Imaging, dosimetry, and radioimmunotherapy with iodine 131-labeled anti-CD37 antibody in B-cell lymphoma.

PURPOSE: This study was undertaken to evaluate the tumor targeting, toxicity, and therapeutic potential of the anti-B-cell-reactive monoclonal antibody MB-1 (anti-CD37) labeled with iodine 131 given in a nonmarrow ablative dose range in B-cell lymphoma patients who relapsed after chemotherapy. PATIENTS AND METHODS: Twelve patients with MB-1-reactive tumors were infused first with 40 mg of trace-labeled (3 to 7 mCi) MB-1. Ten patients who had no serious toxicity postinfusion and who had successful tumor imaging on serial gamma scans then received at least one 40-mg radioimmunotherapy (RIT) dose (25 to 161 mCi). Tracer estimates of delivered whole-body dose (WBD) were used in prescribing a millicurie RIT dose for seven patients. RESULTS: Eleven patients had positive tumor imaging after a tracer dose, including patients with bulky tumors and/or large tumor burdens (> or = 1 kg) +/- splenomegaly. However, overall sensitivity for the detection of known tumor sites was only 39%. In six of eight patients with dose-assessable tumors, the radiation dose to at least one tumor was 1.1 to 3.1 times higher than to any normal organ, excluding the spleen for a 40-mg tracer dose. Tracer-dose toxicities included reversible glossal edema in one patient, grade 3 hepatic transaminasemia in another, and early drops in both circulating B and T cells (with decreases in B cells more pronounced) in nearly all patients. RIT toxicity was primarily myelosuppression (especially thrombocytopenia), which had a delayed onset and protracted recovery (without significant recovery until at least 2 months post-RIT). Grade 3 myelosuppression in two of two patients who were treated at a tracer-projected 50-cGy WBD level (133 and 149 mCi) precluded further planned RIT dose escalation. Less myelosuppression was generally observed in patients who were treated at < or = 40-cGy WBD levels. Antimouse antibodies developed in two patients. Six patients had tumor responses post-RIT. Four had responses that lasted more than 1 month (2 to 6 months), which included one complete response, one partial response, one minor response, and one mixed response. Responses seemed to occur more frequently in imaged tumors than in nonimaged tumors. The most durable response occurred in a patient who had the best antibody targeting to tumor. CONCLUSIONS: Although 131I-MB-1 has limited diagnostic value, it can produce tumor responses at nonmarrow ablative RIT doses. Further studies that focus on improving tumor targeting with this or other B-cell-reactive radiolabeled antibodies and on ameliorating the myelosuppression associated with the RIT-dosing approach used in this trial are warranted.

I-131 anti-B1 therapy/tracer uptake ratio using a new procedure for fusion of tracer images to computed tomography images.

In patients with non-Hodgkin's lymphoma being treated by I-131-radiolabeled anti-B1 monoclonal antibody, we test the hypothesis that the activity taken up in tumors during therapy is the same as that observed during tracer evaluation, except for scaling by the ratio of administered activities. Chemotherapy-relapsed patients are imaged only with planar conjugate views, whereas previously untreated patients are imaged with planar conjugate views and with single-photon emission computed tomography (SPECT). The SPECT tracer activity quantification requires computed tomography (CT) to SPECT image fusion, for which we devised a new procedure: first, the tracer SPECT images are fused to the therapy SPECT images. Then, that transformation is combined with the therapy SPECT-to-CT transformation. We also use (a) the same volumes of interest defined on CT for both tracer and therapy image sets, and (b) a SPECT counts-to-activity conversion factor that adapts to background and rotation radius. We define R as the ratio of therapy activity percentage of infused dose over tracer activity percentage of infused dose at 2-3 days after monoclonal antibody infusion. For 31 chemotherapy-relapsed patients, the R ratio for 60 solitary or composite tumors averages 0.931 +/- 0.031. The hypothesis of R being 1 is rejected with greater than 95% confidence. However, the difference from 1 is only 7.4%. The range of R is 0.43-1.55. For seven previously untreated patients, R averages 1.050 +/- 0.050 for 24 solitary tumors evaluated by SPECT. For six of these patients, R averages 0.946 +/- 0.098 for one of these solitary tumors and for five composite tumors, evaluated by conjugate views. Both results agree with the hypothesis that R is 1. The range of R for the SPECT tumors is 0.71 +/- 0.03 to 1.82 +/- 0.53, and for the conjugate view tumors, it is 0.70-1.35. Plots of R versus tumor volume yield small correlation coefficients. That from SPECT approaches a statistically significant difference from zero correlation (P = 0.06). In summary, on average, the tumor percentage of infused dose following tracer administration is predictive of therapeutic percentage of infused dose within 8%. For greater accuracy with individual tumors, however, an intratherapy evaluation is probably necessary because the range of R is large.

