In vivo quantitation of lesion radioactivity using external counting methods

Radioactive Iodine Therapy in Differentiated Thyroid Cancer: Summary of the Korean Thyroid Association Guidelines 2024 from Nuclear Medicine Perspective, Part-II

Thyroid cancer, one of the most common endocrine tumors, generally has a favorable prognosis but remains a significant medical and societal concern due to its high incidence. Early diagnosis and treatment of differentiated thyroid cancer (DTC) significantly affect long-term outcomes, requiring the selection and application of appropriate initial treatments to improve prognosis and quality of life. Recent advances in technology and health information systems have enhanced our understanding of the molecular genetics of thyroid cancer, facilitating the identification of aggressive subgroups and enabling the accumulation of research on risk factors through big data. The Korean Thyroid Association (KTA) has revised the "KTA Guidelines on the Management of Differentiated Thyroid Cancers 2024" to incorporate these advances, which were developed by a multidisciplinary team and underwent extensive review and approval processes by various academic societies. This article summarizes the 2024 KTA guidelines for radioactive iodine (RAI) therapy in patients with DTC, written by the Nuclear Medicine members of the KTA Guideline Committee, and covers RAI therapy as initial management of DTC and RAI therapy in advanced thyroid cancer.

Dosimetry for Radiopharmaceutical Therapy: Practical Implementation, From the AJR Special Series on Quantitative Imaging

Radiopharmaceutical therapy (RPT) is advancing rapidly and achieving wider clinical application. However, RPT is not yet optimized in practice, as tumor and normal-organ dose estimates and, in turn, dose-response relationships remain poorly defined. Internal dosimetry is evolving to address such issues, transitioning from the estimation of population-average organ-level or tumor-level doses to individualized patient-specific sub-organ or sub-tumor doses. Derivation of patient-specific doses allows the further development of reliable dose-response relationships for diseased tissues and dose-toxicity relationships for normal tissues. Resources such as commercially available or publicly downloadable software are being increasingly developed to facilitate the use of these emerging methods. This review addresses the determination of patient-specific radiation doses for target tissue and at-risk normal tissues in the setting of RPT. Topics covered include: quantities, units, and radionuclides relevant to RPT; dose prescription algorithms; the steps in the dosimetry workflow; and bioeffects modeling. Implementation of patient-specific dosimetry will be essential for this therapeutic modality's optimization and further clinical expansion.

Comparative post-therapeutic dosimetry between 2D planar-based and hybrid-based methods for personalized Lu-177 treatment

PURPOSE

This study aims to compare the calculated absorbed dose in target organs and tumors obtained using the different imaging protocols and the calculation methodologies implemented by HERMES HybridViewer dosimetry software for 177Lu-PSMA I&T and 177Lu-DOTATATE therapy.

METHODS

Multiple time-point whole-body planar images and one SPECT/CT image were acquired from 18 patients including 177Lu-PSMA I&T (13 patients) and 177Lu-DOTATATE treatment (5 patients) after administration of 3.80-8.58 GBq injected activity. The regions of interest were drawn in the whole body, kidneys, liver, urinary bladder, salivary glands, and tumors to determine the time-integrated activity (TIA) in source organs. Absorbed doses in target organs were calculated according to the Medical Internal Radiation Dose (MIRD) scheme using the HERMES HybridViewer dosimetry integrated with OLINDA/EXM V.2.1 that utilizes the non-uniform rational B-splines (NURBS) for computational digital phantom.

RESULTS

The planar-based dosimetry showed a higher dose per injected activity compared to the hybrid-based dosimetry, primarily due to organ overlap. The highest difference in absorbed dose between the imaging scenarios was observed in the spleen with a variation of up to 51.6%, while the difference for other target organs and tumors was less than 40%.

CONCLUSION

The dosimetry calculation derived from the 2D planar-based method consistently demonstrates a significantly higher absorbed dose in organs and tumors compared with the hybrid-based method. However, the hybrid method outperforms the planar method in terms of tumor visualization and overlap-free organ delineation.

Radioiodine therapy in the different stages of differentiated thyroid cancer

Differentiated thyroid cancer is the most frequent type of thyroid cancer with an increasing incidence in the last decades. The initial management is represented by surgical treatment followed by radioactive iodine therapy that includes remnant ablation, adjuvant treatment or treatment of metastatic disease. Radioactive iodine treatment is performed only in selected cases based on the risk of recurrence and mortality during follow up, according to American Joint Committee on Cancer Union for international Cancer Control Tumor, Node, Metastasis (AJCC/TNM) staging system and the 2015 American Thyroid Association (ATA) risk stratification system. This article will review the key factors to consider when planning radioactive iodine therapy in differentiated thyroid cancer patients after surgery and during follow up.

Dosimetry in Radiopharmaceutical Therapy

The application of radiopharmaceutical therapy for the treatment of certain diseases is well established, and the field is expanding. New therapeutic radiopharmaceuticals have been developed in recent years, and more are in the research pipeline. Concurrently, there is growing interest in the use of internal dosimetry as a means of personalizing, and potentially optimizing, such therapy for patients. Internal dosimetry is multifaceted, and the current state of the art is discussed in this continuing education article. Topics include the context of dosimetry, internal dosimetry methods, the advantages and disadvantages of incorporating dosimetry calculations in radiopharmaceutical therapy, a description of the workflow for implementing patient-specific dosimetry, and future prospects in the field.

EANM dosimetry committee recommendations for dosimetry of 177Lu-labelled somatostatin-receptor- and PSMA-targeting ligands

The purpose of the EANM Dosimetry Committee is to provide recommendations and guidance to scientists and clinicians on patient-specific dosimetry. Radiopharmaceuticals labelled with lutetium-177 (177Lu) are increasingly used for therapeutic applications, in particular for the treatment of metastatic neuroendocrine tumours using ligands for somatostatin receptors and prostate adenocarcinoma with small-molecule PSMA-targeting ligands. This paper provides an overview of reported dosimetry data for these therapies and summarises current knowledge about radiation-induced side effects on normal tissues and dose-effect relationships for tumours. Dosimetry methods and data are summarised for kidneys, bone marrow, salivary glands, lacrimal glands, pituitary glands, tumours, and the skin in case of radiopharmaceutical extravasation. Where applicable, taking into account the present status of the field and recent evidence in the literature, guidance is provided. The purpose of these recommendations is to encourage the practice of patient-specific dosimetry in therapy with 177Lu-labelled compounds. The proposed methods should be within the scope of centres offering therapy with 177Lu-labelled ligands for somatostatin receptors or small-molecule PSMA.

I-124 PET/CT image-based dosimetry in patients with differentiated thyroid cancer treated with I-131: correlation of patient-specific lesional dosimetry to treatment response

PURPOSE

The objective of this study is to evaluate the lesion absorbed dose (AD), biological effective dose (BED), and equivalent uniform dose (EUD) to clinical-response relationship in lesional dosimetry for ¹³¹I therapy.

METHODS

Nineteen lesions in four patients with metastatic differentiated thyroid cancer (DTC) were evaluated. The patients underwent PET/CT imaging at 2 h, 24 h, 48 h, 72 h, and 96 h post administration of ~ 33-65 MBq (0.89-1.76 mCi) of ¹²⁴I before undergoing ¹³¹I therapy. The ¹²⁴I PET/CT images were used to perform dosimetry calculations for ¹³¹I therapy. Lesion dose-rate values were calculated using the time-activity data and integrated over the measured time points to obtain AD and BED. The Geant4 toolkit was used to run Monte Carlo on spheres the same size as the lesions to estimate EUD. The lesion AD, BED, and EUD values were correlated with response data (i.e. change in lesion size pre- and post-therapy): complete response (CR, i.e. disappearance of the lesion), partial response (PR, i.e. any decrease in lesion length), stable disease (SD, i.e., no change in length), and progressive disease (PD, i.e., any increase in length).

RESULTS

The lesion responses were CR and PR (58%, 11/19 lesions), SD (21%, 4/19), and PD (21%, 4/19). For CR and PR lesions, the ADs, BEDs and EUDs were > 75 Gy for 82% (9/11) and < 75 Gy for 18% (2/11). The ADs and BEDs were < 75 Gy for SD and PD lesions.

