Quality Assurance Considerations in Radiopharmaceutical Therapy Dosimetry Using PLANETDose: An International Atomic Energy Agency Study.

Implementation of radiopharmaceutical therapy dosimetry varies depending on the clinical application, dosimetry protocol, software, and ultimately the operator. Assessing clinical dosimetry accuracy and precision is therefore a challenging task. This work emphasizes some pitfalls encountered during a structured analysis, performed on a single-patient dataset consisting of SPECT/CT images by various participants using a standard protocol and clinically approved commercial software. Methods: The clinical dataset consisted of the dosimetric study of a patient administered with [177Lu]Lu-DOTATATE at Tygerberg Hospital, South Africa, as a part of International Atomic Energy Agency-coordinated research project E23005. SPECT/CT images were acquired at 5 time points postinjection. Patient and calibration images were reconstructed on a workstation, and a calibration factor of 122.6 Bq/count was derived independently and provided to the participants. A standard dosimetric protocol was defined, and PLANETDose (version 3.1.1) software was installed at 9 centers to perform the dosimetry of 3 treatment cycles. The protocol included rigid image registration, segmentation (semimanual for organs, activity threshold for tumors), and dose voxel kernel convolution of activity followed by absorbed dose (AD) rate integration to obtain the ADs. Iterations of the protocol were performed by participants individually and within collective training, the results of which were analyzed for dosimetric variability, as well as for quality assurance and error analysis. Intermediary checkpoints were developed to understand possible sources of variation and to differentiate user error from legitimate user variability. Results: Initial dosimetric results for organs (liver and kidneys) and lesions showed considerable interoperator variability. Not only was the generation of intermediate checkpoints such as total counts, volumes, and activity required, but also activity-to-count ratio, activity concentration, and AD rate-to-activity concentration ratio to determine the source of variability. Conclusion: When the same patient dataset was analyzed using the same dosimetry procedure and software, significant disparities were observed in the results despite multiple sessions of training and feedback. Variations due to human error could be minimized or avoided by performing intensive training sessions, establishing intermediate checkpoints, conducting sanity checks, and cross-validating results across physicists or with standardized datasets. This finding promotes the development of quality assurance in clinical dosimetry.

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.

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.

Usefulness of low iodine diet in managing patients with differentiated thyroid cancer - initial results.

BACKGROUND: Low iodine diet (LID) is recommended in patients with differentiated thyroid cancer before radioiodine administration. Patients with increased thyroglobulin (Tg) level, but negative (131)I whole body scan present diagnostic and therapeutic dilemma. This study was designed to evaluate the benefit of a two-week LID in patients with elevated serum Tg levels and negative (131)I whole body scans. PATIENTS AND METHODS.: For the impact assessment of two-week LID on radioiodine tissue avidity, radioiodine scans before and after LID were compared. Sixteen patients with serum Tg > 2 µg/L, negative Tg-antibodies, and negative radioiodine scans underwent two-week LID before the (131)I administration. Fourteen patients underwent diagnostic scanning and two patients received radioiodine therapy. Iodine concentration in the morning urine specimens were measured in each patient, a day before and 15(th) day after starting LID. RESULTS: Following self-managed LID, patients were able to significantly reduce their iodine body content by 50% (range 28-65%, p<0,001). 13 patients (82%) accomplished mild iodine deficiency (50-99 µg/L) and one patient (6%) achieved targeted moderate iodine deficient state (<50 µg/L). All diagnostic post-LID scans were negative. Both post-therapy (131)I scans showed radioiodine accumulation outside of normal (131)I distribution (neck region and diffuse hepatic uptake). This study demonstrated that two-week LID is effective way to decrease total body iodine content, although without a visible effect on post-LID diagnostic (131)I scans. CONCLUSIONS: A more stringent dietary protocol and longer iodine restriction period are probably needed to achieve targeted moderate iodine deficiency in patients preparing for (131)I administration. This might result in higher radioiodine avidity of thyroid remnant/metastases.

Planning of 131I therapy for graves disease based on the radiation dose to thyroid follicular cells.

UNLABELLED: We evaluated the effects on the absorbed dose to thyroid follicular cells of self-absorption of (131)I radiation (specifically, beta-rays) in the follicular colloid. METHODS: Thyroid follicles were modeled as colloid-filled spheres, containing a uniform concentration of (131)I and surrounded by a concentric monolayer of cells. Assuming close packing of identical follicles, we used Monte Carlo simulation to assess the absorbed dose to follicular cells. RESULTS: Because of beta-ray self-absorption in colloidal spheres with radii larger than 50 mum, the absorbed dose to follicular cells is less than the average thyroid absorbed dose. CONCLUSION: For the same thyroid mass, radioiodine thyroid uptake, and effective half-life, patients with follicles with colloidal sphere radii of 100, 200, 300, and 400 microm should be administered 9%, 15%, 21%, and 30% more (131)I, respectively, than patients with colloidal sphere radii of less than 50 microm, to yield the same absorbed dose to follicular cells.