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

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

An operator-independent method for background subtraction in adrenal-uptake measurements: concise communication.

A new computer program for adrenal-uptake measurements is presented in which the algorithm identifies the adrenal and background regions automatically after being given a starting point in the image. Adrenal uptakes and results of reproducibility tests are given for patients injected with [131I] 6beta-iodomethyl-19-norcholesterol. The data to date indicate no overlap in the percent-of-dose uptakes for normal patients and patients with Cushing's disease and Cushing's syndrome.

Characterization of scatter and penetration using Monte Carlo simulation in 131I imaging.

UNLABELLED: In 131I SPECT, image quality and quantification accuracy are degraded by object scatter as well as scatter and penetration in the collimator. The characterization of energy and spatial distributions of scatter and penetration performed in this study by Monte Carlo simulation will be useful for the development and evaluation of techniques that compensate for such events in 131I imaging. METHODS: First, to test the accuracy of the Monte Carlo model, simulated and measured data were compared for both a point source and a phantom. Next, simulations to investigate scatter and penetration were performed for four geometries: point source in air, point source in a water-filled cylinder, hot sphere in a cylinder filled with nonradioactive water, and hot sphere in a cylinder filled with radioactive water. Energy spectra were separated according to order of scatter, type of interaction, and gamma-ray emission energy. A preliminary evaluation of the triple-energy window (TEW) scatter correction method was performed. RESULTS: The accuracy of the Monte Carlo model was verified by the good agreement between measured and simulated energy spectra and radial point spread functions. For a point source in air, simulations show that 73% of events in the photopeak window had either scattered in or penetrated the collimator, indicating the significance of collimator interactions. For a point source in a water-filled phantom, the separated energy spectra showed that a 20% photopeak window can be used to eliminate events that scatter more than two times in the phantom. For the hot sphere phantoms, it was shown that in the photopeak region the spectrum shape of penetration events is very similar to that of primary (no scatter and no penetration) events. For the hot sphere regions of interest, the percentage difference between true scatter counts and the TEW estimate of scatter counts was <12%. CONCLUSION: In 131I SPECT, object scatter as well as collimator scatter and penetration are significant. The TEW method provides a reasonable correction for scatter, but the similarity between the 364-keV primary and penetration energy spectra makes it difficult to compensate for these penetration events using techniques that are based on spectral analysis.

Accuracy of 131I tumor quantification in radioimmunotherapy using SPECT imaging with an ultra-high-energy collimator: Monte Carlo study.

