Conjugate View Method Research Articles

Comparison of Accuracy in Calculation of Absorbed Dose to the Kidneys Following Radioligand Therapy with 177Lu-DKFZ-PSMA-617 by Two Different Background Correction Methods.

Background:To improve the accuracy of activity image quality, scatter correction is a critical method. The aim of this study is to compare the accuracy in calculation of absorbed dose to patients following radioligand therapy (RLT) with 177Lu-DKFZ-PSMA-617 by two different methods of background correction in the conjugate view method.Materials and Methods:This study involved 10 patients. The individualized patient dosimetry calculations were based on whole-body planar scintigraphy images acquired in 10 patients with a mean age of 71.4 ± 6.07 years (range 63–85 years) at approximately 0–2 h, 4–6 h, 18–24 h, and 36–48 h after administration of the mean 6253 ± 826.4 MBq (range 5500–7400 MBq) of 177Lu-DKFZ-PSMA-617. Organ activities were calculated using the conjugate view method by Buijs and conventional background correction. Eventually, the absorbed dose of radiation was calculated using Medical Internal Radiation Dose formalism.Results:The dose per unit of injected activity (mGy/MBq) ± standard deviation for kidney using Buijs and conventional methods was 1.05 ± 0.11 and 0.63 ± 0.14, respectively.Conclusion:The Buijs background correction method was more accurate than the conventional method.

PO-1600 Quasi invariance of a point source image response in the conjugate view method with depth

PO-1600 Quasi invariance of a point source image response in the conjugate view method with depth

Assessment of Organ Doses for Patients undergoing 99mTc-MIBI Myocardial Perfusion Imaging.

Radiation absorbed doses associated with myocardial perfusion imaging (MPI) using 99mTc-MIBI has been determined at the Radioisotopes Institute of the Abdou Moumouni University. Thirty patients undergoing MPI were scanned and image quantification was done using Mediso Inter View XP® software. The average administrated activities of 99mTc-Sestamibi were 370 MBq (10 mCi) in stress condition and 1110 MBq (30 mCi) in rest condition with one day protocol. The radionuclide activities in the heart, liver, kidneys and urinary bladder were determined using the conjugate view method. The uptake of 99mTc-MIBI in the heart, liver and kidneys were respectively about 2.17% ±0.64, 6.53% ±1.47 and 5% ±0.52, 10 minutes after injection. The cumulative activities for the heart, liver, kidneys and urinary bladder were respectively 21.97 MBq.h, 63.11 MBq.h, 36.61 MBq.h, 79.48 MBq.h for the stress and 68.23 MBq.h, 239.79 MBq.h, 101.69 MBq.h and 223.1 MBq.hin the rest conditions. The organs absorbed doses in this study were 3.06 mGy for kidneys, 0.75 mGy for liver, 0.75 mGy for the heart, and 20.46mGy for bladder.

Short-term retention of 99mTc activity in bone scintigraphy

The objective of present study is estimation of the radioactivity held within the urinary bladder and kidneys 3 h post radiopharmaceutical (99mTc-MDP) administration, comparison being made via use of MIRDose. Using the conjugate-view method and built-in camera software Mediso XP Interview, urinary bladder and kidney radionuclide activities were calculated using patient clinical data acquired at the aforesaid 3 h post 99mTc-MDP injection juncture. The study was of a cohort of 40 patients attending for staging and cancer follow-up at the Nuclear Medicine Department, Royal Care International Hospital (RCIH), Sudan, Ethical and Research Committee approval for the study having been obtained. The radioisotope biokinetics were simulated using the Medical Internal Radiation Dose (MIRD) biokinetics model, specifically in regard to radionuclide activity in the kidneys and bladder. The results were compared with the data achieved via clinical study, showing the experimentally estimated radionuclide activity in the kidney to be greater by some 50% compared to that of simulation while the bladder radionuclide activity was found to be just 2% of the simulation result. The study also found that 0.9% of the administered radioactivity remained in the urinary bladder and kidneys 3 h post administration of the radionuclide, suggestive of low radiation dose to the urinary bladder and kidneys post 99mTc-MDP bone scans.