CONCLUSION

By performing retrospective dosimetry calculations for ¹³¹I therapy based on ¹²⁴I PET/CT imaging, we evaluated the correlation of three dosimetric quantities to lesional response. When lesion AD, BED, and EUD values were > 75 Gy, 47% (9/19) of the lesions had a CR or PR. The AD, BED, and EUD values for SD and PD lesions were < 75 Gy. The data presented herein suggest that the greater the lesion AD, BED, and/or EUD, the higher the probability of a therapeutic response to ¹³¹I therapy.

Dosimetry for Radiopharmaceutical Therapy: Current Practices and Commercial Resources

With the ongoing dramatic growth of radiopharmaceutical therapy, research and development in internal radiation dosimetry continue to advance both at academic medical centers and in industry. The basic paradigm for patient-specific dosimetry includes administration of a pretreatment tracer activity of the therapeutic radiopharmaceutical; measurement of its time-dependent biodistribution; definition of the pertinent anatomy; integration of the measured time-activity data to derive source-region time-integrated activities; calculation of the tumor, organ-at-risk, and/or whole-body absorbed doses; and prescription of the therapeutic administered activity. This paper provides an overview of the state of the art of patient-specific dosimetry for radiopharmaceutical therapy, including current methods and commercially available software and other resources.

Patient dosimetry of 177Lu-PSMA I&T in metastatic prostate cancer treatment: the experience in Thailand

PURPOSE

This study aimed to determine the radiation dosimetry for 177Lu-PSMA imaging and therapy (I&T) in Thai patients who were treated for metastatic prostate cancer.

METHODS

Whole-body planar images acquired at immediately, 4 and 24 h after 177Lu-PSMA I&T injection (range 4.44-8.51 GBq) were collected from 12 treatment cycles of 8 prostate cancer patients. Region of interests (ROIs) were manually contoured on the whole-body, liver, spleen, urinary bladder, lacrimal glands, parotid, and submandibular glands to determine time-integrated activity (TIA) in source organs and fitted time-activity curves using mono-exponential extrapolation. The S values calculated utilizing non-uniform rational B-splines (NURBS) computational phantoms were extracted from the OLINDA/EXM v. 2.0 to calculate the absorbed dose coefficient in target organs according to the Medical Internal Radiation Dose (MIRD) scheme. The absorbed doses to bone marrow were estimated using the planar two-compartment image-based method by separating the high-uptake and low-uptake compartment. The spherical model was used to calculate the lacrimal gland absorbed doses.

RESULTS

Mean absorbed dose coefficients to the kidneys, bone marrow, liver, urinary bladder, spleen, lacrimal glands, parotid, and submandibular glands were 0.81 ± 0.24, 0.02 ± 0.01, 0.13 ± 0.10, 0.27 ± 0.25, 0.16 ± 0.07, 3.62 ± 1.78, 0.21 ± 0.14, and 0.09 ± 0.07 Gy/GBq, respectively. Dose constraints for the kidneys (23 Gy) and bone marrow (2 Gy) were not reached in any patients. The absorbed dose in lacrimal glands calculated by the NURBS computational phantoms was slightly lower than the calculation based on the Cristy-Eckerman computational phantoms using OLINDA/EXM v. 1.0 by 6.37 ± 0.14%.

CONCLUSION

Dosimetry results in this study suggested that 177Lu-PSMA I&T treatment with higher activities and more cycles is possible without the risk of damaging normal organs in prostate cancer patients.

IPEM topical report: current molecular radiotherapy service provision and guidance on the implications of setting up a dosimetry service

Despite a growth in molecular radiotherapy treatment (MRT) and an increase in interest, centres still rarely perform MRT dosimetry. The aims of this report were to assess the main reasons why centres are not performing MRT dosimetry and provide advice on the resources required to set-up such a service. A survey based in the United Kingdom was developed to establish how many centres provide an MRT dosimetry service and the main reasons why it is not commonly performed. Twenty-eight per cent of the centres who responded to the survey performed some form of dosimetry, with 88% of those centres performing internal dosimetry. The survey showed that a 'lack of clinical evidence', a 'lack of guidelines' and 'not current UK practice' were the largest obstacles to setting up an MRT dosimetry service. More practical considerations, such as 'lack of software' and 'lack of staff training/expertise', were considered to be of lower significance by the respondents. Following on from the survey, this report gives an overview of the current guidelines, and the evidence available demonstrating the benefits of performing MRT dosimetry. The resources required to perform such techniques are detailed with reference to guidelines, training resources and currently available software. It is hoped that the information presented in this report will allow MRT dosimetry to be performed more frequently and in more centres, both in routine clinical practice and in multicentre trials. Such trials are required to harmonise dosimetry techniques between centres, build on the current evidence base, and provide the data necessary to establish the dose-response relationship for MRT.

[Radiological Technology for Targeted Radionuclide Therapy]

Targeted radioisotope therapy (TRT) is a radiotherapy using radioisotope or drug incorporating it and has been used as a treatment for selectively irradiating cancer cells. In recent years, interest in TRT has increased due to improvements in radionuclide production technology, development of new drugs and imaging modalities, and improvements in radiation technology. In order to enhance the effect of TRT, measurement of individual radiation doses to tumor tissue and organs at risk is important using highly quantitative nuclear medicine images. In this paper, we present a review of literature on optimization of TRT, which is a new research area from the perspective of radiation technology.

Conventional Radioiodine Therapy for Differentiated Thyroid Cancer

This article presents an overview of the use of radioactive iodine (131-I) in the treatment of patients with differentiated thyroid cancer. Topics reviewed include definitions; staging; the 2 principal methods for selection of 131-I dosage; the indications for ablation, adjuvant treatment, and treatment; the recommendations for the use of 131-I contained in the guidelines of the American Thyroid Association and the Society of Nuclear Medicine and Molecular Imaging; the dosage recommendations and selection of dosage approach for 131-I by these organizations; the use of recombinant human thyrotropin for radioiodine ablation, adjuvant therapy, or treatment; and the MedStar Washington Hospital Center approach.

Quantification of radioactivity by planar gamma-camera images, a promoted method of absorbed dose in the thyroid after iodine-131 treatment

Iodine-131 (¹³¹I) is an essential and widely used radioisotope in thyroid diseases and animal experiments. Planar imaging has been considered the most popular method for ¹³¹I thyroid uptake radioactive activity quantification. The ROI defining section is essential and can affect the accuracy of quantitative results. However, a consistent method has not been proposed. In this study, a UC-ROI defining method based on ULWL setting and colour display grade was applied. Three steps were performed: image acquisition of five standard activity models and obtaining the exact value that the counts per radioactive activity contributes to the ROI; image acquisition of 20 rat thyroids and obtaining the counts of the ROI (thyroid); and calculating the rat thyroid radioactive activity and comparing these values with the true values. The accuracy of quantification activity of ¹³¹I in rat thyroid reached 2.62% ± 0.41%. The mean quantification within 5% could be achieved in 19 of 20 rat thyroids. No significant difference existed between calculated thyroid ¹³¹I activity and true values with a paired matched-test (t = -0.384, P = 0.706 > 0.05). The results indicated that with the UC-ROI defining method, more accurate thyroid uptake ¹³¹I radioactive activity quantification by SPECT planar imaging can be achieved in vivo rat study.

Quantitative Imaging for Targeted Radionuclide Therapy Dosimetry - Technical Review

Targeted radionuclide therapy (TRT) is a promising technique for cancer therapy. However, in order to deliver the required dose to the tumor, minimize potential toxicity in normal organs, as well as monitor therapeutic effects, it is important to assess the individualized internal dosimetry based on patient-specific data. Advanced imaging techniques, especially radionuclide imaging, can be used to determine the spatial distribution of administered tracers for calculating the organ-absorbed dose. While planar scintigraphy is still the mainstream imaging method, SPECT, PET and bremsstrahlung imaging have promising properties to improve accuracy in quantification. This article reviews the basic principles of TRT and discusses the latest development in radionuclide imaging techniques for different theranostic agents, with emphasis on their potential to improve personalized TRT dosimetry.

