Radiation dosimetry is the science of measuring, calculating, and assessing the dose received by the human body from ionising radiation. In medical imaging and therapy, dosimetry ensures that patients receive the correct diagnostic or therapeutic dose while healthcare workers are kept within safe exposure limits.
This training scenario follows the Radiation Safety Officer (RSO) and staff at St. Mark’s University Hospital, where diagnostic radiology, nuclear medicine, and radiation therapy are part of daily practice. By examining how dosimetry principles are applied in real-world hospital settings, you will learn how concepts such as absorbed dose, equivalent dose, effective dose, dose limits, personal dosimetry, and retrospective monitoring support both patient safety and occupational protection.
At the end of this scenario, you will complete a knowledge check quiz. The questions are directly linked to the material described here, so keep note of the concepts as you progress.
Scenario: Radiation Monitoring at St. Mark’s Hospital
The principle of ALARA
At the start of every induction, the RSO explains the golden rule of radiation protection: the ALARA principle — “As Low As Reasonably Achievable.” This means that all radiation exposures must be kept as low as possible, taking into account economic and social factors, without compromising diagnostic image quality or therapeutic outcomes.
Radiographers and nuclear medicine technologists are reminded to use shielding, optimise scan parameters, and avoid unnecessary repeat exposures to ensure ALARA is maintained.
Dosimetry basics
The RSO reviews the key terms with new staff. Dosimetry refers to the measurement and calculation of the radiation dose absorbed by the body. The SI unit of absorbed dose is the Gray (Gy), which measures the energy deposited in tissue per kilogram of mass.
However, not all types of radiation have the same biological impact. To account for this, the absorbed dose is modified using radiation weighting factors to calculate equivalent dose, expressed in Sieverts (Sv). For X-rays, gamma rays, and beta particles, the weighting factor is 1, while for alpha particles it is 20, reflecting their higher biological effectiveness.
Beyond this, different organs and tissues have different sensitivities to radiation. To reflect this, tissue weighting factors are applied to calculate the effective dose. This allows estimation of the overall risk to the whole body from a non-uniform exposure.
Personal dosimeters
All radiation workers at St. Mark’s wear personal dosimeters. These small devices, clipped to the torso, measure cumulative occupational exposure over a set monitoring period.
The hospital primarily uses thermoluminescent dosimeters (TLDs), which contain lithium fluoride crystals. When exposed to radiation, electrons in the crystal lattice are trapped at higher energy states. On heating, they release light proportional to the dose received.
The dosimeters are exchanged every month. The RSO reviews results to ensure no staff exceed dose limits. For context, the recommended annual occupational dose limit for radiation workers is 20 mSv averaged over 5 years, with no single year exceeding 50 mSv.
One radiographer’s dosimeter showed an exposure of 0.2 mSv for the monitoring period — well below the limit and typical of diagnostic radiology staff when appropriate protection is used.
Deep dose equivalent
The RSO explains that dosimeter reports may include quantities such as deep dose equivalent (DDE). This refers to the dose equivalent at a tissue depth of 1 cm in soft tissue, representing exposure to penetrating radiation such as X-rays or gamma rays. Other quantities include lens dose equivalent and shallow dose equivalent for skin.
Ionisation chambers and survey meters
Besides personal dosimetry, the physics team uses field instruments. The ionisation chamber is the standard device for measuring radiation exposure in a calibration or survey setting. It measures the ionisation produced in a known volume of air, which directly relates to exposure. Ionisation chambers are particularly useful for accurate measurement of dose rates in diagnostic X-ray beams.
For quick checks of contamination or presence of radiation, a Geiger–Müller counter is used. However, the RSO notes its limitation: it cannot accurately measure high dose rates or provide reliable energy information. For low-level environmental radiation, a scintillation detector is more appropriate due to its higher sensitivity.
Internal dosimetry
In nuclear medicine, the RSO discusses internal dosimetry. Unlike external X-ray exposure, internal dosimetry concerns the intake of radioactive materials into the body. The primary concern is the committed dose to specific organs — for example, iodine-131 taken up by the thyroid. Patients receiving therapeutic radionuclides are carefully monitored, and staff are trained to minimise the risk of accidental intake.