Extension of the International Atomic Energy Agency phantom study in image quantification: results of multicentre evaluation in Croatia.

To make quantitative methods of nuclear medicine more available, four centres in Croatia participated in the national intercomparison study, following the materials and methods used in the previous international study organized by the International Atomic Energy Agency (IAEA). The study task was to calculate the activities of four Ba sources (T 1/2=10.54 years; E γ=356 keV) using planar and single-photon emission computed tomography (SPECT) or SPECT/CT acquisitions of the sources inside a water-filled cylindrical phantom. The sources were previously calibrated by the US National Institute of Standards and Technology. Triple-energy window was utilized for scatter correction. Planar studies were corrected for attenuation correction (AC) using the conjugate-view method. For SPECT/CT studies, data from X-ray computed tomography were used for attenuation correction (CT-AC), whereas for SPECT-only acquisition, the Chang-AC method was applied. Using the lessons learned from the IAEA study, data were acquired according to the harmonized data acquisition protocol, and the acquired images were then processed using centralized data analysis. The accuracy of the activity quantification was evaluated as the ratio R between the calculated activity and the value obtained from National Institute of Standards and Technology. For planar studies, R=1.06±0.08; for SPECT/CT study using CT-AC, R=1.00±0.08; and for Chang-AC, R=0.89±0.12. The results are in accordance with those obtained within the larger IAEA study and confirm that SPECT/CT method is the most appropriate for accurate activity quantification.

Determination of Uterus Absorbed Dose by Patients following Myocardial Perfusion Scan using TLD and Conjugate View Methods.

Introduction:The determination of patient’s absorbed dose is the first step of radiation protection which depends on the quantification of organ activity in nuclear medicine. The aim of the present study was to determine the absorbed dose by patient’s uterus following myocardial perfusion scan with 99mTC-sestamibi using Theroluminescnce dosimetry (TLD) and conjugate-view methods. Material and Method: In this study, each patient was injected 15 to 20 mCi (based on their weight) of 99mTC-sestamibi. Myocardial perfusion scan from twenty two patients (females) were acquired by gamma camera at 15, 60 and 90 minutes after 99mTC-sestamibi injection. To determine the amount of activity in uterus, conjugate view method was applied on images. Then, MIRD equation was used to estimate absorbed dose in uterus of patients. Moreover, uterus absorbed dose was determined using TLD method. At the end, absorbed dose values obtained in conjugate view method were compared with the data obtained from TLD method. Results: The average amount of uptake for 99mTC-sestamibi by heart was calculated 3.077±0.067 percent of injected dose. The uterus activity at the intervals of 15, 60 and 90 minutes after injection of 99mTC-sestamibi was 0.044±0.015, 0.031±0.014 and 0.026±0.013 mCi, repectively. The uterus absorbed dose per unit of injected activity (mGy/MBq × 10−4) obtained 5.258±0.500 using TLD method. Conclusion: The results of this study were in good agreement with similar studies. Dosimetry using TLD, in comparison with the conjugate view method, demonstrates more accurate results.

A novel planar image-based method for bone marrow dosimetry in (177)Lu-DOTATATE treatment correlates with haematological toxicity.