131I activity quantification of gamma camera planar images

A procedure to estimate the activity in target tissues in patients during the therapeutic administration of ¹³¹I radiopharmaceutical treatment for thyroid conditions (hyperthyroidism and differentiated thyroid cancer) using a gamma camera (GC) with a high energy (HE) collimator, is proposed. Planar images are acquired for lesions of different sizes r, and at different distances d, in two HE GC systems. Defining a region of interest (ROI) on the image of size r, total counts n _g are measured. Sensitivity S (cps MBq^-1) in each acquisition is estimated as the product of the geometric G and the intrinsic efficiency η ₀. The mean fluence of 364 keV photons arriving at the ROI per disintegration G, is calculated with the MCNPX code, simulating the entire GC and the HE collimator. Intrinsic efficiency η ₀ is estimated from a calibration measurement of a plane reference source of ¹³¹I in air. Values of G and S for two GC systems-Philips Skylight and Siemens e-cam-are calculated. The total range of possible sensitivity values in thyroidal imaging in the e-cam and skylight GC measure from 7 cps MBq^-1 to 35 cps MBq^-1, and from 6 cps MBq^-1 to 29 cps MBq^-1, respectively. These sensitivity values have been verified with the SIMIND code, with good agreement between them. The results have been validated with experimental measurements in air, and in a medium with scatter and attenuation. The counts in the ROI can be produced by direct, scatter and penetration photons. The fluence value for direct photons is constant for any r and d values, but scatter and penetration photons show different values related to specific r and d values, resulting in the large sensitivity differences found. The sensitivity in thyroidal GC planar imaging is strongly dependent on uptake size, and distance from the GC. An individual value for the acquisition sensitivity of each lesion can significantly alleviate the level of uncertainty in the measurement of thyroid uptake activity for each patient.

Strengths and Weaknesses of a Planar Whole-Body Method of (153)Sm Dosimetry for Patients with Metastatic Osteosarcoma and Comparison with Three-Dimensional Dosimetry

PURPOSE

Dosimetric accuracy depends directly upon the accuracy of the activity measurements in tumors and organs. The authors present the methods and results of a retrospective tumor dosimetry analysis in 14 patients with a total of 28 tumors treated with high activities of (153)Sm-ethylenediaminetetramethylenephosphonate ((153)Sm-EDTMP) for therapy of metastatic osteosarcoma using planar images and compare the results with three-dimensional dosimetry.

MATERIALS AND METHODS

Analysis of phantom data provided a complete set of parameters for dosimetric calculations, including buildup factor, attenuation coefficient, and camera dead-time compensation. The latter was obtained using a previously developed methodology that accounts for the relative motion of the camera and patient during whole-body (WB) imaging. Tumor activity values calculated from the anterior and posterior views of WB planar images of patients treated with (153)Sm-EDTMP for pediatric osteosarcoma were compared with the geometric mean value. The mean activities were integrated over time and tumor-absorbed doses were calculated using the software package OLINDA/EXM.

RESULTS

The authors found that it was necessary to employ the dead-time correction algorithm to prevent measured tumor activity half-lives from often exceeding the physical decay half-life of (153)Sm. Measured half-lives so long are unquestionably in error. Tumor-absorbed doses varied between 0.0022 and 0.27 cGy/MBq with an average of 0.065 cGy/MBq; however, a comparison with absorbed dose values derived from a three-dimensional analysis for the same tumors showed no correlation; moreover, the ratio of three-dimensional absorbed dose value to planar absorbed dose value was 2.19. From the anterior and posterior activity comparisons, the order of clinical uncertainty for activity and dose calculations from WB planar images, with the present methodology, is hypothesized to be about 70%.

CONCLUSION

The dosimetric results from clinical patient data indicate that absolute planar dosimetry is unreliable and dosimetry using three-dimensional imaging is preferable, particularly for tumors, except perhaps for the most sophisticated planar methods. The relative activity and patient kinetics derived from planar imaging show a greater level of reliability than the dosimetry.

Multi-centre evaluation of accuracy and reproducibility of planar and SPECT image quantification: An IAEA phantom study

Accurate quantitation of activity provides the basis for internal dosimetry of targeted radionuclide therapies. This study investigated quantitative imaging capabilities at sites with a variety of experience and equipment and assessed levels of errors in activity quantitation in Single-Photon Emission Computed Tomography (SPECT) and planar imaging. Participants from 9 countries took part in a comparison in which planar, SPECT and SPECT with X ray computed tomography (SPECT-CT) imaging were used to quantify activities of four epoxy-filled cylinders containing ¹³³Ba, which was chosen as a surrogate for ¹³¹I. The sources, with nominal volumes of 2, 4, 6 and 23mL, were calibrated for ¹³³Ba activity by the National Institute of Standards and Technology, but the activity was initially unknown to the participants. Imaging was performed in a cylindrical phantom filled with water. Two trials were carried out in which the participants first estimated the activities using their local standard protocols, and then repeated the measurements using a standardized acquisition and analysis protocol. Finally, processing of the imaging data from the second trial was repeated by a single centre using a fixed protocol. In the first trial, the activities were underestimated by about 15% with planar imaging. SPECT with Chang's first order attenuation correction (Chang-AC) and SPECT-CT overestimated the activity by about 10%. The second trial showed moderate improvements in accuracy and variability. Planar imaging was subject to methodological errors, e.g., in the use of a transmission scan for attenuation correction. The use of Chang-AC was subject to variability from the definition of phantom contours. The project demonstrated the need for training and standardized protocols to achieve good levels of quantitative accuracy and precision in a multicentre setting. Absolute quantification of simple objects with no background was possible with the strictest protocol to about 6% with planar imaging and SPECT (with Chang-AC) and within 2% for SPECT-CT.

Novel Approaches to Thyroid Cancer Treatment and Response Assessment

The incidence of thyroid cancer has been increasing. After total thyroidectomy of well-differentiated thyroid tumors with intermediate- or high-risk features on pathology, radioiodine remains one of the mainstays of therapy for both thyroid remnant ablation as well as for treatment of metastatic disease. SPECT/CT, a relatively new modality, has been shown to play a pivotal role predominantly in the post-therapy setting by changing the risk stratification of patients with thyroid cancer. In the case of radioiodine treatment failure, FDG-PET/CT may provide prognostic information based on extent and intensity of metabolically active metastatic sites as well as serve as an important imaging test for response assessment in patients treated with chemotherapy, targeted therapies, or radiotherapy, thereby affecting patient management in multiple ways. The role of newer redifferentiation drugs has been evaluated with the use of I-124 PET/CT.

2015 American Thyroid Association Management Guidelines for Adult Patients with Thyroid Nodules and Differentiated Thyroid Cancer: The American Thyroid Association Guidelines Task Force on Thyroid Nodules and Differentiated Thyroid Cancer

BACKGROUND

Thyroid nodules are a common clinical problem, and differentiated thyroid cancer is becoming increasingly prevalent. Since the American Thyroid Association's (ATA's) guidelines for the management of these disorders were revised in 2009, significant scientific advances have occurred in the field. The aim of these guidelines is to inform clinicians, patients, researchers, and health policy makers on published evidence relating to the diagnosis and management of thyroid nodules and differentiated thyroid cancer.

METHODS

The specific clinical questions addressed in these guidelines were based on prior versions of the guidelines, stakeholder input, and input of task force members. Task force panel members were educated on knowledge synthesis methods, including electronic database searching, review and selection of relevant citations, and critical appraisal of selected studies. Published English language articles on adults were eligible for inclusion. The American College of Physicians Guideline Grading System was used for critical appraisal of evidence and grading strength of recommendations for therapeutic interventions. We developed a similarly formatted system to appraise the quality of such studies and resultant recommendations. The guideline panel had complete editorial independence from the ATA. Competing interests of guideline task force members were regularly updated, managed, and communicated to the ATA and task force members.

RESULTS

The revised guidelines for the management of thyroid nodules include recommendations regarding initial evaluation, clinical and ultrasound criteria for fine-needle aspiration biopsy, interpretation of fine-needle aspiration biopsy results, use of molecular markers, and management of benign thyroid nodules. Recommendations regarding the initial management of thyroid cancer include those relating to screening for thyroid cancer, staging and risk assessment, surgical management, radioiodine remnant ablation and therapy, and thyrotropin suppression therapy using levothyroxine. Recommendations related to long-term management of differentiated thyroid cancer include those related to surveillance for recurrent disease using imaging and serum thyroglobulin, thyroid hormone therapy, management of recurrent and metastatic disease, consideration for clinical trials and targeted therapy, as well as directions for future research.

