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International Code of Practice for
Dosimetry of Radiotherapy Beams
Absorbed Dose Determination in External
Beam Radiotherapy
This document is Technical Reports Series No. 398 (Rev. 1), an international code of practice for dosimetry based on standards of absorbed dose to water. It is endorsed by the European Society for Radiotherapy and Oncology (ESTRO). The International Atomic Energy Agency (IAEA) published this document in Vienna in February 2024.
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Foreword
Following recommendations from the IAEA/WHO Network of Secondary Standards Dosimetry Laboratories, a coordinated research project led to the publication of IAEA Technical Reports Series No. 398 (TRS‑398) in
- TRS-398 facilitated the transition from air kerma-based calibrations to absorbed dose to water-based calibrations. Absorbed dose to water directly relates to radiotherapy treatments and offers a more robust system with reduced uncertainty. Today, absorbed dose to water is the basis for reference dosimetry in external beam radiotherapy worldwide. This publication addresses the need for a unified approach to calibrating ionization chambers and determining absorbed dose to water for various radiation beams used in radiotherapy. It is intended for users with calibrations traceable to a primary standards dosimetry laboratory.
Acknowledgements
The IAEA expresses gratitude to contributors including P. Andreo, D. Burns, R.‑P. Kapsch, M. McEwen, and S. Vatnitsky, as well as T. Ackerly, D. Butler, A. Fukumura, G. Hartmann, A. Nisbet, H. Nyström, M. Pimpinella, and J. Seuntjens for their comments and suggestions. The IAEA and its Member States do not assume responsibility for consequences arising from the use of this publication. Guidance provided represents expert opinion and not necessarily a consensus of all Member States. The use of country designations does not imply any judgement by the IAEA. Mention of specific companies or products does not imply endorsement. The IAEA is not responsible for the persistence or accuracy of external website URLs.
INTRODUCTION
The International Commission on Radiation Units and Measurements (ICRU) concluded that an accuracy of ±5% is needed in delivering absorbed dose to a target volume for tumor eradication. Some clinicians requested even tighter limits, such as ±2%, but this was considered virtually impossible to achieve in 1976. The 5% accuracy requirement corresponds to a combined uncertainty of 2.5% at one standard deviation (k=1). The first edition of the international code of practice in 2000 considered a combined uncertainty of approximately one standard deviation of 5%. A later review proposed 3.5% for the combined standard uncertainty of dose delivery at the specification point. An uncertainty close to 3% (k=1) is considered the current acceptable accuracy requirement for the difference between the prescribed dose and the dose delivered to the patient at the specification point under optimal conditions, specifically regarding the dosimetry component of radiotherapy treatment. The uncertainty in the dose delivered, starting with the beam calibration uncertainty, is a primary concern from a dosimetry perspective.
In 1987, the IAEA published an international code of practice for absorbed dose determination in photon and electron beams. A second edition was published in 1997, updating dosimetry for photon beams, mainly kilovoltage X rays. A code of practice on the use of plane parallel ionization chambers
At secondary standards dosimetry laboratories (SSDLs), calibration coefficients from a PSDL or from the International Bureau of Weights and Measures (BIPM) are transferred to hospital users. For 60Co gamma ray beams, most SSDLs can provide users with a calibration coefficient in terms of absorbed dose to water, as all SSDLs have such beams. However, it is generally not feasible for SSDLs to supply experimentally determined calibration coefficients at high energy photon and electron beams.
A major advance in radiotherapy has been the increasing use of proton and heavier ion irradiation facilities. Practical dosimetry in these fields is also based on the use of ionization chambers that are provided with calibrations in terms of absorbed dose to water. Therefore, the dosimetry procedures developed for high energy photons and electrons can also be applicable to protons and heavier ions. At the other end of the range of available teletherapy beams are kilovoltage X ray beams, and for these the use of standards of absorbed dose to water was introduced. However, for kilovoltage X rays there are currently few laboratories providing ND,w calibrations because most PSDLs have not yet established primary standards of absorbed dose to water for such radiation qualities. Nevertheless ND,w calibrations in kilovoltage X ray beams may be provided by PSDLs and SSDLs on the basis of their standards of air kerma and one of the current dosimetry protocols for X ray beams. A coherent dosimetry system based on standards of absorbed dose to water is now possible for practically all radiotherapy beams.
