Residency · Residency · Radiation Oncology
Radiation Dosimetry: Principles and Measurement
Overview
Radiation dosimetry is the scientific discipline focused on measuring the absorbed dose, which is the fundamental quantity connecting the delivery of radiation to its biological effects. Accurate dosimetry is essential for every aspect of radiation therapy, including machine calibration, treatment planning, and ensuring patient safety. In North America, the TG-51 protocol serves as the standard for clinical reference dosimetry of high-energy photon and electron beams.
Fundamental Quantities
Exposure
Exposure is defined exclusively for photon beams in air and represents the amount of charge of one sign produced per unit mass of air by ionizing radiation. Its unit is coulombs per kilogram (C/kg), with the historical unit being the Roentgen (1 R = 2.58 x 10^-4 C/kg). This quantity is limited to photon energies below approximately 3 MeV and is not applicable to megavoltage beams used in modern radiotherapy.
Kerma (Kinetic Energy Released per Unit Mass)
Kerma quantifies the sum of the initial kinetic energies of all charged particles liberated by uncharged particles per unit mass of material. It is measured in Gray (Gy), where 1 Gy equals 1 joule per kilogram. Kerma includes energy that may leave the local volume, such as bremsstrahlung photons produced by secondary electrons. Under conditions of charged particle equilibrium (CPE) and negligible bremsstrahlung losses, kerma equals the absorbed dose.
Absorbed Dose
Absorbed dose is the energy deposited per unit mass of a medium by ionizing radiation, also measured in Gray (Gy). One Gray corresponds to 100 centigray (cGy) or 100 rad. This is the most clinically relevant dosimetric quantity because it directly relates to the energy imparted to tissue. Absorbed dose is independent of the type of radiation or medium, making it a pure measure of energy deposition.
Dose Equivalent and Effective Dose
Dose equivalent (H) is calculated by multiplying the absorbed dose by a radiation weighting factor and is primarily used in radiation protection rather than therapy. Effective dose further accounts for organ-specific sensitivity weighting factors. Both are measured in Sievert (Sv) and are relevant for protecting staff and estimating stochastic risks but are not used for treatment dosimetry.
Ionization Chamber Dosimetry
Principles
Ionization chambers operate by collecting charge produced by radiation ionizing a gas, typically air, within a known volume. The collected charge is proportional to the dose in the air cavity. Cavity theory, including Bragg-Gray and Spencer-Attix models, relates the dose measured in the air cavity to the dose in the surrounding medium, usually water.
Types of Ionization Chambers
The Farmer-type chamber is cylindrical with an active volume of approximately 0.6 cc and is the standard for photon beam calibration. Parallel-plate chambers have a flat, thin air gap and are preferred for electron beam calibration, surface dose measurements, and small fields. Micro-chambers, with volumes ranging from 0.01 to 0.05 cc, are used for small field dosimetry and stereotactic beams. Well-type chambers are specialized for brachytherapy source calibration.
Bragg-Gray Cavity Theory
A cavity is considered "Bragg-Gray" if it is sufficiently small so as not to perturb the charged particle fluence in the medium. Under this condition, the dose in the medium equals the dose in the cavity multiplied by the mass stopping power ratio of the medium to the cavity gas. This requires that the secondary electron spectrum within the cavity matches that in the undisturbed medium. The Spencer-Attix modification refines this theory by accounting for delta-ray production and using restricted stopping powers.
TG-51 Calibration Protocol
Overview
The AAPM Task Group 51 protocol, published in 1999, is the standard for clinical reference dosimetry in North America, replacing the earlier TG-21 protocol that was based on exposure calibration. TG-51 uses an absorbed dose to water calibration coefficient (N_D,w) provided by an accredited dosimetry calibration laboratory (ADCL). The absorbed dose to water (D_w) is calculated using the formalism: D_w = M × k_Q × N_D,w, where M is the corrected charge reading, k_Q is the beam quality conversion factor, and N_D,w is the calibration coefficient.
Key Components
Calibration Coefficient (N_D,w)
The calibration coefficient N_D,w is obtained by sending the ionization chamber to an ADCL, such as the University of Wisconsin. It represents the absorbed dose to water per unit charge collected and is determined at Co-60 beam quality. This coefficient is traceable to national standards, such as those maintained by NIST.
Beam Quality Conversion Factor (k_Q)
The factor k_Q converts the Co-60-based calibration to the user's specific beam quality. For photon beams, k_Q is tabulated as a function of %dd(10)x, which is the percentage depth dose at 10 cm depth for a 10x10 cm field with electron contamination removed. For electron beams, k_Q is calculated as the product of k_ecal, an energy-independent factor, and k'_R50, which depends on the R50 parameter (depth at which dose falls to 50%).
Corrected Charge Reading (M)
The raw electrometer reading must be corrected for several factors: temperature and pressure (P_TP), with standard conditions defined as 22°C and 101.325 kPa; ion recombination (P_ion), typically determined by the two-voltage technique; polarity effects (P_pol), calculated as the average of readings at positive and negative collection voltages; and the electrometer calibration factor (P_elec).
Reference Conditions
For photon beams, reference conditions specify a 10x10 cm field size, source-to-surface distance (SSD) of 100 cm, and chamber placement at 10 cm depth in water. For electron beams, the field size must be at least 10x10 cm at the surface (or 20x20 cm for higher energies), SSD is 100 cm, and the chamber is positioned at the reference depth d_ref = 0.6 R50 - 0.1 cm. A water phantom is used as the reference medium.
TG-51 Addendum (2014)
The 2014 addendum to TG-51 provided updated recommendations addressing practical implementation issues. It clarified the measurement of %dd(10)x using the lead foil technique, updated k_Q values, and included uncertainty analysis. The addendum also recommended annual calibration checks with a tolerance of ±1%.
