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Principles of Medical Internal Dosimetry in Clinical Practice

Tuesday 02/07/2023

Room: Memorial Union 49

13:00 - 17:00

Chair(s): Darrell Fisher

TPM-A.1  13:00  Medical Internal Radiation Dosimetry: Introduction and Overview. Liverett M*, Versant Medical Physics and Radiation Safety

Radiation dosimetry provides the fundamental quantities used for radiation protection, risk assessment, and treatment planning. Every practicing health physicist in medicine should become familiar with the general principles of medical internal dosimetry in clinical practice, even though one’s specific responsibilities may not directly involve the need to perform patient dose assessments. The increasing use of radiopharmaceuticals for diagnostic exams and therapeutic treatment requires patient-specific approaches to absorbed dose calculations using modern methods, models, and calculational tools. Absorbed dose calculations are performed prior to radionuclide therapy based on direct measurements of radionuclide uptake, retention, translocation, and clearance. However, the need for clinical internal dosimetry extends beyond the mere calculation of organ or tissue dose per unit administered activity. Internal dosimetry serves other fundamental purposes, including evaluation of agent safety and efficacy, as an information resource for patients, predicting short-term biological effects, evaluating dose-related biological endpoints, and correlating dose and response. Internal dosimetry also supports a system of complete patient medical records and helps to fulfill institutional legal and regulatory requirements.

TPM-A.2  13:20  Current Radiopharmaceuticals of Interest in Nuclear Medicine . Erwin W*, University of Texas M.D. Anderson Cancer Center

Both the number and variety of radiopharmaceuticals either approved for clinical use or in clinical trials continues to expand, particularly those related to cancer diagnosis and treatment. These radiopharmaceuticals may be labeled with either a gamma emitting radionuclide for planar gamma camera and SPECT imaging, a positron emitter for PET scanning, or a beta or alpha emitter for therapy. Radiopharmaceuticals fall into three classes: 1) diagnostic, 2) therapeutic and 3) so-called theranostic. The development of oncologic theranostic agents in particular, where the pharmaceutical is labeled with one radionuclide for imaging and another for therapy, is becoming common. The diagnostic and therapeutic radionuclides and radiopharmaceuticals that are currently of most interest, including those labeled with paired diagnostic and therapeutic radionuclides and emerging alpha-emitting agents, will be reviewed in this session.

TPM-A.3  13:50  Fundamental Principles of Internal Dosimetry for Health Physics Professionals . Fisher D*, Versant Medical Physics and Radiation Safety

This session reviews the mathematical formalism for calculating internal doses from patient imaging data. The Medical Internal Radiation Dose (MIRD) formalism provides standard methods, models, assumptions, and mathematical schema for assessing internal radiation doses from administered radiopharmaceuticals. In practice, this schema simplifies the problem of assessing dose for many different radionuclides—each with its unique radiological characteristics and chemical properties as labeled compounds—in the highly diverse biological environment represented by the human body, internal organs, tissues, fluid compartments, and cells. The major challenge in radiation dose assessment is to determine the time-dependent biokinetics of radionuclide uptake, retention, redistribution within, and excretion from the body. In clinical practice, direct patient measurements are obtained using calibrated imaging systems. Detected counts are translated to absolute activity resident in the major organs and tissues through complete decay. Dosimetry accounts for radionuclide nuclear emission properties, energy absorbed fractions, and the geometry and density of body tissues, and cross-organ irradiations. Stylized, voxel, and mesh human anatomical models have been developed to facilitate dose calculations. The virtue of the MIRD approach is that it systematically reduces complex dosimetric analyses to methods that are relatively simple to use, including software tools for experimental and clinical use.