UNLABELLED: Accuracy of 131I tumor quantification after radioimmunotherapy (RIT) was investigated for SPECT imaging with an ultra-high-energy (UHE) collimator designed for imaging 511-keV photons. METHODS: First, measurements and Monte Carlo simulations were carried out to compare the UHE collimator with a conventionally used, high-energy collimator. On the basis of this comparison, the UHE collimator was selected for this investigation, which was carried out by simulation of spherical tumors in a phantom. Reconstruction was by an expectation-maximization algorithm that included scatter and attenuation correction. Keeping the tumor activity constant, simulations were carried out to assess how volume-of-interest (VOI) counts vary with background activity, radius of rotation (ROR), tumor location, and size. The constant calibration factor for quantification was determined from VOI counts corresponding to a 3.63-cm-radius sphere of known activity. Tight VOIs corresponding to the physical size of the spheres or tumors were used. RESULTS: Use of the UHE collimator resulted in a large reduction in 131I penetration, which is especially significant in RIT where background uptake is high. With the UHE collimator, typical patient images showed an improvement in contrast. Considering the desired geometric events, sensitivity was reduced, but only by a factor of 1.6. Simulation results for a 3.63-cm-radius tumor showed that VOI counts vary with background, location, and ROR by less than 3.2%, 3%, and 5.3%, respectively. The variation with tumor size was more significant and was a function of the background. Good quantification accuracy (<6.5% error) was achieved when tumor size was the same as the sphere size used in the calibration, irrespective of the other parameters. For smaller tumors, activities were underestimated by up to -15% for the 2.88-cm-radius sphere, -23% for the 2.29-cm-radius sphere, and -47% for the 1.68-cm-radius sphere. CONCLUSION: Reasonable accuracy can be achieved for VOI quantification of 131I using SPECT with an UHE collimator and a constant calibration factor. Difference in tumor size relative to the size of the calibration sphere had the biggest effect on accuracy, and recovery coefficients are needed to improve quantification of small tumors.

A measure of Anger-camera linearity: results with and without a corrector.

A method for measuring the X and Y linearity of an Anger camera coupled to a computer is presented. It has similarities to, and differences from, the method recommended by the National Electrical Manufactures Association (NEMA). Test images are taken through a lead plate with parallel and equally spaced slots, and the locations of the lines in the images are fitted by least-squares with an equation that allows for slight misalignment. Discrepancies from the fit are calculated and displayed as a distribution over the camera field. The maximum and average discrepancies are tabulated. The field of view that is of interest is selectable within the analysis program. Among four large-field-of-view uncorrected cameras, three 37-tube types (two measured using Tc-99m and one with Au-195) show a similar degree of nonlinearity. However, the maximum and average discrepancies from linearity for a 61-tube prototype camera, measured using Tc-99m over a 15-in. field of view, are 40% of those for the other three. For the four cameras, an event-shifting on-line corrector with best-case sampling improves linearity by an average factor of 5.5, including both 15-and 11.25-in. fields of view.

Digital tomographic imaging with time-modulated pseudorandom coded aperture and Anger camera.

The properties of a time-modulated pseudorandom coded aperture with digital reconstruction are compared with those of conventional collimators used in gamma-ray imaging. The theory of this coded aperture is given and the signal-to-noise ratio in an element of the reconstructed image is shown to depend on the entire source distribution. Experimental results with a preliminary 4 X 4-cm pseudorandom coded aperture and an Anger camera are presented. These results include phantom and human thyroid images and tomographic images of a rat bone scan. The experimental realization of the theoretical advantages of the time-modulated coded aperture gives reason for continuing the clinical implementation and further development of the method.

Thyroid scintigraphy with time-coded aperture.

Coded aperture imaging (CAI) and multiple-view pinhole imaging (MVPI) of the thyroid were compared in 19 patients to determine whether CAI's theoretical advantages of high resolution, high efficiency, freedom from distortion, accurate size representation, and tomographic presentation could be realized in the clinical setting, and to determine whether CAI offers any advantage over conventional MVPI. The coded aperture images were judged better than the pinhole images in five cases, equal in 13 cases, and worse in one case. The major problem with CAI was the long reconstruction time. Further development and an extended clinical trial appear warranted.

Iodine-131 treatment of thyroid cancer: absorbed dose calculated from post-therapy scans.

The radiation absorbed dose for nine neck lesions distributed among four thyroid-cancer patients was measured directly from images taken after administration of a treatment dose of 131I. The tumor volume was measured with anterior plus lateral pinhole images by determining magnification and assuming an ellipsoidal shape. Uptake and effective half-life were determined from serial anterior images by use of a calibration curve. Dose lower limits ranged from 2,400 to 29,900 rad. Response to treatment was judged on the basis of one or more follow-up scans at least 8 mo later. All lesions responded to the therapy administration which ranged from 150 to 175 mCi.

Field-flood requirements for emission computed tomography with an Anger camera.