In-Vivo Dosimetry
Purpose
In-vivo dosimetry aims to verify that the dose delivered to the patient matches the planned dose and to detect gross errors in treatment delivery. It is especially valuable for complex treatments, new techniques, or initial treatment sessions.
Detector Types
Diodes are semiconductor detectors offering real-time readout and small size but have energy-dependent responses. MOSFETs (metal-oxide-semiconductor field-effect transistors) are very small but exhibit angular dependence. Thermoluminescent dosimeters (TLDs), commonly made of lithium fluoride (LiF), are passive detectors read after irradiation. Optically stimulated luminescence dosimeters (OSLDs), composed of Al2O3:C crystals, are reusable and passive. Radiochromic film, such as EBT3, provides two-dimensional dose distribution, is energy-independent in the megavoltage range, but requires careful handling and calibration. Electronic portal imaging devices (EPIDs) can be used for transit dosimetry by capturing megavoltage imaging.
Practical Considerations
Entrance dose measurements can detect setup errors, incorrect field sizes, or missing accessories, while exit dose measurements provide information about patient thickness and beam transmission. In-vivo dosimetry is increasingly mandated by accreditation bodies and regulatory agencies. Typical tolerance levels are ±5% for routine treatments and ±3% for high-precision techniques.
Small Field Dosimetry
Fields smaller than approximately 4x4 cm pose unique dosimetric challenges due to loss of lateral electronic equilibrium, source occlusion, and detector volume averaging effects. The IAEA TRS-483 and AAPM TG-155 provide guidance for small field measurements. Appropriate detectors include micro-chambers, stereotactic diodes, synthetic diamond detectors, and radiochromic film. Output factors for small fields require field-specific correction factors, denoted k_fclin,fmsr, to ensure accurate dosimetry.
Commissioning and Ongoing QA
Beam Commissioning
Beam commissioning involves full characterization of all beam energies, including percentage depth dose (PDD) curves, beam profiles, output factors, wedge factors, tray factors, and multileaf collimator (MLC) transmission. This requires extensive measurements in a water phantom. The collected data are entered into the treatment planning system (TPS) for beam modeling, which is then validated through independent point dose checks and plan measurements.
Ongoing QA Program
Daily quality assurance includes output constancy checks, typically within ±3%. Monthly QA involves output calibration within ±2%, beam symmetry and flatness assessments, and mechanical checks. Annual QA includes a full TG-51 calibration with a tolerance of ±1% and a comprehensive TG-142 program. Independent verification through external audit programs, such as IROC Houston (formerly RPC), is strongly recommended.
<image>A detailed diagram of a Farmer-type ionization chamber cross-section, showing the thimble-shaped air cavity, central collecting electrode, guard ring, stem, triaxial cable connection, and the build-up cap. Labels indicate the active volume (0.6 cc), wall material (graphite or C-552), and the electric field lines within the cavity. An inset shows the chamber positioned in a water phantom at reference depth with the effective point of measurement marked.</image>
<image>A flowchart illustrating the TG-51 calibration procedure. Starting with the chamber's N_D,w certificate from the ADCL, proceeding through measurement of raw charge M, application of correction factors (P_TP, P_ion, P_pol, P_elec), determination of beam quality specifier (%dd(10)x for photons or R50 for electrons), lookup of k_Q, and final calculation of D_w = M_corrected x k_Q x N_D,w. The result in cGy/MU is shown at the bottom with the clinical standard of 1 cGy/MU at reference conditions.</image>
<image>A comparison panel showing various in-vivo dosimetry detectors: a p-type diode on the patient's skin surface, a MOSFET detector taped near the eye for lens dose measurement, a TLD chip embedded in a wax phantom, and a radiochromic film sheet placed under the patient. Each detector is labeled with its key advantages and limitations in a table below.</image>
Key Clinical Pearls
The TG-51 protocol establishes the fundamental link between linear accelerator output, measured in monitor units (MU), and absorbed dose, measured in centigray (cGy). The standard convention is that 1 cGy corresponds to 1 MU under reference conditions (10 cm depth, 10x10 cm field, 100 cm SSD in water). Temperature and pressure corrections are critical for unsealed ion chambers; a 1°C error can cause approximately a 0.3% dose error, and omitting the correction entirely can lead to errors of several percent. Ion recombination correction is especially important at high dose rates, such as flattening filter-free (FFF) beams and stereotactic body radiotherapy (SBRT), as well as for pulsed beams; the two-voltage technique is the standard method to determine this correction. Small field dosimetry remains a significant source of clinical error; therefore, appropriate detectors must always be used, and field-size-specific correction factors per TRS-483 must be applied. Participation in external audit programs like IROC Houston is strongly recommended and often required for clinical trial participation, as it provides an independent verification of an institution's dosimetric accuracy.
References
- Almond PR et al. "AAPM's TG-51 protocol for clinical reference dosimetry of high-energy photon and electron beams." Med Phys. 1999;26(9):1847-1870.
- McEwen M et al. "Addendum to the AAPM's TG-51 protocol for clinical reference dosimetry of high-energy photon beams." Med Phys. 2014;41(4):041501.
- Andreo P et al. IAEA TRS-398: "Absorbed Dose Determination in External Beam Radiotherapy." IAEA, 2000.
- Alfonso R et al. "A new formalism for reference dosimetry of small and nonstandard fields." Med Phys. 2008;35(11):5179-5186 (basis for TRS-483).
- AAPM TG-142: Klein EE et al. Med Phys. 2009;36(9):4197-4212.