The development of key data for measurement standards in radiation dosimetry followed a specific request by CCRI, established by CIPM, which supervises the work of BIPM. The materials considered are air, graphite, and water.
ND,W BASED FORMALISM
Correction for the radiation quality of the beam, kQ,Qₒ
Calibration of ionization chambers
Reference dosimetry in the user beam
Cross‑calibration of ionization chambers
CODE OF PRACTICE FOR 60Co GAMMA RAY
BEAMS
Dosimetry equipment
Beam quality specification
Determination of absorbed dose to water
Cross‑calibration of field ionization chambers
Measurements under non‑reference
conditions
Estimated uncertainty in the determination
of absorbed dose to water under reference
conditions
CODE OF PRACTICE FOR HIGH ENERGY
PHOTON BEAMS
Dosimetry equipment
Beam quality specification
Determination of absorbed dose to water
Values for kQ Q,o
Cross‑calibration of field ionization chambers
Measurements under non‑reference
conditions
Estimated uncertainty in the determination
of absorbed dose to water under reference
conditions
CODE OF PRACTICE FOR MEDIUM ENERGY
KILOVOLTAGE X RAY BEAMS
Dosimetry equipment
Beam quality specification
Determination of absorbed dose to water
Measurements under non‑reference
conditions
Estimated uncertainty in the determination
of absorbed dose to water under reference
conditions
CODE OF PRACTICE FOR PROTON BEAMS
Dosimetry equipment
Beam quality specification
Determination of absorbed dose to water
Values for kQ Q,o
Cross‑calibration of ionization chambers
Measurements under non‑reference
conditions
Estimated uncertainty in the determination
of absorbed dose to water under reference
conditions
CODE OF PRACTICE FOR LIGHT ION BEAMS
Dosimetry equipment
Beam quality specification
Determination of absorbed dose to water
Values for kQ Q,o
Measurements under non‑reference
conditions
Estimated uncertainty in the determination
of absorbed dose to water under reference
conditions
APPENDIX I: FORMALISM FOR THE
DOSIMETRY OF KILOVOLTAGE X RAY BEAMS
APPENDIX II: DETERMINATION OF kQ,Qo
AND ITS UNCERTAINTY
APPENDIX III: BEAM QUALITY
SPECIFICATION
APPENDIX IV: EXPRESSION OF
UNCERTAINTIES
Dosimetry Recommendations and Practices
The IAEA and ICRU recommend classifying nuclei with an atomic number (Z) of 10 or less as 'light ions' and heavier nuclei as 'heavy ions'. This international code offers guidance for reference and relative dosimetry for protons and light ions. The code is based on absorbed dose to water. Neutron therapy beam dosimetry is not included due to the complexity of
Impact of New Data
The impact of the new data on measurement standards and ionization chamber calibrations varies depending on the radiation modality and type of standard used. Changes are up to ~1% for air kerma standards for kilovoltage X ray and 60Co beams (also for some brachytherapy sources; e.g. 192Ir). A similar change could have been expected for the ionometric absorbed dose to water BIPM standard for 60Co, but the implementation of the new data is assessed in the context of known changes to other correction factors, resulting in a practically negligible change. For graphite calorimetry standards there are only small changes, mostly associated with one of the transfer methods used for converting dose in graphite to dose in water, which depends on the particular standard used at each laboratory. No changes occur for water calorimetry.
Data Sources
This international code of practice is based on the stopping power data tables for charged particles in Ref. [32] for graphite, water and air, whereas data from Refs [48–51] are used for other materials. For photons, the adopted cross‑sections and μen/ρ values are those in the 2014 version of the PENELOPE Monte Carlo system [52] using the same theory as Scofield did [36]; the dense energy grid of this database allows an accurate description of the variation of the cross‑section near absorption edges.
Consistency and Updates
The resulting dosimetric data (e.g. the factors that correct for the difference between the response of an ionization chamber in the reference beam quality used for calibrating the chamber and its response in the user beam quality — i.e. the kQ values) given in this publication do not differ substantially from those included in the first edition of this international code of practice, being generally within the uncertainties stated in the earlier report, but the present update is necessary to maintain consistency with the data used for measurement standards and to incorporate data for new ionization chambers made available since the publication of the first edition.