Background177Lu-DOTATATE is a valuable treatment option for patients with advanced neuroendocrine tumours overexpressing somatostatin receptors. Though well tolerated in general, bone marrow toxicity can, besides renal exposure, become dose limiting and affect the ability to sustain future therapies. The aim of this study was to develop a novel planar image-based method for bone marrow dosimetry and evaluate its correlation with haematological toxicity during 177Lu-DOTATATE treatment. In this study, 46 patients with advanced neuroendocrine tumours were treated with 7.2 GBq (3.5–8.3 GBq) of 177Lu-DOTATATE on two to five occasions. Planar gamma camera images were acquired at 2, 24, 48 and 168 h post-injection. Whole-body regions of interest were created in the images, and a threshold-based segmentation algorithm was applied to separate the uptake of 177Lu-DOTATATE into high and low uptake compartments. The conjugate view method was used to quantify the activity, the accumulated activity was calculated and the absorbed dose to the bone marrow was estimated according to the MIRD scheme. Patients were monitored for haematological toxicity based on haemoglobin (Hb), white blood cell (WBC) and platelet (PLT) counts every other week during the treatment period.ResultsThe mean absorbed dose to the bone marrow was estimated to 0.20 Gy (0.11–0.37 Gy) per 7.4 GBq of 177Lu-DOTATATE, and the mean dose per fraction correlated with a decrease in Hb (p = 0.01), WBC (p < 0.01) and PLT (p < 0.01) counts. The total mean absorbed dose to the bone marrow was 0.64 Gy (0.30–1.5 Gy) per 24 GBq (8.2–37 GBq) of 177Lu-DOTATATE and also correlated with a decrease in Hb (p < 0.01), WBC (p = 0.01) and PLT (p < 0.01) counts.ConclusionsThe planar image-based method developed in this study resulted in similar absorbed doses to the bone marrow as reported in earlier studies with blood-based bone marrow dosimetry. The results correlated with haematological toxicity, making it a promising method for estimating bone marrow doses in 177Lu-DOTATATE treatment without the need for blood and urine sampling.

Comparison of accuracy in calculation of absorbed dose to patients following bone scan with 99mTc-marked diphosphonates by two different background correction methods

To improve the accuracy of the activity quantification and the image quality in scintigraphy, scatter correction is a vital procedure. The aim of this study is to compare the accuracy in calculation of absorbed dose to patients following bone scan with (99m)Tc-marked diphosphonates ((99m)Tc-MDP) by two different methods of background correction in conjugate view method. This study involved 22 patients referring to the Nuclear Medicine Center of Shahid Chamran Hospital, Isfahan, Iran. After the injection of (99m)Tc-MDP, whole-body images from patients were acquired at 10, 60, 90, and 180 min. Organ activities were calculated using the conjugate view method by Buijs and conventional background correction. Finally, the absorbed dose was calculated using the Medical Internal Radiation Dosimetry (MIRD) technique. The results of this study showed that the absorbed dose per unit of injected activity (rad/mCi) ± standard deviation for pelvis bone, bladder, and kidneys by Buijs method was 0.19 ± 0.05, 0.08 ± 0.01, and 0.03 ± 0.01 and by conventional method was 0.13 ± 0.04, 0.08 ± 0.01, and 0.024 ± 0.01, respectively. This showed that Buijs background correction method had a high accuracy compared to conventional method for the estimated absorbed dose of bone and kidneys whereas, for the bladder, its accuracy was low.

Measurement of Linear Attenuation Coefficient of Tc-99m using Planar Gamma Camera Image

