This knowledge check has been designed to consolidate your understanding of fundamental principles and calculations used in radiation dosimetry. It focuses on both diagnostic and therapeutic applications of radiopharmaceuticals, as well as external radiation exposure scenarios.
Through a series of clinically relevant questions, you will review key concepts such as specific absorbed fraction, the MIRD schema, equivalent and effective dose, external dose rate calculations, and the use of dosimeters in radiation protection. Each question encourages you to apply theoretical knowledge to practical situations that arise in nuclear medicine and radiotherapy practice.
The aim of this exercise is not only to test recall of definitions and formulae but also to strengthen problem-solving skills in dose assessment and radiation safety. On completion, you should have greater confidence in interpreting dosimetric quantities, performing basic calculations, and understanding their relevance to patient care and occupational protection.
Scenario: Dosimetry in a Radiopharmaceutical Therapy and Imaging Study
Patient Case
Dr. Harris, a consultant in nuclear medicine, was supervising a group of postgraduate trainees during their clinical rotation at a large university hospital. The group had just received a referral for a 62-year-old male patient, Mr. Thompson, who had metastatic prostate cancer. He was scheduled to undergo both diagnostic imaging and subsequent radionuclide therapy as part of his treatment. The case provided an ideal opportunity for the trainees to review and apply their knowledge of dosimetry principles, calculations, and radiation safety procedures.
Diagnostic Imaging Stage
The first stage involved diagnostic imaging using F-18 fluorodeoxyglucose (FDG) to assess the metabolic activity of the lesions before therapy planning. Mr. Thompson was administered 150 MBq of F-18 FDG, and the trainees were asked to calculate the absorbed dose to the patient’s organs. Dr. Harris emphasised that the concept of “specific absorbed fraction” was central here, as it described the fraction of energy emitted in a source region that is absorbed in a target region. This parameter underpins the Medical Internal Radiation Dose (MIRD) methodology, which was the framework the team would follow.
Following the injection, a whole-body PET/CT scan was performed. The scanner reported an average whole-body dose of 5 mSv for the procedure. The trainees noted that this figure, although modest, had to be considered in the cumulative dose burden for the patient, since he was also scheduled to receive therapeutic activity. One of the students asked about the purpose of the MIRD schema, which led Dr. Harris to explain that it provides a standardised approach for translating administered activity into absorbed dose estimates, using factors such as cumulated activity, specific absorbed fractions, and S values.
Therapeutic Application
Later in the day, the discussion turned to the upcoming therapy with Ir-192 brachytherapy for metastatic lesions in the pelvic region. Dr. Harris reminded the trainees that the dose constant for Ir-192 is 4.8 R·cm²/mCi·h, a key factor in calculating external dose rates around the patient for radiation protection purposes. They were tasked with estimating the dose rate at 1 m from a source of known activity, reinforcing the importance of inverse square law considerations.
As the calculations proceeded, one trainee asked about the mathematical methods used in voxel-based internal dosimetry. Dr. Harris explained that Monte Carlo simulations were often employed, allowing for detailed modelling of radiation transport and energy deposition within heterogeneous anatomical structures. This was especially relevant for personalised dosimetry, where patient-specific CT data were used to define voxel geometries.
Decay and Dosimetry Factors
The conversation shifted to decay parameters. The group discussed how the effective half-life of a radionuclide most significantly impacts the time-integrated activity within a patient’s organ. Biological clearance and physical decay together determine the residence time of the radiopharmaceutical, which in turn affects the absorbed dose. For example, the clearance of F-18 FDG from the kidneys could influence the renal dose calculation.
Dr. Harris then presented a safety-related case study. If a 10 mCi Tc-99m source were accidentally dropped in a clinical environment, what would be the likely dose to staff if they remained close to the source for several minutes? This prompted the trainees to apply their knowledge of kerma (kinetic energy released in matter) and absorbed dose calculations. They recalled that the SI unit for kerma is the gray (Gy), and that dose assessments depend on photon energy fluence and the mass energy absorption coefficient of the exposed medium.