CONCLUSIONS

We have developed evidence-based recommendations to inform clinical decision-making in the management of thyroid nodules and differentiated thyroid cancer. They represent, in our opinion, contemporary optimal care for patients with these disorders.

Initial radioiodine administration: when to use it and how to select the dose

All published guidelines on the use of radioactive iodine for the treatment of well-differentiated thyroid cancer agree that an individualized assessment of the risk of cancer-related mortality and of disease recurrence should direct the decision of whether radioiodine treatment is needed and how much to administer. At the author's institution, they mostly follow the American Thyroid Association's risk stratification system, with the addition of a category of very-low-risk patients that do not receive radioactive iodine.

Patient-specific whole-body attenuation correction maps from a CT system for conjugate-view-based activity quantification: method development and evaluation

For activity quantification based on planar scintillation camera measurements, photon attenuation is an important factor that needs to be corrected for in a patient- and organ-specific manner. One possibility for obtaining attenuation correction maps is to use X-ray CT scout images. Since the intensity of scout images is in relative numbers, their image values need to be multiplied by a factor to become quantitative and thus useful for attenuation correction. The calibration factor can for our current imaging system be obtained from a scanner system file, but is generally not available. For this purpose, a method based on the patient weight has been developed. Results based on 79 patient scout images show that the calibration factor thus determined correlates well with values that, in this case, are independently specified by the system. The accuracy of attenuation correction factors (ACFs) derived from the scout-based attenuation correction maps is evaluated by comparison to ACFs derived from three-dimensional CT studies. For photon energies of 208, 245, and 364 keV, scout-based ACFs are on average 1.2% and 0.5% from the CT-derived values, using the system-based and the weight-based values of the scout-image calibration factor, respectively. The imprecision is somewhat higher for the weight-based method, due to variability in the delineation of the patient contour used as a part of this method. In conclusion, X-ray scouts are found useful for attenuation correction with a satisfactory accuracy obtained, both using the new, weight-based method, and using the previous, system-based method, for determining the required calibration factor.

Lung dose calculation with SPECT/CT for ⁹⁰Yittrium radioembolization of liver cancer

PURPOSE

To propose a new method to estimate lung mean dose (LMD) using technetium-99m labeled macroaggregated albumin ((99m)Tc-MAA) single photon emission CT (SPECT)/CT for (90)Yttrium radioembolization of liver tumors and to compare the LMD estimated using SPECT/CT with clinical estimates of LMD using planar gamma scintigraphy (PS).

METHODS AND MATERIALS

Images of 71 patients who had SPECT/CT and PS images of (99m)Tc-MAA acquired before TheraSphere radioembolization of liver cancer were analyzed retrospectively. LMD was calculated from the PS-based lung shunt assuming a lung mass of 1 kg and 50 Gy per GBq of injected activity shunted to the lung. For the SPECT/CT-based estimate, the LMD was calculated with the activity concentration and lung volume derived from SPECT/CT. The effect of attenuation correction and the patient's breathing on the calculated LMD was studied with the SPECT/CT. With these effects correctly taken into account in a more rigorous fashion, we compared the LMD calculated with SPECT/CT with the LMD calculated with PS.

RESULTS

The mean dose to the central region of the lung leads to a more accurate estimate of LMD. Inclusion of the lung region around the diaphragm in the calculation leads to an overestimate of LMD due to the misregistration of the liver activity to the lung from the patient's breathing. LMD calculated based on PS is a poor predictor of the actual LMD. For the subpopulation with large lung shunt, the mean overestimation from the PS method for the lung shunt was 170%.

CONCLUSIONS

A new method of calculating the LMD for TheraSphere and SIR-Spheres radioembolization of liver cancer based on (99m)Tc-MAA SPECT/CT is presented. The new method provides a more accurate estimate of radiation risk to the lungs. For patients with a large lung shunt calculated from PS, a recalculation of LMD based on SPECT/CT is recommended.

Efficacy of dosimetric versus empiric prescribed activity of 131I for therapy of differentiated thyroid cancer

BACKGROUND

The optimal management of high-risk patients with differentiated thyroid cancer (DTC) consists of thyroidectomy followed by radioiodine ((131)I) therapy. The prescribed activity of (131)I can be determined using two approaches: 1) empiric prescribed activity of (131)I (E-Rx); and 2) dosimetry-based prescribed activity of (131)I (D-Rx).

AIM

The aim of the study was to compare the relative treatment efficacy and side effects of D-Rx vs. E-Rx.

METHODS

A retrospective analysis was performed of patients with distant metastases and/or locoregionally advanced radioiodine-avid DTC who were treated with either D-Rx or E-Rx. Response to treatment was based on RECIST (Response Evaluation Criteria in Solid Tumors) 1.1 criteria.

RESULTS

The study group consisted of 87 patients followed for 51 ± 35 months, of whom 43 were treated with D-Rx and 44 with E-Rx. Multivariate analysis, controlling for age, gender, and status of metastases revealed that the D-Rx group tended to be 70% less likely to progress (odds ratio, 0.29; 95% confidence interval, 0.087-1.02; P = 0.052) and more likely to obtain complete response (CR) compared to the E-Rx group (odds ratio, 8.2; 95% confidence interval, 1.2-53.5; P = 0.029). There was an association in the D-Rx group between the observed CR and percentage of maximum tolerable activity given as a first treatment of (131)I (P = 0.030). The advantage of D-Rx was specifically apparent in the locoregionally advanced group because CR was significantly higher in D-Rx vs. E-Rx in this group of patients (35.7 vs. 3.3%; P = 0.009). The rates of partial response, stable disease, and progression-free survival, as well as the frequency of side effects, were not significantly different between the two groups.

CONCLUSION

Higher efficacy of D-Rx with a similar safety profile compared to E-Rx supports the rationale for employing individually prescribed activity in high-risk patients with DTC.

Biodistribution and dosimetry of In-111/Y-90-HuCC49ΔCh2 (IDEC-159) in patients with metastatic colorectal adenocarcinoma

BACKGROUND

CC49, an antibody (mAb) reactive to tumor-associated glycoprotein (TAG-72), has been extensively studied for radioimmunotherapy for colon cancer. Myelotoxicity has been dose-limiting because of prolonged circulation time in the plasma, and human anti-mouse antibody responses were observed in the majority of patients. A CH2 domain deleted and humanized CC49 (HuCC49ΔCh2) was developed to ameliorate these problems. This study reports biodistribution and dosimetry of (111)In/(90)Y-HuCC49ΔCh2 (IDEC-159).

MATERIALS AND METHODS

Five (5) patients with colon cancer were enrolled. Each patient received intravenous administration of 185 MBq (111)In-HuCC49ΔCh2, followed by sequential gamma camera imaging, and blood counting. Uptakes and clearance half-lives for organs and tumors were quantified from images. Absorbed doses for (90)Y-HuCC49ΔCh2 were derived from (111)In-HuCC49ΔCh2 kinetic data.

RESULTS

Compared to reported (111)In/(90)Y-CC49 data in the literature, median blood circulation T(1/2β) was less at 38 (31-43) hours for (90)Y-HuCC49ΔCh2, than 50 hours for (90)Y-CC49. Median tumor-to-marrow absorbed dose ratio was 18 for (90)Y-HuCC49ΔCh2, and 9.53 for (90)Y-CC49. Median tumor-to-liver absorbed dose ratio was 3.14 for (90)Y-HuCC49ΔCh2, and 1.0 for (90)Y-CC49. Median tumor-to-spleen absorbed dose was 3.19 for (90)Y-HuCC49ΔCh2, and 1.07 for (90)Y-CC49.

CONCLUSIONS

A humanized and CH2 domain-deleted CC49 antibody radiolabeled with (111)In/(90)Y showed improved tumor-to-normal dose ratios over those reported from studies with intact CC49.