Emission computed tomography with a rotating camera places stringent requirements on camera uniformity and the stability of camera response. In terms of clinical tomographic imaging, we have studied the statistical accuracy required for camera flood correction, the requirements for flood accuracy, the utility and validity of flood and data image smoothing to reduce random noise effects, and the magnitude and effect of camera variations as a function of angular position, energy window, and tuning. Uniformity of the corrected flood response must be held to better than 1% to eliminate image artifacts that are apparent in a million-count image of a liver slice. This requires calibration with an accurate, well-mixed flood source. Both random fluctuations and variations in camera response with rotation must be kept below 1%. To meet the statistical limit, one requires at least 30 million counts for the flood-correction image. Smoothing the flood image alone introduces unacceptable image artifacts. Smoothing both the flood image and data, however, appears to be a good approach toward reducing noise effects. Careful camera tuning and magnetic shield design provide camera stability suitable for present clinical applications.

SPECT Compton-scattering correction by analysis of energy spectra.

The hypothesis that energy spectra at individual spatial locations in single photon emission computed tomographic projection images can be analyzed to separate the Compton-scattered component from the unscattered component is tested indirectly. An axially symmetric phantom consisting of a cylinder with a sphere is imaged with either the cylinder or the sphere containing 99mTc. An iterative peak-erosion algorithm and a fitting algorithm are given and employed to analyze the acquired spectra. Adequate separation into an unscattered component and a Compton-scattered component is judged on the basis of filtered-backprojection reconstruction of corrected projections. In the reconstructions, attenuation correction is based on the known geometry and the total attenuation cross section for water. An independent test of the accuracy of separation is not made. For both algorithms, reconstructed slices for the cold-sphere, hot-surround phantom have the correct shape as confirmed by simulation results that take into account the measured dependence of system resolution on depth. For the inverse phantom, a hot sphere in a cold surround, quantitative results with the fitting algorithm are accurate but with a particular number of iterations of the erosion algorithm are less good. (A greater number of iterations would improve the 26% error with the algorithm, however.) These preliminary results encourage us to believe that a method for correcting for Compton-scattering in a wide variety of objects can be found, thus helping to achieve quantitative SPECT.

Conjugate view gamma camera method for estimating tumor uptake of iodine-131 metaiodobenzylguanidine.

Therapy with [131I]MIBG has produced partial remissions of malignant pheochromocytomas but not all patients respond. Responses correlate with the quantity of radiation delivered. We developed the conjugate-view method of imaging using 131I reference sources of known radioactivity placed on the surface of the patient and standard nuclear medicine equipment (gamma camera and computer), to estimate tumor uptake of [131I]MIBG. Such an estimate is a first step toward calculating radiation absorbed dose. Three different methods of background subtraction were evaluated with an anthropomorphic phantom and in five patients. In phantom results, measured tumor activity decreased exponentially with a half-life in agreement with that of 131I to within 3%. However, in the phantom studies, in which non-tumor activity is zero, no single method of background subtraction is superior. In patients, two background subtraction methods, which take their estimate from regions immediately surrounding or adjacent to the tumor and reference source, are less sensitive to reference source position and appear more accurate than a third method which uses a background region of interest displaced from the tumor. The agreement of the calculated activity concentration (nCi/g) with that measured by counting portions of the excised tumors gives validation to the method.

Simulated ECT of the left ventricle using rotating slant-hole collimator and two camera positions.

Limited-angular-range tomography leads to an elongating distortion of the object in the direction of the z axis (perpendicular to the camera face). Two-view tomography appends to the usual data set another set of projections taken after the camera is rotated 90 degrees about an axis perpendicular to z. We investigated two-view tomography using a rotating-slant-hole collimator, 12 projections per view and the SMART iterative algorithm. Computer simulations extended previous results to include noise and attenuation. Phantoms imaged were the Au-rings in air and a ventricle phantom angled with respect to the z axis and placed in a water bath. Two-view results were generally superior compared to one-view results, were subject to some artifact in imaging defects, but could detect defects by looking at the differences between two sets of images, and were fairly insensitive to ventricle angulation. Therefore, two-view tomography has promise for thallium-type imaging.