Retrospective dosimetry
On rare occasions, retrospective dose assessments may be required after an accidental exposure. One technique used in such cases is retrospective dosimetry using tooth enamel or fingernails, which trap radiation-induced signals. More commonly, biological dosimetry involves analysing chromosomal aberrations in lymphocytes, which reflect the radiation dose received.
Radiation quality factors and weighting
The RSO explains the relative risks of different types of radiation. Alpha particles have the highest radiation weighting factor (20), reflecting their intense ionisation density and biological effect if deposited internally. Neutrons have variable weighting depending on energy, while X-rays, gamma rays, and beta particles all have a weighting factor of 1.
Monitoring a new CT installation
The hospital had recently installed a new multislice CT scanner. Before clinical use, the medical physics team performed extensive dosimetry checks. Phantom studies measured dose distributions and were compared with diagnostic reference levels. The team demonstrated how iterative reconstruction reduced noise and allowed lower tube current, exemplifying how dosimetry principles support ALARA.
Patient dose and radiosensitive organs
During quality checks, the physicists highlighted that CT contributes significantly to the population’s radiation dose. Organs such as the thyroid and gonads are particularly radiosensitive. Shielding and protocol optimisation are key to reducing unnecessary exposure.
They also explained that air kerma is the quantity describing the energy released in air by ionisation, while the exposure quantity is the measure of ionisation produced in air, traditionally expressed in coulombs per kilogram. These quantities are fundamental to linking machine output to patient dose.
Unit conversions and interpretation
Staff were reminded that the Sievert (Sv) is the unit of equivalent and effective dose, incorporating both radiation type and tissue sensitivity. Absorbed dose is in Grays, and exposure in air is measured in C/kg. Understanding these distinctions allows clinicians to interpret dose reports and regulatory limits correctly.
Reinforcing ALARA in daily work
Throughout the hospital, posters reminded staff of ALARA. Practical steps included:
- Using shielding such as lead aprons and thyroid collars.
- Collimating beams to restrict the field size.
- Using appropriate exposure factors.
- Avoiding repeat exposures unless clinically essential.
- Maintaining distance from radiation sources whenever possible.
By embedding these practices, St. Mark’s ensured both staff and patients were protected.
Conclusion
The St. Mark’s Hospital dosimetry scenario demonstrates how radiation measurement and monitoring safeguard both patients and staff. Concepts such as ALARA, absorbed dose, equivalent dose, effective dose, and dose limits are not abstract definitions but everyday tools that underpin safe imaging and therapy.
Devices like TLDs, ionisation chambers, and scintillation detectors provide accurate monitoring, while awareness of internal exposures and retrospective methods ensures preparedness for accidents. By applying these principles, the hospital achieves the right balance between clinical benefit and radiation safety.
Knowledge Check
You have now reviewed the essential principles of dosimetry through a hospital-based training scenario. The following knowledge check quiz is designed to test your understanding of key topics, including ALARA, dosimetry units, personal monitoring devices, deep dose equivalent, internal dosimetry, and radiation weighting factors.
Instruction: Select the best answer from the options provided. Refer back to the scenario if needed, and use it to guide your responses. Completing the quiz will help you consolidate your knowledge and ensure confidence in applying dosimetry principles to clinical practice.
Disclaimer
This training scenario has been created for educational purposes only. It is designed to support learning in radiation safety and dosimetry by illustrating core concepts in a clinical context. The content does not replace official regulations, institutional protocols, or professional judgement.
Radiation protection practices must always follow the standards and recommendations of recognised authorities such as the International Commission on Radiological Protection (ICRP), International Atomic Energy Agency (IAEA), and relevant national regulatory bodies. Learners and professionals are responsible for applying the correct procedures as defined by their institution, local legislation, and professional guidelines.
Any clinical examples in this scenario are fictional and intended solely to reinforce learning objectives.
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