Development and evaluation of a model-based downscatter compensation method for quantitative I-131 SPECT

PURPOSE

The radionuclide 131I has found widespread use in targeted radionuclide therapy (TRT), partly due to the fact that it emits photons that can be imaged to perform treatment planning or posttherapy dose verification as well as beta rays that are suitable for therapy. In both the treatment planning and dose verification applications, it is necessary to estimate the activity distribution in organs or tumors at several time points. In vivo estimates of the 131I activity distribution at each time point can be obtained from quantitative single-photon emission computed tomography (QSPECT) images and organ activity estimates can be obtained either from QSPECT images or quantification of planar projection data. However, in addition to the photon used for imaging, 131I decay results in emission of a number of other higher-energy photons with significant abundances. These higher-energy photons can scatter in the body, collimator, or detector and be counted in the 364 keV photopeak energy window, resulting in reduced image contrast and degraded quantitative accuracy; these photons are referred to as downscatter. The goal of this study was to develop and evaluate a model-based downscatter compensation method specifically designed for the compensation of high-energy photons emitted by 131I and detected in the imaging energy window.

METHODS

In the evaluation study, we used a Monte Carlo simulation (MCS) code that had previously been validated for other radionuclides. Thus, in preparation for the evaluation study, we first validated the code for 131I imaging simulation by comparison with experimental data. Next, we assessed the accuracy of the downscatter model by comparing downscatter estimates with MCS results. Finally, we combined the downscatter model with iterative reconstruction-based compensation for attenuation (A) and scatter (S) and the full (D) collimator-detector response of the 364 keV photons to form a comprehensive compensation method. We evaluated this combined method in terms of quantitative accuracy using the realistic 3D NCAT phantom and an activity distribution obtained from patient studies. We compared the accuracy of organ activity estimates in images reconstructed with and without addition of downscatter compensation from projections with and without downscatter contamination.

RESULTS

We observed that the proposed method provided substantial improvements in accuracy compared to no downscatter compensation and had accuracies comparable to reconstructions from projections without downscatter contamination.

CONCLUSIONS

The results demonstrate that the proposed model-based downscatter compensation method is effective and may have a role in quantitative 131I imaging.

Three-dimensional radiobiological dosimetry (3D-RD) with 124I PET for 131I therapy of thyroid cancer

Radioiodine therapy of thyroid cancer was the first and remains among the most successful radiopharmaceutical (RPT) treatments of cancer although its clinical use is based on imprecise dosimetry. The positron emitting radioiodine, (124)I, in combination with positron emission tomography (PET)/CT has made it possible to measure the spatial distribution of radioiodine in tumors and normal organs at high resolution and sensitivity. The CT component of PET/CT has made it simpler to match the activity distribution to the corresponding anatomy. These developments have facilitated patient-specific dosimetry (PSD), utilizing software packages such as three-dimensional radiobiological dosimetry (3D-RD), which can account for individual patient differences in pharmacokinetics and anatomy. We highlight specific examples of such calculations and discuss the potential impact of (124)I PET/CT on thyroid cancer therapy.

A pretherapy biodistribution and dosimetry study of indium-111-radiolabeled trastuzumab in patients with human epidermal growth factor receptor 2-overexpressing breast cancer

PURPOSE

The purposes of this study were to evaluate the organ biodistribution, pharmacokinetics, immunogenicity, and tumor uptake of (111)Indium ((111)In)-MxDTPA-trastuzumab in patients with human epidermal growth factor receptor 2 (HER2)-overexpressing breast cancers and to determine whether (90)Y-MxDTPA-trastuzumab should be evaluated in subsequent clinical therapy trials.

EXPERIMENTAL DESIGN

Patients with HER2-overexpressing breast cancers who were to undergo planned trastuzumab therapy first received unlabeled trastuzumab (4-8 mg/kg IV), followed 4 hours later by 5 mCi (111)In-MxDTPA-trastuzumab (10 mg antibody). Serial blood samples, 24-hour urine collections, and nuclear scans were performed at defined time points for 7 days.

RESULTS

Eight (8) patients received (111)In-MxDTPA-trastuzumab, which was well tolerated with no adverse side-effects. Three (3) of 7 patients with known lesions demonstrated positive imaging on nuclear scans. No antiantibody responses were observed for 2 months postinfusion. Organ doses (cGy/mCi) assuming radiolabeling with (90)Y were 19.9 for heart wall, 17.6 for liver, 4.6 for red marrow, and 2.8 for the whole body. Tumor doses ranged from 24 to 172 cGy/mCi.

CONCLUSIONS

In summary, results from this study indicate that (90)Y-MxDTPA-trastuzumab is an appropriate agent to evaluate in therapy trials. No evidence of an immune response to (111)In-MxDTPA-trastuzumab was detected, predicting for the ability to administer multiple cycles. With the exception of cardiac uptake, pharmacokinetics and organ biodistribution were comparable to other (90)Y-labeled monoclonal antibodies previously evaluated in the clinic. Cardiac uptake was comparable to hepatic uptake and therefore predicted to not be prohibitively high as to result in dose-limiting cardiotoxicity.

The effect of volume-of-interest misregistration on quantitative planar activity and dose estimation

In targeted radionuclide therapy (TRT), dose estimation is essential for treatment planning and tumor dose response studies. Dose estimates are typically based on a time series of whole-body conjugate view planar or SPECT scans of the patient acquired after administration of a planning dose. Quantifying the activity in the organs from these studies is an essential part of dose estimation. The quantitative planar (QPlanar) processing method involves accurate compensation for image degrading factors and correction for organ and background overlap via the combination of computational models of the image formation process and 3D volumes of interest defining the organs to be quantified. When the organ VOIs are accurately defined, the method intrinsically compensates for attenuation, scatter and partial volume effects, as well as overlap with other organs and the background. However, alignment between the 3D organ volume of interest (VOIs) used in QPlanar processing and the true organ projections in the planar images is required. The aim of this research was to study the effects of VOI misregistration on the accuracy and precision of organ activity estimates obtained using the QPlanar method. In this work, we modeled the degree of residual misregistration that would be expected after an automated registration procedure by randomly misaligning 3D SPECT/CT images, from which the VOI information was derived, and planar images. Mutual information-based image registration was used to align the realistic simulated 3D SPECT images with the 2D planar images. The residual image misregistration was used to simulate realistic levels of misregistration and allow investigation of the effects of misregistration on the accuracy and precision of the QPlanar method. We observed that accurate registration is especially important for small organs or ones with low activity concentrations compared to neighboring organs. In addition, residual misregistration gave rise to a loss of precision in the activity estimates that was on the order of the loss of precision due to Poisson noise in the projection data. These results serve as a lower bound on the effects of misregistration on the accuracy and precision of QPlanar activity estimate and demonstrate that misregistration errors must be taken into account when assessing the overall precision of organ dose estimates.

The impact of 3D volume of interest definition on accuracy and precision of activity estimation in quantitative SPECT and planar processing methods

Accurate and precise estimation of organ activities is essential for treatment planning in targeted radionuclide therapy. We have previously evaluated the impact of processing methodology, statistical noise and variability in activity distribution and anatomy on the accuracy and precision of organ activity estimates obtained with quantitative SPECT (QSPECT) and planar (QPlanar) processing. Another important factor impacting the accuracy and precision of organ activity estimates is accuracy of and variability in the definition of organ regions of interest (ROI) or volumes of interest (VOI). The goal of this work was thus to systematically study the effects of VOI definition on the reliability of activity estimates. To this end, we performed Monte Carlo simulation studies using randomly perturbed and shifted VOIs to assess the impact on organ activity estimates. The 3D NCAT phantom was used with activities that modeled clinically observed (111)In ibritumomab tiuxetan distributions. In order to study the errors resulting from misdefinitions due to manual segmentation errors, VOIs of the liver and left kidney were first manually defined. Each control point was then randomly perturbed to one of the nearest or next-nearest voxels in three ways: with no, inward or outward directional bias, resulting in random perturbation, erosion or dilation, respectively, of the VOIs. In order to study the errors resulting from the misregistration of VOIs, as would happen, e.g. in the case where the VOIs were defined using a misregistered anatomical image, the reconstructed SPECT images or projections were shifted by amounts ranging from -1 to 1 voxels in increments of with 0.1 voxels in both the transaxial and axial directions. The activity estimates from the shifted reconstructions or projections were compared to those from the originals, and average errors were computed for the QSPECT and QPlanar methods, respectively. For misregistration, errors in organ activity estimations were linear in the shift for both the QSPECT and QPlanar methods. QPlanar was less sensitive to object definition perturbations than QSPECT, especially for dilation and erosion cases. Up to 1 voxel misregistration or misdefinition resulted in up to 8% error in organ activity estimates, with the largest errors for small or low uptake organs. Both types of VOI definition errors produced larger errors in activity estimates for a small and low uptake organs (i.e. -7.5% to 5.3% for the left kidney) than for a large and high uptake organ (i.e. -2.9% to 2.1% for the liver). We observed that misregistration generally had larger effects than misdefinition, with errors ranging from -7.2% to 8.4%. The different imaging methods evaluated responded differently to the errors from misregistration and misdefinition. We found that QSPECT was more sensitive to misdefinition errors, but less sensitive to misregistration errors, as compared to the QPlanar method. Thus, sensitivity to VOI definition errors should be an important criterion in evaluating quantitative imaging methods.