TPM-A.4  14:30  Scientific Advances in Nuclear Medicine Imaging . Erwin William*, University of Texas M.D. Anderson Cancer Center

The science of nuclear medicine imaging has advanced tremendously over the past two decades, beginning with the approval of 18F-FDG PET for oncologic indications. Shortly thereafter, the first hybrid PET/CT scanner was introduced. SPECT/CT and PET/MR followed soon after PET/CT, and very recently, whole body PET/CT scanners have become available. Dedicated, small field-of-view cardiac and multi-element detector breast imaging devices were developed within that time frame as well. The introduction of new detector materials such as cadmium-zinc-telluride solid-state and lutetium orthosilicate scintillator, as well as silicon photomultipliers, occurred in tandem with scanner technology developments. Finally, activity quantification capability has been incorporated into both PET and SPECT imaging, not only for diagnosis but also for radiopharmaceutical therapy treatment planning dosimetry. These nuclear medicine imaging scientific advances and how they impact management of cancer patients will be discussed in this session.

TPM-A.5  15:00  Break.    

TPM-A.6  15:15  Quantitative measurement quality assurance . Liverett Misty*

Clinical nuclear medicine focuses on acquisition of high-quality diagnostic images. By comparison, quantitative imaging encompasses all the techniques needed for multiple (serial) data acquisition and interpreting medical images to determine absolute activities of the radionuclides in the organs and tissues of the body. Quantitative imaging requires precise preplanned standard processes, including equipment calibrations and set up, patient preparation, and calculation methods. Unlike qualitative imaging, the quantitative assessment process should maintain certain constants without modifications from patient to patient. Assessments include measurement quality assurance, attenuation correction, photon scatter correction, and background subtraction. Consequently, the nuclear medicine staff must be trained to understand the essential differences between conventional diagnostic imaging and quantitative data acquisition. Training should emphasize the importance of calibrations and standards, correction factors, and translating counts to absolute activity in the imageable organs and tissues.

TPM-A.7  15:45  Interpreting Time-activity Data . Fisher Darrell R*

Time-activity data plotted as percent or fraction of administered activity versus time post-infusion may suggest an organ or tissue time-activity curve represented by a mathematical function that may be fitted to the acquired data points. Although many different functional forms may fit the acquired time-activity data, the most common are single or double-exponentials. Integration of the area under the resulting time-activity curve through complete decay yields the time-integrated activity, that is, the total number of radioactive decays in the source organ. The time-integrated activity coefficient is the most important input to dosimetry software. This workshop session describes methods for (1) evaluating plotted time-activity data for organs and tissues, tumors, or the total-body activity, (2) fitting an appropriate mathematical function to these data, and (3) integrating the fitted time-activity function to determine total number of radioactive decays (time-integrated activity) as required for absorbed dose calculations.

TPM-A.8  15:55  Software Tools for Calculating Internal Radiation Dose . Fisher Darrell R*

Internal dosimetry calculations account for all radiation energy imparted to organs and tissues, including both self-organ dose and cross-organ dose contributions. These calculations are applied to human models representing male and female phantoms of many different ages and sizes. Given the many radionuclide choices available, and extensive differences in radiation emission properties, internal dosimetry becomes a computationally intensive effort that lends itself to computerization. Software solutions efficiently and conveniently implement the MIRD schema. Several new computer programs have been developed for use in medical internal dosimetry, both free to the user and commercially available. Some computer programs calculate time-integrated activities but have no dose-calculation functionality. The opposite is true for other software tools, while still others provide a complete suite of capabilities for tools for co-registering multiple clinical image formats at various timepoints to analyze biokinetic measurement data, compute time-integrated activity coefficients, and perform absorbed-dose calculations.

TPM-A.9  16:20  Roles and Responsibilities . Erwin William*

Clinical applications of medical internal dosimetry include those for clinical trials of investigational agents, standard-of-care nuclear medicine procedures (including hybrid), radiopharmaceutical misadministration, fetal and treatment planning. This final session will focus on the roles and responsibilities of the medical physicist, radiation safety officer and staff, and institutional review board, with regard to the above applications of internal dosimetry for both investigational and approved radiopharmaceuticals.

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