Introduction: Attenuation is one of the most important factors which have an impact on the accuracy on quantitation of activity when using gamma camera image in a medium. Due to attenuation a number of photons are lost and there is also loss of counts which otherwise would have been included in the images. The purpose of this study is to estimate the linear attenuation coefficient for Tc-99m with water filled phantom using conjugate view method and transmission method and compare the obtained measurements with the reference values. Materials and Methods: To determine the linear attenuation coefficient of radionuclide Tc-99m using conjugate view method, the sensitivity of the gamma camera was determined. 30 MBq of Tc-99m was taken which was measured in dose calibrator and gamma camera images were acquired. Then the sensitivity was calculated as CPS/MBq. A Jaszczak phantom was filled up with water and a small sized cylindrical container filled with activity of 30 MBq Tc-99m liquid was placed inside the Jaszczak phantom centrally. The phantom was scanned on a dual head gamma camera and the images were taken and counts from the images were estimated. After estimation of all values, the linear attenuation coefficient was calculated using conjugate view formula. The linear attenuation coefficient was also measured from the transmission imaging. The transmission factor (TF) was determined by measuring the ratio of count rates obtained with Tc-99m under gamma camera, with and without the phantom. Results: The linear attenuation coefficient was calculated 0.1467 ± 0.0057 from conjugate view method and 0.1483 ± 0.0023 from transmission measurements. Both results were found within ±5% of the established value (0.15 cm-1). Conclusions: Correction for attenuation is required for the accurate estimation of activity. For conjugate view method, the attenuation coefficient was measured with a known activity of Tc-99m. The attenuation coefficient was also estimated by transmission measurements. The accuracy of the activity measurement is highly dependent on the accuracy of the attenuation coefficient used in the calculation. DOI: http://dx.doi.org/10.3329/bjnm.v17i1.22493 Bangladesh J. Nuclear Med. 17(1): 61-66, January 2014

Renal function affects absorbed dose to the kidneys and haematological toxicity during ¹⁷⁷Lu-DOTATATE treatment.

PurposePeptide receptor radionuclide therapy (PRRT) has become an important treatment option in the management of advanced neuroendocrine tumours. Long-lasting responses are reported for a majority of treated patients, with good tolerability and a favourable impact on quality of life. The treatment is usually limited by the cumulative absorbed dose to the kidneys, where the radiopharmaceutical is reabsorbed and retained, or by evident haematological toxicity. The aim of this study was to evaluate how renal function affects (1) absorbed dose to the kidneys, and (2) the development of haematological toxicity during PRRT treatment.MethodsThe study included 51 patients with an advanced neuroendocrine tumour who received 177Lu-DOTATATE treatment during 2006 – 2011 at Sahlgrenska University Hospital in Gothenburg. An average activity of 7.5 GBq (3.5 – 8.2 GBq) was given at intervals of 6 – 8 weeks on one to five occasions. Patient baseline characteristics according to renal and bone marrow function, tumour burden and medical history including prior treatment were recorded. Renal and bone marrow function were then monitored during treatment. Renal dosimetry was performed according to the conjugate view method, and the residence time for the radiopharmaceutical in the whole body was calculated.ResultsA significant correlation between inferior renal function before treatment and higher received renal absorbed dose per administered activity was found (p < 0.01). Patients with inferior renal function also experienced a higher grade of haematological toxicity during treatment (p = 0.01). The residence time of 177Lu in the whole body (range 0.89 – 3.0 days) was correlated with grade of haematological toxicity (p = 0.04) but not with renal absorbed dose (p = 0.53).ConclusionPatients with inferior renal function were exposed to higher renal absorbed dose per administered activity and developed a higher grade of haematological toxicity during 177Lu-DOTATATE treatment. The study confirms the tolerability of PRRT in patients with an advanced neuroendocrine tumour but indicates that patients with inferior renal function are at risk of being exposed to higher absorbed doses to normal tissue on treatment.

Estimation of organ activity using four different methods of background correction in conjugate view method