Effects of shortened acquisition time on accuracy and precision of quantitative estimates of organ activity

PURPOSE

Quantitative estimation of in vivo organ uptake is an essential part of treatment planning for targeted radionuclide therapy. This usually involves the use of planar or SPECT scans with acquisition times chosen based more on image quality considerations rather than the minimum needed for precise quantification. In previous simulation studies at clinical count levels (185 MBq 111In), the authors observed larger variations in accuracy of organ activity estimates resulting from anatomical and uptake differences than statistical noise. This suggests that it is possible to reduce the acquisition time without substantially increasing the variation in accuracy.

METHODS

To test this hypothesis, the authors compared the accuracy and variation in accuracy of organ activity estimates obtained from planar and SPECT scans at various count levels. A simulated phantom population with realistic variations in anatomy and biodistribution was used to model variability in a patient population. Planar and SPECT projections were simulated using previously validated Monte Carlo simulation tools. The authors simulated the projections at count levels approximately corresponding to 1.5-30 min of total acquisition time. The projections were processed using previously described quantitative SPECT (QSPECT) and planar (QPlanar) methods. The QSPECT method was based on the OS-EM algorithm with compensations for attenuation, scatter, and collimator-detector response. The QPlanar method is based on the ML-EM algorithm using the same model-based compensation for all the image degrading effects as the QSPECT method. The volumes of interests (VOIs) were defined based on the true organ configuration in the phantoms. The errors in organ activity estimates from different count levels and processing methods were compared in terms of mean and standard deviation over the simulated phantom population.

RESULTS

There was little degradation in quantitative reliability when the acquisition time was reduced by half for the QSPECT method (the mean error changed by < 1%, e.g., 0.9%-0.3% = 0.6% for the spleen). The magnitude of the errors and variations in errors for large organ with high uptake were still acceptable for 1.5 min scans, even though the errors were slightly larger than those for the 30 min scans (i.e., < 2% for liver, < 3% for heart). The errors over the ranges of scan times studied for the QPlanar method were all within 0.3% for all organs.

CONCLUSIONS

These data indicate that, for the purposes of organ activity estimation, acquisition times could be reduced at least by a factor of 2 for the QSPECT and QPlanar methods with little effect on the errors in organ activity estimates. The acquisition time can be further reduced for the QPlanar method, assuming well-registered VOIs are available and the activity distribution in organs can be treated as uniform. Although the differences in accuracy and precision were statistically significant for all the acquisition times shorter than 30 min, the magnitude of the changes in accuracy and precision were small and likely not clinically important. The reduction in SPECT acquisition time justified by this study makes the use of SPECT a more clinically practical alternative to conventional planar scanning for targeted radiotherapy treatment planning.

A gamma camera count rate saturation correction method for whole-body planar imaging

Whole-body (WB) planar imaging has long been one of the staple methods of dosimetry, and its quantification has been formalized by the MIRD Committee in pamphlet no 16. One of the issues not specifically addressed in the formalism occurs when the count rates reaching the detector are sufficiently high to result in camera count saturation. Camera dead-time effects have been extensively studied, but all of the developed correction methods assume static acquisitions. However, during WB planar (sweep) imaging, a variable amount of imaged activity exists in the detector's field of view as a function of time and therefore the camera saturation is time dependent. A new time-dependent algorithm was developed to correct for dead-time effects during WB planar acquisitions that accounts for relative motion between detector heads and imaged object. Static camera dead-time parameters were acquired by imaging decaying activity in a phantom and obtaining a saturation curve. Using these parameters, an iterative algorithm akin to Newton's method was developed, which takes into account the variable count rate seen by the detector as a function of time. The algorithm was tested on simulated data as well as on a whole-body scan of high activity Samarium-153 in an ellipsoid phantom. A complete set of parameters from unsaturated phantom data necessary for count rate to activity conversion was also obtained, including build-up and attenuation coefficients, in order to convert corrected count rate values to activity. The algorithm proved successful in accounting for motion- and time-dependent saturation effects in both the simulated and measured data and converged to any desired degree of precision. The clearance half-life calculated from the ellipsoid phantom data was calculated to be 45.1 h after dead-time correction and 51.4 h with no correction; the physical decay half-life of Samarium-153 is 46.3 h. Accurate WB planar dosimetry of high activities relies on successfully compensating for camera saturation which takes into account the variable activity in the field of view, i.e. time-dependent dead-time effects. The algorithm presented here accomplishes this task.

Impact of rituximab treatment on (90)Y-ibritumomab dosimetry for patients with non-Hodgkin lymphoma

To determine whether the therapeutic effect of (90)Y-ibritumomab might be enhanced by a full course of rituximab followed by single dose of (90)Y-ibritumomab, the trial included pre- and post-rituximab treatment imaging with (111)In-ibritumomab and blood pharmacokinetics. Comparison of the pre- and post-rituximab imaging and blood data allowed for the assessment of impact of rituximab on (90)Y-ibritumomab dosimetry.

METHODS

Seventeen patients with relapsed B cell non-Hodgkin lymphoma first received 250 mg/m(2) of rituximab plus 185 MBq of (111)In-ibritumomab for initial dosimetry evaluation. In weeks 2-4, patients received 3 weekly 375 mg/m(2) doses of rituximab. In week 6, patients received a 250 mg/m(2) dose of rituximab plus 185 MBq of (111)In-ibritumomab for a second dosimetry evaluation. Five sequential, whole-body gamma-camera images were acquired after the (111)In-ibritumomab injection. Calculated radiation doses were based on patient-specific organ masses. For each patient, paired comparison of radiation doses before and after rituximab treatment was performed. Paired comparison of residence times for spleen and tumor was also performed.

RESULTS

Before rituximab treatment, the median radiation dose (mGy/MBq) was 0.48 (range, 0.24-0.86) for total body, 3.7 (range, 2.1-11.6) for liver, 6.1 (range, 1.8-17.8) for spleen, 3.3 (range, 2.0-4.7) for kidneys, 2.4 (range, 1.3-3.7) for heart wall, 1.1 (range, 0.4-2.3) for lungs, 0.79 (range, 0.32-1.22) for marrow from blood, and 18.1 (range, 4.7-98.9) for tumor. Paired comparisons were performed in 16 patients only because human antimurine antibody developed in 1 patient. The median change was 0.007 mGy/MBq for body, -0.14 mGy/MBq for liver, -0.31 mGy/MBq for kidneys, 0.38 mGy/MBq for heart wall, -0.17 mGy/MBq for lungs, and 0.046 mGy/MBq for marrow from blood. The median change in residence time was -0.92 h for spleen and -0.24 h for tumor. The changes were statistically insignificant for total body, liver, kidneys, lungs, and marrow from blood. The median residence times, or mGy/MBq if there were no volume changes, decreased 24% for spleen (P = 0.0005) and 28% for tumor (P = 0.005). The median radiation dose to heart wall increased 16%, which was statistically significant (P = 0.002).

CONCLUSION

Changes in (90)Y-ibritumomab dosimetry after 4 wk of rituximab treatment were not significant for most organs, except for the heart wall. The reduction of spleen and tumor residence times is more likely to be due to the therapeutic effects of rituximab.