CT-SPECT fusion plus conjugate views for determining dosimetry in iodine-131-monoclonal antibody therapy of lymphoma patients.

UNLABELLED: A method for performing 131I quantitative SPECT imaging is described which uses the superimposition of markers placed on the skin to accomplish fusion of computed tomography (CT) and SPECT image sets. METHODS: To calculate mean absorbed dose after administration of one of two 131I-labeled monoclonal antibodies (Mabs), the shape of the time-activity curve is measured by daily diagnostic conjugate views, the y-axis of that curve is normalized by a quantitative SPECT measurement (usually intra-therapy), and the tumor mass is deduced from a concurrent CT volume measurement. The method is applied to six B-cell non-Hodgkin's lymphoma patients. RESULTS: For four tumors in three patients treated with the MB1 Mab, a correlation appears to be present between resulting mean absorbed dose and disease response. Including all dosimetric estimates for both antibodies, the range for the specific absorbed dose is within that found by others in treating B-cell lymphoma patients. Excluding a retreated anti-B1 patient, the tumor-specific absorbed dose during anti-B1 therapy is from 1.4 to 1.7 mGy/MBq. For the one anti-B1 patient, where quantitative SPECT and conjugate-view imaging was carried out back to back, the quantitative SPECT-measured activity was somewhat less for the spleen and much less for the tumor than that from conjugate views. CONCLUSION: The quantitative SPECT plus conjugate views method may be of general utility for macro-dosimetry of 131I therapies.

Coded-aperture imaging of the heart.

Coded-aperture imaging of the heart combines the advantages of tomography with good sensitivity, high resolution, and accurate size scaling. Since the images are multiplexed, the method may be adapted to small, portable cameras for bed-side use without sacrificing image resolution. A new coded aperture designed especially for cardiac imaging has been constructed and tested. This aperture incorporates significant improvements over previous designs. Longitudinal tomograms are calculated at 1-cm intervals using a modified ART algorithm. Experimental lateral resulution at 140 keV with a portable scintillation camera is 3.8 mm FWHM at 4 cm, and 7.8 mm FWHM at 12 cm. Dpth resolution determined from a sloping line source is 1.1 cm FWHM at 4 cm, and 2.9 cm at 12 cm. The calculated point-source sensitivities in air at 4 cm and 12 cm, respectively, are 20 and 8 cps/microCi. Images of good diagnostic quality have been obtained in phantoms and in a dog model of acute myocardial infarction, using thallium-201, technetium-99m pyrophosphate, and gated ventricular blood-pool imaging with Tc-labeled red blood cells. Preliminary studies in humans confirm the good results in animals.

SPECT dual-energy-window Compton correction: scatter multiplier required for quantification.

The dual-energy window Compton-scattering correction technique is defined here especially for accurate quantification of focal regions having higher than average uptake. The quantification is relative to a known-activity reference source. The scatter multiplier ("k" value) is determined for a radioactive 99mTc sphere on or off the axis of a cylinder containing water with or without background. Both maximum likelihood and filtered-backprojection reconstruction are employed. Either projections or tomograms are corrected. With tight regions of interest, there is a tendency for the requisite "k" value to be slightly lower as the diameter of the cylinder is increased. Neither sphere location nor background perturbs "k", however, so a constant value is a good, first approximation. Then a two-sphere validation test yields an accuracy of 8% with subtracted-tomograms ("k" = 1.30) and 2% with subtracted-projections ("k" = 1.20). With a reference-source region of interest which is four times larger, "k" is reduced and also now depends on background. Although equivalent quantitatively, maximum likelihood is preferable to filtered backprojection with Chang attenuation correction since it produces a less-noisy image.

Performance evaluation of SPRINT, a single photon ring tomography for brain imaging.

SPRINT, a prototype single photon tomograph, has been designed primarily for high-resolution brain imaging in humans with I-123-labeled compounds such as iodoamphetamine, hydroxyiodopropyldiamine (HIPDM), and iodobenzene (IBZ). SPRINT uses a ring of stationary, discrete Nal detectors, and fan-beam sampling is accomplished with a rotating eight-slit aperture ring that acquires a complete projection set in 1/8 revolution. In-plane and cross-plane resolutions are 8mm and 10mm FWHM, respectively, measured on axis. Sensitivity with an 18% energy window is 1000 cprs per microCi/cc for Tc-99m in a 20 cm diameter phantom. A detailed evaluation of system performance has been completed, and preliminary human brain blood flow images have been obtained using HIPDM.