To make an accurate estimation of the uptake of radioactivity in an organ using the conjugate view method, corrections of physical factors, such as background activity, scatter, and attenuation are needed. The aim of this study was to evaluate the accuracy of four different methods for background correction in activity quantification of the heart in myocardial perfusion scans. The organ activity was calculated using the conjugate view method. A number of 22 healthy volunteers were injected with 17-19 mCi of (99m)Tc-methoxy-isobutyl-isonitrile (MIBI) at rest or during exercise. Images were obtained by a dual-headed gamma camera. Four methods for background correction were applied: (1) Conventional correction (referred to as the Gates' method), (2) Buijs method, (3) BgdA subtraction, (4) BgdB subtraction. To evaluate the accuracy of these methods, the results of the calculations using the above-mentioned methods were compared with the reference results. The calculated uptake in the heart using conventional method, Buijs method, BgdA subtraction, and BgdB subtraction methods was 1.4 ± 0.7% (P < 0.05), 2.6 ± 0.6% (P < 0.05), 1.3 ± 0.5% (P < 0.05), and 0.8 ± 0.3% (P < 0.05) of injected dose (I.D) at rest and 1.8 ± 0.6% (P > 0.05), 3.1 ± 0.8% (P > 0.05), 1.9 ± 0.8% (P < 0.05), and 1.2 ± 0.5% (P < 0.05) of I.D, during exercise. The mean estimated myocardial uptake of (99m)Tc-MIBI was dependent on the correction method used. Comparison among the four different methods of background activity correction applied in this study showed that the Buijs method was the most suitable method for background correction in myocardial perfusion scan.

Biological Tissue Modeling with Agar Gel Phantom for Radiation Dosimetry of &amp;lt;sup&amp;gt;99m&amp;lt;/sup&amp;gt;Tc

The biological tissue has been mimicked and replaced by other materials, which have shown certain radiological similarity determined by attenuation coefficient (μ), density and atomic number. Specifically, in molecular imaging and radiation therapy have been developed multifunctional radiopharmaceuticals which contain beta/gamma and/or light emitters to chronic degenerative diseases treatment. Therefore, it is necessary to develop phantoms that allow optical and radiometric characterization. Since the agar gel has shown to be a medium which allows to model biological tissue in phototherapy studies, the aim of this study is to determine whether the agar gel may be used as biological tissue substitutes in 99mTc dosimetry. Agar gel was prepared to 1% and 2.3% (water:agar) and its radiologicalproperties as: linear attenuation coefficient obtained by narrow beam geometry and XCOM software, density and effective atomic number (Zeff) were determined. Using the determined μ, photontransmission was calculated by Monte Carlosimulation. The 99mTc source region was immersed in a water phantom, two source regions were used, one source region was filled with water and another with agar gel. For both cases; the cumulated activity () by conjugate view method, the absorbed doseper unitcumulated activity (S) and absorbed dose (D) were determined. The 2.3% concentration gel consistency facilitated its handling during a bigger irradiation time. A was obtained and also this value was corroborated with the XCOM software. The agar gel density was and . The calculated cumulated activity presented 1% difference in both phantoms. The absorbed doseper unitcumulated activity was the same in both media, therefore the D too. Agar gel showed to be equivalent to water in terms of radiological properties for 140 keV photons, thus it can substitute soft tissue in 99mTc dosimetry.

Estimation of absorbed dose to the kidneys in patients after treatment with 177Lu-octreotate: comparison between methods based on planar scintigraphy.

BackgroundLu-[DOTA0, Tyr3]-octreotate (177Lu-octreotate) is used to treat neuroendocrine tumors with high somatostatin-receptor expression. 177Lu-octreotate is mainly excreted via the kidneys, but to some extent, accumulates in the kidney cortex due to, e.g., tubular reabsorption. Renal toxicity is one of the main limiting factors in 177Lu-octreotate treatment. Further knowledge of the biodistribution and dosimetry of 177Lu-octreotate in individual patients is needed. The aim of this study was to estimate the absorbed dose to the kidneys and compare the results obtained with planar imaging and different dosimetric methods: (1) conjugate-view (CV) method using patient-specific kidney sizes, (2) PA method, based on posterior images only, (3) CV method with reduced number of time points (CVreduced data), and (4) CV method using standard kidney sizes (CVstandard size).MethodsTotally, 33 patients each received 3.4 to 8.2 GBq of 177Lu-octreotate up to five times, with infusion of lysine and arginine to block the renal uptake. Whole-body planar gamma camera images were acquired on days 0, 1, 2, and 7. The 177Lu concentration in the kidneys was determined by the CV method, and the absorbed dose was estimated with patient-specific organ sizes. Comparison to the CV method was made using posterior images only, together with the influence of the number of time points and with standard organ sizes.ResultsLarge interindividual variations were found in the time-activity curve pattern and in the absorbed dose to the kidneys using the CV method: 0.33 to 2.4 Gy/GBq (mean = 0.80 Gy/GBq, SD = 0.30). In the individual patient, the mean deviation of all subsequent kidney doses compared to that of the first administration was 1% (SD = 19%) and 5% (SD = 23%) for the right and left kidneys, respectively. Excluding data for day 7 resulted in large variations in the absorbed dose.ConclusionLarge interindividual variations in kidney dose were found, demonstrating the need for patient-specific dosimetry and treatment planning.