Evaluation of a method for activity estimation in Sm-153 EDTMP imaging

Absolute activity evaluation is fundamental for internal radionuclide dosimetry when patient-specific therapy optimization is wanted. Often, quantification is attempted with 3D SPECT image based (IB) methods, but the true concentration values can be underestimated due to the partial volume effect (PVE). This is especially true when small diffuse lesions are present. In this paper, we describe a 3D region of interest (ROI) based quantification method (LS-ROI), which estimates the ROI concentration values directly from the projection data acquired in the tomographic scan once ROIs have been segmented on a CT and/or a SPECT image. The method, which has inherent PVE correction capabilities, was applied both on simulated and on real phantom data. Simulations reflected the case of a patient with bone metastases treated with 153Sm-EDTMP: Both the activity in the metastases and the total retention in the skeleton were evaluated. Thirty noisy data sets were produced in order to evaluate the accuracy and precision of the method. The effect of region segmentation errors on estimated concentrations was thoroughly investigated. Real data were acquired on a NEMA phantom, where a cylindrical central region (283 cm3) simulated the bone and two spheres (10.3 and 25.5 cm3) simulated the metastases. The results obtained with the LS-ROI method were compared with those of a conventional 3D IB method and those of a quantitative conjugate view approach derived from LS-ROI and applied to the anterior and posterior views acquired in the tomographic scan (LS-ROI anterior-posterior: LS-ROI-AP). Simulations showed that when the geometry of regions is known, the LS-ROI method recovered the simulated concentration values within 20%, while the IB method underestimated the concentration in high activity small lesions by as much as 49%. Segmentation errors, up to 44% of the true region volume, produced a higher variation in LS-ROI estimates than in IB ones; however, the overall bias of the LS-ROI estimates (< or = 25%) remained lower than that of IB estimates. In the case of the evaluation of the total retention in the skeleton, the LS-ROI method recovered the simulated value within 2%, while IB underestimated it up to 13%. In all the cases, the LS-ROI-AP method showed an accuracy comparable with that of the LS-ROI one, and a worse precision just because of the lower number of counts used in the analysis. However, a worsening of LS-ROI-AP performances was demonstrated in the case of strong overlap of regions: In this case, a bias of up to 40% was observed. The results obtained on real phantom data confirmed the simulation results: The IB method underestimated activity up to 47% in the smallest sphere, while the bias was reduced to 13% with LS-ROI and LS-ROI-AP estimates. The good quantification capabilities of the LS-ROI method can be useful for absolute activity quantification in the case of small active diffused lesions and constitute the basis for the development of an accurate patient-specific planning strategy in internal radionuclide treatments, provided there is a reliable segmentation of lesions.

Comparison of organ residence time estimation methods for radioimmunotherapy dosimetry and treatment planning--patient studies

The estimation of organ residence time is essential for high-dose myeloablative regimens in radioimmunotherapy (RIT). Frequently, this estimation is based on a series of simple planar scans and planar processing. The authors previously performed a simulation study which demonstrated that the accuracy of this methodology is limited compared to a hybrid planar/SPECT residence time estimation method. In this work the authors applied this hybrid method to data from a clinical trial of high-dose myeloablative yttrium-90 ibritumomab tiuxetan therapy. Image data acquired from 18 patients were comprised of planar scans at five time points ranging from 1 to 144 h postinjection and abdominal and thoracic SPECT/CT scans obtained at 24 h postinjection. The simple planar processing method used in this work was based on the geometric mean method with energy window based scatter compensation. No explicit background subtraction nor object or source thickness corrections were performed. The SPECT projections were reconstructed using iterative reconstruction with compensations for attenuation, scatter, and full collimator-detector response. Large differences were observed when residence times were estimated using the simple planar method compared to the hybrid method. The differences were not constant but varied in magnitude and sign. For the dose-limiting organ (liver), the average difference was -18% and variation in the difference was 19%, similar to the differences observed in a previously reported simulation study. The authors also looked at the relationship between the weight of the patient and the liver residence time and found that there was no meaningful correlation for either method. This indicates that weight would not be an adequate proxy for an experimental estimate of residence time when choosing the activity to administer for therapy. The authors conclude that methods such as the simple planar method used here are inadequate for RIT treatment planning. More sophisticated methods, such as the hybrid SPECT/planar method investigated here, are likely to be better predictors of organ dose and, as a result, organ toxicities.

A phase I study of a combination of yttrium-90-labeled anti-carcinoembryonic antigen (CEA) antibody and gemcitabine in patients with CEA-producing advanced malignancies

PURPOSE

To determine the maximum tolerated dose of combined therapy using an yttrium-90-labeled anti-carcinoembryonic antigen (CEA) antibody with gemcitabine in patients with advanced CEA-producing solid tumors.

EXPERIMENTAL DESIGN

The chimeric human/murine cT84.66 is an anti-CEA intact IgG1, with high affinity and specificity to CEA. This was given at a fixed yttrium-90-labeled dose of 16.6 mCi/m(2) to subjects who had and an elevated CEA in serum or in tumor by immunohistochemistry. Also required was a tumor that imaged with an (111)In-labeled cT84.66 antibody. Patients were treated with escalating doses of gemcitabine given i.v. over 30 minutes on day 1 and 3 after the infusion of the yttrium-90-labeled antibody. Patients were treated in cohorts of 3. The maximum tolerated dose was determined as the highest level at which no >1 of 6 patients experienced a dose limiting toxicity.

RESULTS

A total of 36 patients were enrolled, and all but one had prior systemic therapy. The maximum tolerated dose of gemcitabine in this combination was 150 mg/m(2). Dose limiting toxicities at a gemcitabine dose of 165 mg/m(2) included a grade 3 rash and grade 4 neutropenia. One partial response was seen in a patient with colorectal cancer, and 4 patients had a >50% decrease in baseline CEA levels associated with stable disease. Human antichimeric antibody responses were the primary reason for stopping treatment in 12 patients.

CONCLUSIONS

Feasibility of combining gemcitabine with an yttrium-90-labeled anti-CEA antibody is shown with preliminary evidence of clinical response.

MIRD dose estimate report No. 20: radiation absorbed-dose estimates for 111In- and 90Y-ibritumomab tiuxetan

Absorbed-dose calculations provide a scientific basis for evaluating the biologic effects associated with administered radiopharmaceuticals. In cancer therapy, radiation dosimetry supports treatment planning, dose-response analyses, predictions of therapy effectiveness, and completeness of patient medical records. In this study, we evaluated the organ radiation absorbed doses from intravenously administered (111)In- and (90)Y-ibritumomab tiuxetan.

METHODS

Ten patients (6 men and 4 women) with non-Hodgkin lymphoma, cared for at 3 different medical centers, were administered the tracer (111)In-ibritumomab tiuxetan and assessed using planar scintillation camera imaging at 5 time points and CT-organ volumetrics to determine patient-specific organ biokinetics and dosimetry. Explicit attenuation correction based on the transmission scan or transmission measurements provided the fraction of (111)In-administered activity in 7 major organs, the whole body, and remainder tissues over time through complete decay. Time-activity curves were constructed, and radiation doses were calculated using MIRD methods and implementing software.

RESULTS

Mean radiation absorbed doses for (111)In- and for (90)Y-ibritumomab tiuxetan administered to 10 cancer patients are reported for 24 organs and the whole body. Biologic uptake and retention data are given for 7 major source organs, remainder tissues, and the whole body. Median absorbed dose values calculated by this method were compared with previously published dosimetry for ibritumomab tiuxetan and the product package insert.

CONCLUSION

In high-dose radioimmunotherapy, the importance of patient-specific dosimetry becomes obvious when the objective of treatment planning is to achieve disease cures, safely, by limiting radiation dose to any critical normal organ to its maximum tolerable value. Compared with the current package insert, we found differences in median absorbed dose by multiples of 24 in the kidneys, 1.8 in the red marrow, 0.65 in the liver, 0.077 in the intestinal wall, 0.30 in the lungs, 0.46 in the spleen, and 0.34 in the heart wall.

