotherapy. Proc. Natn. Acad. Sri. U.S.A. 83, 414. Lambrecht R. M. and Mirzadeh S. (1985) Cyclotron isotopes and radiopharmaceuticals-XXXV. Astatine-211. Int. J. Appl. Radiat. Isot. 36, 443.

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