Estimation of organ absorbed doses in patients from 99m Tc-diphosphonate using the data of MIRDose software

The purpose of this study was to compare estimation of radiation absorbed doses to patients following bone scans with technetium-99m-labeled methylene diphosphonate (MDP) with the estimates given in MIRDose software. In this study, each patient was injected 25 mCi of (99m)Tc-MDP. Whole-body images from thirty patients were acquired by gamma camera at 10, 60, 90, 180 minutes after (99m)Tc-MDP injection. To determine the amount of activity in each organ, conjugate view method was applied on images. MIRD equation was then used to estimate absorbed doses in different organs of patients. At the end, absorbed dose values obtained in this study were compared with the data of MIRDose software. The absorbed doses per unit of injected activity (mGy/MBq × 10(-4)) for liver, kidneys, bladder wall and spleen were 3.86 ± 1.1, 38.73 ± 4.7, 4.16 ± 1.8 and 3.91 ± 1.3, respectively. The results of this study may be useful to estimate the amount of activity that can be administered to the patient and also showed that methods used in the study for absorbed dose calculation is in good agreement with the data of MIRDose software and it is possible to use by a clinician.

177Lu‐DOTA0‐Tyr3]‐Octreotate Treatment in Patients with Disseminated Gastroenteropancreatic Neuroendocrine Tumors: The Value of Measuring Absorbed Dose to the Kidney

Peptide receptor radiation therapy (PRRT) using [(177)Lu-DOTA(0)-Tyr(3)]-octreotate is a new, promising option for treatment of disseminated gastroenteropancreatic neuroendocrine tumors (GEPNETs). During 2006-2008, 26 patients with disseminated GEPNETs were treated with (177)Lu-octreotate. Radiologic response (RECIST), biochemical response [plasma chromogranin-A (P-CgA)], hematologic toxicity [Common Toxicity Criteria (CTC)], absorbed dose to the kidneys (conjugate view method), and glomerular filtration rate (GFR) were analyzed. (177)Lu-octreotate (8 GBq) was given one to five times (median = 3) with a 6-week interval between each. Sixteen of the 26 patients were evaluated radiologically; 6 (38%) had partial response (PR), 8 (50%) had stable disease (SD), and 2 (13%) had progressive disease (PD). Seventeen of the 26 patients were evaluated biochemically; 6 (35%) showed a >or=30% decrease, 8 (47%) showed a >or=20% increase, and 3 (18%) showed neither a >or=30% decrease nor a >or=20% increase. The mean absorbed dose to the kidneys was 24 Gy. With a dose limit of 27 Gy to the kidneys, 10 patients did not receive the planned four treatments, while four patients had the potential to receive additional treatment. A significant reduction (p = 0.0013) of GFR was observed at follow-up. Three patients experienced CTC grade 3 hematologic toxicity. By using the absorbed dose to the kidneys as a limiting factor, treatment with (177)Lu-octreotate can be individualized, e.g., overtreatment can be avoided and patients with the potential to receive additional treatment can be identified. Further studies are needed to define tolerance doses to the kidneys so that treatment can be optimized.

