10.5 Medical dosimetry

Medical dosimetry applies nuclear physics to measure and calculate radiation doses in medical settings. It ensures safe and effective use of ionizing radiation for diagnostic and therapeutic purposes, playing a crucial role in radiation oncology, nuclear medicine, and diagnostic radiology.

Key concepts include absorbed dose, equivalent dose, and effective dose. Units like Gray and Sievert quantify radiation exposure. Detection methods such as ionization chambers and thermoluminescent dosimeters form the foundation of dosimetry measurements, essential for treatment planning and radiation protection.

Principles of medical dosimetry

• Medical dosimetry applies nuclear physics principles to measure and calculate radiation doses in medical settings
• Ensures safe and effective use of ionizing radiation for diagnostic and therapeutic purposes
• Plays a crucial role in radiation oncology, nuclear medicine, and diagnostic radiology

Radiation dose concepts

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• Absorbed dose measures energy deposited per unit mass of tissue
• Equivalent dose accounts for biological effectiveness of different radiation types
• Effective dose considers radiation sensitivity of various organs and tissues
• Dose rate describes the amount of radiation delivered per unit time

Units of radiation measurement

• Gray (Gy) represents absorbed dose, defined as 1 joule of energy per kilogram of matter
• Sievert (Sv) measures equivalent and effective doses, accounting for biological impact
• Radiation absorbed dose (rad) equals 0.01 Gy, commonly used in older literature
• Roentgen equivalent man (rem) equals 0.01 Sv, used for equivalent dose in some countries

Absorbed dose vs effective dose

• Absorbed dose quantifies physical energy deposition in tissue
• Effective dose estimates overall health risk from radiation exposure
• Conversion from absorbed to effective dose involves tissue weighting factors
• Effective dose allows comparison of different types of radiation exposures

Radiation detection methods

• Radiation detection techniques form the foundation of medical dosimetry measurements
• Various detectors exploit different physical principles to quantify radiation interactions
• Selection of appropriate detector depends on radiation type, energy, and measurement purpose

Ionization chambers

• Gas-filled detectors measure ionization produced by radiation in a gas volume
• Consist of a chamber with two electrodes and a gas-filled sensitive volume
• Collect charge produced by radiation-induced ionization of gas molecules
• Provide accurate, real-time measurements of exposure and absorbed dose
• Used in radiation therapy beam calibration and quality assurance procedures

Thermoluminescent dosimeters

• Utilize crystalline materials that store energy from radiation exposure
• Release stored energy as light when heated, proportional to absorbed radiation dose
• Common materials include lithium fluoride and calcium fluoride
• Provide cumulative dose measurements over extended periods
• Widely used for personal dosimetry and environmental monitoring

Semiconductor detectors

• Employ solid-state materials (silicon or germanium) to detect ionizing radiation
• Radiation creates electron-hole pairs in the semiconductor material
• Offer high energy resolution and compact size compared to gas-filled detectors
• Used in spectroscopy applications and advanced imaging systems
• Silicon diodes find applications in in vivo dosimetry during radiotherapy

Dose calculation techniques

• Accurate dose calculations essential for treatment planning and radiation protection
• Combine physical models, patient anatomy, and radiation source characteristics
• Continual advancements in computational methods improve accuracy and efficiency

Monte Carlo simulations

• Stochastic method simulates individual particle interactions in matter
• Provides highly accurate dose distributions, especially in heterogeneous media
• Accounts for complex geometries and material compositions
• Computationally intensive, traditionally limiting clinical use
• Recent advances in hardware and algorithms enable faster Monte Carlo calculations

Analytical methods

• Use mathematical models to approximate radiation transport and energy deposition
• Include pencil beam, convolution/superposition, and collapsed cone algorithms
• Offer faster calculation times compared to Monte Carlo simulations
• Trade-off between calculation speed and accuracy in complex geometries
• Widely implemented in commercial treatment planning systems

Treatment planning systems

• Integrate imaging data, dose calculation algorithms, and optimization tools
• Allow creation and evaluation of radiation therapy treatment plans
• Incorporate beam modeling, tissue heterogeneity corrections, and plan optimization
• Provide dose-volume histograms and other quantitative plan evaluation metrics
• Evolving to include artificial intelligence and automated planning features

Dosimetry in radiotherapy

• Ensures accurate delivery of prescribed radiation dose to target volumes
• Minimizes dose to surrounding healthy tissues and critical structures
• Involves pre-treatment planning, in vivo measurements, and post-treatment verification

External beam radiation therapy

• Utilizes radiation beams delivered from outside the body to treat tumors
• Includes techniques like 3D conformal radiotherapy, IMRT, VMAT, and SBRT
• Dosimetry involves beam data acquisition, patient-specific calculations, and QA
• Incorporates multi-leaf collimators for precise beam shaping and modulation
• Requires accurate patient positioning and immobilization for dose delivery

Brachytherapy dosimetry

• Involves placement of radioactive sources within or near the target volume
• Includes high-dose-rate (HDR) and low-dose-rate (LDR) techniques
• Dosimetry calculations based on TG-43 formalism or model-based dose calculation algorithms
• Requires precise source positioning and dwell time optimization
• Challenges include high dose gradients and inter-seed attenuation effects

Proton therapy dosimetry

• Exploits the unique depth-dose characteristics of proton beams (Bragg peak)
• Requires specialized dosimetry equipment and procedures
• Involves complex range verification and robustness analysis
• Considers uncertainties in relative biological effectiveness (RBE) of protons
• Emerging techniques include in vivo range verification using prompt gamma imaging

Imaging in medical dosimetry

• Integrates anatomical and functional imaging for improved treatment planning
• Enables adaptive radiotherapy strategies based on tumor response
• Facilitates image-guided radiation therapy (IGRT) for precise dose delivery