Tomographic thyroid scintigraphy: comparison with standard pinhole imaging: concise communication.

Coded-aperture imaging (CAI) and multiple-view pinhole imaging (PI) of the thyroid were compared in a prospective study in 136 consecutive patients. Following 10 mCi of pertechnetate, 200K-count pinhole images were obtained in the anterior, RAO, and LAO projections, and CAI data were obtained in the anterior position. Four coronal tomographic sections were reconstructed by computer. Five observers read the studies separately, and ROC curves were constructed. Based on 109 pairs of studies, the ROC curves revealed similar performance for all observers for both techniques. When four observers compared the studies subjectively they rated the CAI more useful in 36% of cases, the PI in 6%, and the two equal in 58%. The advantages offered by the tomograms included improved contrast, accurate size representation of the gland at all depths, freedom frm pinhole-type distortion, and faster data acquisition. The major disadvantage to tomography was the 2-hr computer-processing time required. It this can be reduced, CAI offers sufficient advantages over conventional pinhole imaging to warrant its routine use.

Initial results for Hybrid SPECT--conjugate-view tumor dosimetry in 131I-anti-B1 antibody therapy of previously untreated patients with lymphoma.

UNLABELLED: A study of the use of 131I-labeled anti-B1 monoclonal antibody, proceeded by an unlabeled predose, for therapy of previously untreated non-Hodgkin's lymphoma patients has recently been completed at the University of Michigan, Ann Arbor. More than half of the patients treated were imaged intratherapy with SPECT to separate apparently large tumors, unresolved by conjugate views, into individual ones specified by CT scan. The dosimetry of these tumors is reported here. METHODS: The activity-quantification procedure used 3-dimensional CT-to-SPECT fusion so that attenuation maps could be computed from CT and that volumes of interest could be drawn on the CT slices and transferred to the SPECT images. Daily conjugate-view images after a tracer dose of labeled anti-B1 antibody followed by an unlabeled predose provided the shape of the time-activity curve for the calculation of therapy dosimetry. Reconstructed SPECT counts that were within a volume of interest were converted to activity by using a background-and-radius-adaptive conversion factor. Activities were increased for tumors less than 200 g using a recovery-coefficient factor derived from activity measurements for a set of spheres with volumes ranging from 1.6 to 200 cm3. The calculated tumor radiation absorbed dose was based, in part, on the CT volume and on the intratherapy-SPECT activity. RESULTS: The mean of the radiation dose values for 131 abdominal or pelvic tumors in 31 patients was 616 cGy with a standard deviation of +/- 50 cGy. The largest dose was 40 Gy and the smallest dose was 73 cGy. The mean volume for the tumors was 59.2 +/- 11.2 cm3. The correlation coefficient between absorbed dose and tumor volume was small (r2 = 0.007), and the slope of the least-squares fit represented a decrease of only 36.4 cGy per 100 cm3 increase in volume. This small slope may reflect a characteristic of anti-B1 antibody therapy that is important for its success. The mean absorbed dose per unit administered activity was 1.83 +/- 0.145 Gy/GBq. The largest value was 12.6 Gy/GBq, and the smallest value was 0.149 Gy/GBq. The mean dose for 9 axillary tumors in 5 patients was significantly lower than the average dose for abdominal and pelvic tumors (P = 0.01). Therefore, axillary tumors should be grouped separately in assessing dose-response relationships. Anecdotal patient results tended to verify the validity of using the shape of the conjugate-view time-activity curve for the average SPECT-intratherapy curve. However, there was also an indication that the shape varies somewhat for individual tumors with respect to time to peak. CONCLUSION: Hybrid SPECT-conjugate-view dosimetry provided radiation absorbed dose estimates for the individual patient tumors that were resolved by CT.