Evaluation of quantitative planar 90Y bremsstrahlung whole-body imaging

With high-dose administration of 90Y labeled antibodies, it is possible to image 90Y without an admixture of 111In. We have earlier shown that it is possible to perform quantitative 90Y bremsstrahlung SPECT for dosimetry purposes with reasonable accuracy. However, whole-body (WB) activity quantification with the conjugate view method is not as time consuming as SPECT and has been the method of choice for dosimetry. We have investigated the possibility of using a conjugate view method where scatter-, backscatter- and septal-penetration compensations are performed by inverse filtering and attenuation correction is performed with a WB x-ray image, for total-body and organ activity quantification of 90Y. The method was evaluated using both Monte Carlo simulated scintillation camera images using realistic source distributions, and by an experimental phantom study. The method was evaluated in terms of image quality and accuracy of the activity quantification. The experimental phantom study was performed using the RSD torso phantom with 90Y activity uniformly distributed in the liver insert. A GE Discovery VH/Hawkeye system was used to acquire the image. The simulation study was performed for a realistic activity distribution in the NCAT anthropomorphic phantom where 90Y bremsstrahlung images were generated using the SIMIND MC program. Two different phantom configurations and two activity distributions were simulated. To mimic the RSD phantom experiment one simulation study was also made with 90Y activity located only in the liver. The SIMIND program was configured to resemble a GE Discovery VH/Hawkeye system. An x-ray projector program was used to generate whole-body x-ray images from the NCAT phantom for attenuation correction in the conjugate view method. Organ activities were calculated from ROIs that exactly covered the organs. Corrections for background activity, overlapping activity and source extension in the depth direction were applied on the ROI data. The total-body activities for the simulated images were generally overestimated by around 10%, which is reasonable since the correction for source extension was not applied on the total-body values. The accuracy of the organ activities was mostly within 15% for both the simulation study and the experimental study. The results suggest that it is possible to quantify 90Y activity in ROIs with reasonable accuracy using this method.

Patient-Specific Radiation Dosimetry for Radionuclide Therapy of Liver Tumors With Intrahepatic Artery Rhenium-188 Lipiodol

A clinically practical algorithm has been developed for the treatment of liver cancer by the administration of rhenium-188 ((188)Re)-labeled lipiodol via the hepatic artery. This algorithm is based on the "maximum tolerated-activity" paradigm for radionuclide therapy. A small "scout" activity of (188)Re-labeled lipiodol is administered to the patient before the actual therapeutic administration. At approximately 3 hours after administration, the activities in the normal liver, liver tumors, lungs, and total body are measured by gamma camera imaging using the conjugate-view method, with first-order corrections for attenuation (using a (188)Re transmission scan) and scatter (using the "dual-window" method). At the same time, peripheral blood samples are counted, and the activity concentrations in whole blood are calculated. The blood activity concentrations are then converted to red marrow activity concentrations and then total red marrow activity using anatomic data from Standard Man anthropomorphic models. Next, the cumulated activities in the normal liver, liver tumors, lungs, red marrow, and total body are calculated using the measured activities in the respective source regions and conservatively assuming elimination of activity only by physical decay in situ. The absorbed doses to the therapy-limiting normal tissues, liver, lung, and red marrow, are then calculated using the Medical Internal Radiation Dose Committee schema, adjusting the pertinent S factors for differences in total body and organ masses between the patient and the anthropomorphic model and including the dose contribution from the liver tumors. Finally, based on maximum tolerated absorbed doses of 3,000, 1,200, and 150 rad (cGy) to liver, lung, and red marrow, the respective absorbed doses per unit administered activity are used to calculate the therapy activity. Although not required for treatment planning, tumor absorbed dose may also be estimated. This algorithm has been automated using an Excel (Microsoft, Redmond, WA) spreadsheet.