CT-based dosimetry

• Provides electron density information for dose calculations
• Enables creation of digitally reconstructed radiographs (DRRs) for patient positioning
• Allows delineation of target volumes and organs at risk
• Dual-energy CT improves tissue characterization and proton range estimation
• Iterative reconstruction techniques reduce imaging dose while maintaining image quality

PET and SPECT applications

• Provide functional information for target volume definition
• Enable dose painting based on metabolic activity or hypoxia
• PET/CT and SPECT/CT fusion improves anatomical localization
• Challenges include motion artifacts and standardization of uptake values
• Emerging applications in treatment response monitoring and adaptive planning

MRI-guided radiation therapy

• Offers superior soft tissue contrast compared to CT
• Enables real-time tumor tracking and adaptive planning
• Requires specialized MRI-linear accelerator systems
• Presents challenges in electron density estimation and geometric distortion correction
• Facilitates functional imaging techniques (diffusion, perfusion) for treatment adaptation

Quality assurance in dosimetry

• Ensures accuracy, consistency, and safety in radiation measurements and dose delivery
• Involves regular equipment checks, patient-specific verifications, and independent audits
• Critical for maintaining high standards of care in radiation oncology and diagnostic imaging

Calibration of dosimetry equipment

• Establishes traceability to national or international standards
• Includes ionization chamber calibration factors and electrometer calibration
• Requires regular cross-calibration of field and reference instruments
• Involves energy and dose-rate dependent correction factors
• Adheres to protocols such as AAPM TG-51 or IAEA TRS-398

Patient-specific QA procedures

• Verify accuracy of treatment plans before delivery
• Include phantom measurements and independent dose calculations
• Gamma analysis compares measured and calculated dose distributions
• EPID-based dosimetry enables in vivo dose verification
• Machine log file analysis provides additional verification of delivery parameters

Dosimetry audits and protocols

• External audits ensure consistency across institutions
• Include on-site visits and mailed dosimeter programs
• Follow standardized protocols (IROC, EQUAL-ESTRO)
• Verify beam output, depth dose characteristics, and small field dosimetry
• Identify systematic errors and improve overall quality of radiotherapy treatments

Radiation protection principles

• Aims to prevent deterministic effects and minimize stochastic risks of radiation exposure
• Applies to patients, medical staff, and the general public
• Balances the benefits of medical procedures against potential radiation risks

ALARA concept

• Stands for "As Low As Reasonably Achievable"
• Guides radiation protection practices in medical and occupational settings
• Involves time, distance, and shielding as key protective measures
• Requires ongoing education and training of radiation workers
• Implemented through dose constraints and reference levels

Shielding calculations

• Determine appropriate barrier thicknesses for radiation facilities
• Consider primary and secondary radiation, including scatter and leakage
• Utilize workload, use factor, and occupancy factor in calculations
• Follow national guidelines (NCRP reports) and local regulations
• Account for different shielding properties of materials (concrete, lead, steel)

Occupational vs public exposure

• Occupational limits typically higher than public exposure limits
• Occupational exposure monitored through personal dosimetry programs
• Public exposure controlled through facility design and operational procedures
• Pregnant radiation workers subject to additional dose restrictions
• Emergency exposure situations may allow higher doses for life-saving actions

Biological effects of radiation

• Encompasses cellular, tissue, and whole-body responses to ionizing radiation
• Informs radiation protection standards and therapeutic applications
• Considers both deterministic (threshold) and stochastic (probabilistic) effects

Linear energy transfer (LET)

• Describes energy deposition density along a particle track
• High LET radiation (alpha particles, neutrons) produces dense ionizations
• Low LET radiation (x-rays, gamma rays) creates sparse ionizations
• Influences the biological effectiveness of different radiation types
• Affects DNA damage patterns and cellular repair mechanisms

Relative biological effectiveness (RBE)

• Compares biological effect of test radiation to reference radiation (usually 250 kVp x-rays)
• Varies with radiation type, energy, dose rate, and biological endpoint
• Generally higher for high-LET radiations
• Used in radiation protection (radiation weighting factors) and hadron therapy
• Challenges in determining RBE for proton and heavy ion therapies

Dose-response relationships

• Describe the relationship between radiation dose and biological effect
• Linear no-threshold (LNT) model used for radiation protection purposes
• Deterministic effects show threshold doses and severity increases with dose
• Stochastic effects (cancer induction) assumed to have no threshold
• Factors like dose rate, fractionation, and tissue radiosensitivity influence response

Emerging technologies in dosimetry

• Drive improvements in accuracy, efficiency, and personalization of radiation treatments
• Integrate advanced computational methods and novel detection technologies
• Enable adaptive and real-time treatment modifications

Real-time dosimetry systems

• Provide immediate feedback on delivered radiation doses
• Include in vivo dosimetry using diodes, MOSFETs, or optical fibers
• Enable detection and correction of treatment delivery errors
• Facilitate adaptive radiotherapy based on daily dose measurements
• Challenges include detector positioning and interpretation of real-time data

3D dosimetry techniques

• Measure full 3D dose distributions in phantom materials
• Include gel dosimetry, plastic scintillator detectors, and radiochromic 3D dosimeters
• Enable comprehensive verification of complex treatment techniques
• Provide high spatial resolution for small field and steep gradient dosimetry
• Challenges in readout techniques and stability of 3D dosimeters

Artificial intelligence in dose prediction

• Utilizes machine learning algorithms to improve dose calculation accuracy and speed
• Enables rapid plan generation and automated treatment planning
• Facilitates knowledge-based planning using historical patient data
• Assists in organ-at-risk segmentation and target volume delineation
• Challenges include data quality, model interpretability, and regulatory approval