Evaluation of quantitative imaging methods for organ activity and residence time estimation using a population of phantoms having realistic variations in anatomy and uptake

Estimating organ residence times is an essential part of patient-specific dosimetry for radioimmunotherapy (RIT). Quantitative imaging methods for RIT are often evaluated using a single physical or simulated phantom but are intended to be applied clinically where there is variability in patient anatomy, biodistribution, and biokinetics. To provide a more relevant evaluation, the authors have thus developed a population of phantoms with realistic variations in these factors and applied it to the evaluation of quantitative imaging methods both to find the best method and to demonstrate the effects of these variations. Using whole body scans and SPECT/CT images, organ shapes and time-activity curves of 111In ibritumomab tiuxetan were measured in dosimetrically important organs in seven patients undergoing a high dose therapy regimen. Based on these measurements, we created a 3D NURBS-based cardiac-torso (NCAT)-based phantom population. SPECT and planar data at realistic count levels were then simulated using previously validated Monte Carlo simulation tools. The projections from the population were used to evaluate the accuracy and variation in accuracy of residence time estimation methods that used a time series of SPECT and planar scans, Quantitative SPECT (QSPECT) reconstruction methods were used that compensated for attenuation, scatter, and the collimator-detector response. Planar images were processed with a conventional (CPlanar) method that used geometric mean attenuation and triple-energy window scatter compensation and a quantitative planar (QPlanar) processing method that used model-based compensation for image degrading effects. Residence times were estimated from activity estimates made at each of five time points. The authors also evaluated hybrid methods that used CPlanar or QPlanar time-activity curves rescaled to the activity estimated from a single QSPECT image. The methods were evaluated in terms of mean relative error and standard deviation of the relative error in the residence time estimates taken over the phantom population. The mean errors in the residence time estimates over all the organs were < 9.9% (pure QSPECT), < 13.2% (pure QPLanar), < 7.2% (hybrid QPlanar/QSPECT), < 19.2% (hybrid CPlanar/QSPECT), and 7%-159% (pure CPlanar). The standard deviations of the errors for all the organs over all the phantoms were < 9.9%, < 11.9%, < 10.8%, < 22.0%, and < 107.9% for the same methods, respectively. The processing methods differed both in terms of their average accuracy and the variation of the accuracy over the population of phantoms, thus demonstrating the importance of using a phantom population in evaluating quantitative imaging methods. Hybrid CPlanar/QSPECT provided improved accuracy compared to pure CPlanar and required the addition of only a single SPECT acquisition. The QPlanar or hybrid QPlanar/QSPECT methods had mean errors and standard deviations of errors that approached those of pure QSPECT while providing simplified image acquisition protocols, and thus may be more clinically practical.

Anniversary paper: nuclear medicine: fifty years and still counting

The history, present status, and possible future of nuclear medicine are presented. Beginning with development of the rectilinear scanner and gamma camera, evolution to the present forms of hybrid technology such as single photon emission computed tomography/computed tomography (CT) and positron emission tomography/CT is described. Both imaging and therapy are considered and the recent improvements in dose estimation using hybrid technologies are discussed. Future developments listed include novel radiopharmaceuticals created using short chains of nucleic acids and varieties of nanostructures. Patient-specific radiotherapy is an eventual outcome of this work. Possible application to proving the targeting of potential chemotherapeutics is also indicated.

Comparison of residence time estimation methods for radioimmunotherapy dosimetry and treatment planning--Monte Carlo simulation studies

Estimating the residence times in tumor and normal organs is an essential part of treatment planning for radioimmunotherapy (RIT). This estimation is usually done using a conjugate view whole body scan time series and planar processing. This method has logistical and cost advantages compared to 3-D imaging methods such as Single photon emission computed tomography (SPECT), but, because it does not provide information about the 3-D distribution of activity, it is difficult to fully compensate for effects such as attenuation and background and overlapping activity. Incomplete compensation for these effects reduces the accuracy of the residence time estimates. In this work we compare residence times estimates obtained using planar methods to those from methods based on quantitative SPECT (QSPECT) reconstructions. We have previously developed QSPECT methods that provide compensation for attenuation, scatter, collimator-detector response, and partial volume effects. In this study we compared the use of residence time estimation methods using QSPECT to planar methods. The evaluation was done using the realistic NCAT phantom with organ time activities that model (111)In ibritumomab tiuxetan. Projection data were obtained using Monte Carlo simulations (MCS) that realistically model the image formation process including penetration and scatter in the collimator-detector system. These projection data were used to evaluate the accuracy of residence time estimation using a time series of QSPECT studies, a single QSPECT study combined with planar scans and the planar scans alone. The errors in the residence time estimates were 3.8%, 15%, and 2%-107% for the QSPECT, hybrid planar/QSPECT, and planar methods, respectively. The quantitative accuracy was worst for pure planar processing and best for pure QSPECT processing. Hybrid planar/QSPECT methods, where a single QSPECT study was combined with a series of planar scans, provided a large and statistically significant improvement in quantitative accuracy for most organs compared to the planar scans alone, even without sophisticated attention to background subtraction or thickness corrections in planar processing. These results indicate that hybrid planar/QSPECT methods are generally superior to pure planar methods and may be an acceptable alternative to performing a time series of QSPECT studies.

Radioiodine in the treatment of thyroid cancer

This article presents an overview of the use of radioactive iodine (131-I) in the treatment of patients who have well differentiated thyroid cancer. We review definitions; staging; the two-principal methods for selection of a dosage of 131-I for ablation and treatment; the objectives of ablation and treatment; the indications for ablation and treatment; the recommendations for the use of 131-I for ablation and treatment contained in the Guidelines of the American Thyroid Association, the European Consensus, the Society of Nuclear Medicine, and the European Association of Nuclear Medicine; the dosage recommendations and selection of dosage for 131-I by the these organizations; and the Washington Hospital Center approach.

Software package for integrated data processing for internal dose assessment in nuclear medicine (SPRIND)

PURPOSE

Internal radiation dose calculations are normally carried out using the Medical Internal Radiation Dose (MIRD) schema. This requires residence times of radiopharmaceutical activity and S-values for all organs of interest. Residence times can be obtained by quantitative nuclear imaging modalities. For dealing with S-values, the freeware packages MIRDOSE and, more recently, OLINDA/EXM are available. However, these software packages do not calculate residence times from image data.

METHODS AND RESULTS

For this purpose, we developed an IDL-based software package for integrated data processing for internal dose assessment in nuclear medicine (SPRIND). SPRIND allows reading and viewing of planar whole-body scintigrams. Organ and background regions of interest (ROIs) can be drawn and are automatically mirrored from the anterior to the posterior view. ROI statistics are used to obtain anterior-posterior averaged counts for each organ, corrected for background activity and attenuation. Residence times for each organ are calculated based on effective decay. The total body biological half-time is calculated for use in the voiding bladder model. Red bone marrow absorbed dose can be calculated using bone regions in the scintigrams or by a blood-derived method. Finally, the results are written to a file in MIRDOSE-OLINDA/EXM format. Using scintigrams in DICOM, the complete analysis is gamma camera vendor independent, and can be performed on any computer using an IDL virtual machine.

CONCLUSION

SPRIND is an easy-to-use software package for radiation dose assessment studies. It has made these studies less time consuming and less error prone.

Comparison of conventional, model-based quantitative planar, and quantitative SPECT image processing methods for organ activity estimation using In-111 agents

Accurate quantification of organ radionuclide uptake is important for patient-specific dosimetry. The quantitative accuracy from conventional conjugate view methods is limited by overlap of projections from different organs and background activity, and attenuation and scatter. In this work, we propose and validate a quantitative planar (QPlanar) processing method based on maximum likelihood (ML) estimation of organ activities using 3D organ VOIs and a projector that models the image degrading effects. Both a physical phantom experiment and Monte Carlo simulation (MCS) studies were used to evaluate the new method. In these studies, the accuracies and precisions of organ activity estimates for the QPlanar method were compared with those from conventional planar (CPlanar) processing methods with various corrections for scatter, attenuation and organ overlap, and a quantitative SPECT (QSPECT) processing method. Experimental planar and SPECT projections and registered CT data from an RSD Torso phantom were obtained using a GE Millenium VH/Hawkeye system. The MCS data were obtained from the 3D NCAT phantom with organ activity distributions that modelled the uptake of (111)In ibritumomab tiuxetan. The simulations were performed using parameters appropriate for the same system used in the RSD torso phantom experiment. The organ activity estimates obtained from the CPlanar, QPlanar and QSPECT methods from both experiments were compared. From the results of the MCS experiment, even with ideal organ overlap correction and background subtraction, CPlanar methods provided limited quantitative accuracy. The QPlanar method with accurate modelling of the physical factors increased the quantitative accuracy at the cost of requiring estimates of the organ VOIs in 3D. The accuracy of QPlanar approached that of QSPECT, but required much less acquisition and computation time. Similar results were obtained from the physical phantom experiment. We conclude that the QPlanar method, based on 3D organ VOIs and accurate models of the projection process, provided a substantial increase in accuracy of organ activity estimates from planar images compared to CPlanar processing and had accuracy approaching that of QSPECT.