A New Method to Obtain Transmission Images for Planar Whole-Body Activity Quantification

Dosimetry is important in the evaluation of new radiopharmaceuticals and in radionuclide therapy treatment planning. The base is an accurate in vivo measurement of the spatial and temporal distribution of the administered radioactivity. This can be made by planar whole-body activity quantification using the conjugate-view method where a geometrical-mean image is corrected for attenuation by a measured transmission image. We present a new method to measure whole-body transmission images using the X-ray unit on a single photon emission computed tomography/computed tomography (SPECT/CT) scintillation system. Two general problems are associated with this method: (1) the divergence of the X-ray beam and (2) the scaling of the resulting image. We have applied linear transformations and scale by a system-generated factor to obtain images corresponding to a radiological depth at 70 keV. We validated the method by measurements of the radiological depth for different types of phantoms, and in terms of activity quantification accuracy when applying the method for attenuation correction. An accuracy of within +/-4% was found for both the radiological depth measurements and for the activity-quantification measurements. The advantage with the proposed method is that it is fast (2 minutes for a 2-minute scan), it has good spatial resolution and signal-to-noise ratio, and it is easy to carry out for the patient.

Accuracy of the quantification of organ activity from planar gamma camera images.

The accuracy in determination of organ activity of (99m)Tc was investigated, with activity estimated from gamma camera images of phantoms, using the conjugate view method. The accuracy depends on several parameters such as the choice of background correction method, the accuracy in determination of the effective attenuation coefficient and the thickness of the body and organs and on the determination of the gamma camera sensitivity. The background correction method has a major influence on the quantification of the activity. Methods which take the volume of the source organ into consideration are recommended. The discrepancy in the determined organ activity varied between an underestimation of 26% and an overestimation of 16% in the MIRD phantom, depending on which organ was studied and on the correction method used. To correct for absorption and scattering, an effective attenuation coefficient was used. A theoretical analysis showed that a change in the effective attenuation coefficient of 0.01 cm(-1) resulted in a 15% change in the calculated activity. Also the thickness of the body and the organ of interest influences the calculated activity. A 2 cm deviation in the body thickness causes a deviation of approximately 10% in the calculated activity. The quantification is improved if the attenuation coefficient is determined by transmission measurements.

Quantitative studies of monoclonal antibody targeting to disialoganglioside GD2 in human brain tumors.

Iodine-131 3F8, a murine IgG3 monoclonal antibody that targets to GD2-bearing tumors, was administered intravenously to 12 patients with brain tumors. Six patients received 2 mCi (0.74 Bq) of 131I-3F8, five patients 10 mCi (3.7 Bq)/1.73 m2 of 131I-3F8, and one patient 2.6 mCi (0.96 Bq) of 124I-3F8, with no side-effects. Nine of 11 malignant gliomas and the single metastatic melanoma showed antibody localization, with the best tumor delineation on single-photon emission tomography (SPET) following 10 mCi (3.7 Bq)/1.73 m2 dose. No nonspecific uptake in the normal craniospinal axis was detected. There was no difference in the pharmacokinetics of low-dose versus the higher-dose antibody groups; plasma and total-body half-lives were 18 h and 49 h, respectively. Surgical sampling and time-activity curves based on quantitative imaging showed peak uptake in high-grade glioma at 39 h, with a half-life of 62 h. Tumor uptake at time of surgery averaged 3.5 x 10(-3) %ID/g and peak activity by the conjugate view method averaged 9.2 x 10(-3) %ID/g (3.5-17.8). Mean radiation absorption dose was 3.9 rad per mCi injected (range 0.7-9.6) or 10.5 cGy/Bq (range 1.9-26). There was agreement on positive sites when immunoscintigraphy was compared with technetium-99m glucoheptonate/diethylene triamine penta-acetic acid planar imaging, thallium-201 SPET, and fluorine-18 fluorodeoxyglucose positron emission tomography. Taken together, these data suggest that quantitative estimates of antibody targeting to intracranial tumors can be made using the modified conjugate view method.