Radiation Protection and Monitoring
To reinforce external dosimetry, the group reviewed thermoluminescent dosimeters (TLDs), which all staff wore routinely. They discussed which factors affected TLD response, such as photon energy, angular dependence, and fading of the stored signal. Dr. Harris emphasised that correct calibration of TLDs was essential for accurate dose monitoring.
Turning back to the patient case, the trainees considered risk assessment. One question raised was: Which quantity is most relevant for assessing cancer risk from radiation exposure? Dr. Harris explained that the effective dose was the appropriate measure, as it incorporates both the equivalent dose to different tissues and their relative sensitivities (tissue weighting factors).
Applied Examples
To solidify their understanding, the group worked through an example: a patient received an administered activity of 3.7 GBq of a therapeutic radiopharmaceutical. They were asked to calculate the absorbed dose to a specific organ given known biokinetic and physical decay data. This exercise highlighted the importance of both cumulated activity and energy emitted per decay.
The conversation also explored committed effective dose, especially relevant in occupational exposures. Dr. Harris described a scenario where a worker accidentally inhaled 1 MBq of I-131. The group estimated the committed dose, noting that inhalation and ingestion pathways involved long-term residence of radionuclides within the body, making dose assessments more complex than external exposure.
To illustrate external exposure further, Dr. Harris gave another calculation: if a source emits 10⁶ photons per second with a specified energy, what would be the air kerma rate at a set distance? This exercise required the group to apply fluence rate, energy transfer, and geometric considerations.
The trainees also revisited the concept of equivalent dose, which is calculated by multiplying the absorbed dose by a radiation weighting factor (WR). This distinction was made clear when comparing exposures from photons (WR = 1) and alpha particles (WR = 20), stressing the different biological impacts of various radiations.
Finally, the group considered a voxel phantom simulation that produced an absorbed dose distribution across multiple organs for Mr. Thompson. They were asked: What is the absorbed dose in grays to a tissue receiving 2 J of energy from a mass of 1 kg? This basic physics exercise reminded them of the direct definition of the gray: joules per kilogram.
Session Conclusion
To close the session, Dr. Harris asked the group to consider which factors do not affect absorbed dose calculations. After discussion, the trainees concluded that while factors such as decay data, organ mass, and biokinetics are crucial, external shielding materials outside the body do not alter internal dose estimates once the radionuclide is administered.
By the end of the session, the students had not only worked through real patient data but also revisited fundamental dosimetry questions ranging from specific absorbed fraction to kerma, effective dose, equivalent dose, and committed effective dose. The clinical context of Mr. Thompson’s imaging and therapy had provided them with a rich framework to apply theoretical knowledge, linking each concept to practical scenarios encountered in nuclear medicine.
Transition to the Quiz
To reinforce these lessons, the trainees were then invited to complete a knowledge check quiz based on the scenario. The quiz tested their understanding of concepts such as specific absorbed fraction, the MIRD schema, external dose rate calculations, equivalent and effective dose, and the use of dosimeters.
The exercise was not intended as a formal examination but as a learning tool—an opportunity for self-assessment, reflection, and consolidation of knowledge. By working through the questions, the trainees could confirm their grasp of essential calculations, identify areas for further revision, and strengthen their ability to apply dosimetric principles in both diagnostic and therapeutic nuclear medicine practice.
Knowledge Check
Disclaimer
The Radiation in Action: A Clinical Dosimetry Challenge is an educational resource developed to facilitate the consolidation of theoretical knowledge and applied principles in radiation dosimetry. The material, including case scenarios, calculations, and discussion points, is intended solely for instructional purposes within academic and professional training settings.
This resource does not constitute clinical advice, treatment recommendations, or formal guidance on patient management. All clinical scenarios presented are illustrative in nature and are not based on individual patient records. Calculations and values provided are for learning purposes only and should not be applied directly to clinical decision-making.
Users are reminded that the safe and appropriate application of radiation dosimetry in practice requires adherence to established institutional protocols, professional standards, and applicable local, national, and international regulations.
No responsibility or liability is accepted by the authors, contributors, or affiliated institutions for any outcome arising from reliance on the content of this resource.
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