Motivation for Updating the Code of Practice
The first edition of this international code of practice was written in the mid‑1990s. Since then, a number of developments in radiotherapy and radiation dosimetry have taken place. New technologies for radiotherapy have been implemented, mostly related to megavoltage photon beams, as well as proton and heavier ion beams, whose reference dosimetry requires guidance and data for end users. New detectors have become commercially available that require data for their clinical application. The ICRU published a report on key data for measurement standards in radiation dosimetry [32], which reviewed the quantities and correction factors that play a fundamental role in dosimetry, estimated the uncertainties of key data and analysed the implications of using the data in Ref. [32] on measurements
and calculations. [53] and then implemented in standards laboratories for the calibration of ionization chambers. The impact of the new data on measurement standards, and therefore on ionization chamber calibrations by standards laboratories, varies depending on the radiation modality and type of standard used. Monte Carlo simulation of radiation transport has become a widely used technique for the accurate calculation of dosimetric quantities for all beam types, superseding many of the approximations used to determine the data in the previous edition of this international code of practice. The cross‑sections and coefficients in the most commonly used Monte Carlo systems have been updated following Ref. Comprehensive sets of dosimetric quantities have been calculated that reflect the impact of the new key data on the reference dosimetry of high energy radiotherapy beams. For the dosimetry of kilovoltage X rays, the predictions in the first edition of this international code of practice regarding the availability of absorbed dose to water calibrations (ND,w) have not been realized; in addition, no specific data were recommended. Given that changes in cross‑sections and coefficients for the photoelectric effect have resulted in a major change in key data, new data consistent with Ref. [32] have become available [54] for this type of beam and their insertion in an updated international code of practice is deemed necessary. The first edition of this international code of practice included recommendations for the dosimetry of radiotherapy beams in non‑standard conditions, that is for beams that are not 10 cm × 10 cm. Recent developments, particularly for the dosimetry of small megavoltage photon beams (see Ref. [12]), need to be considered in a general international code of practice. The feedback received from users after years of application of the first edition of this international code of practice in clinical practice needs to be taken into account.
Advantages of Absorbed Dose to Water Standards
The absorbed dose to water is of main interest in radiotherapy because it relates closely to the biological effects of radiation. Calibrations in terms of absorbed dose to water and dosimetry procedures using the associated calibration coefficients have advantages.
Reduced Uncertainty
The drive towards an improved basis for dosimetry in radiotherapy has motivated PSDLs to devote much effort to developing primary standards of absorbed dose to water for the various beam modalities. The rationale for changing the basis of calibrations from air kerma to absorbed dose to water was the expectation that the calibration of ionization chambers in terms of absorbed dose to water would considerably reduce the uncertainty in determining the absorbed dose to water in radiotherapy beams. Measurements based on calibration in air in terms of air kerma require chamber dependent conversion factors to determine the absorbed dose to water. These conversion factors do not account for differences between individual chambers of a particular type, which have been found to be significant for some chamber models. Calibrations in terms of absorbed dose to water can be performed under similar conditions to those of subsequent measurements in the user beam, so that the response of each
determining absorbed dose to water. The general equation for determining absorbed dose to water demonstrates the simplicity of this formalism.
Advantages of ND,w,Q Calibrations
Calibrations based on ND,w,Q (calibration coefficient in terms of absorbed dose to water for a dosimeter at a user beam quality Q) are favored for radiotherapy dosimetry of high-energy photon and electron beams and are implemented globally. This methodology has been extended to medium- energy kilovoltage X-ray and brachytherapy standards in some PSDLs and is under development for proton beams in other standards laboratories. Air kerma calibrations remain important in various radiotherapy applications and other areas of radiation medicine. This is particularly true for kilovoltage X-ray dosimetry, where 'true' absorbed dose to water standards are lacking for low-energy beams or are still being implemented for medium- energy beams. In proton and heavier ion beams, ND,w,60Co coefficients are common, but Wair values are often derived from NK chamber calibrations. Brachytherapy source calibrations, radiation protection, radiodiagnostic, and interventional radiology applications also rely on air kerma standards.
Scope of the International Code of Practice
This international code of practice provides a methodology for determining absorbed dose to water in photon, electron, proton, and heavier ion beams used in external radiotherapy. It relies on ionization chambers or dosimeters with an ND,w calibration coefficient and is applicable in hospitals and facilities providing radiation treatment. This publication aims to promote uniformity and consistency in radiation dose delivery worldwide and serves as a valuable resource for the medical physics and radiotherapy community. It can also assist the IAEA/WHO SSDL Network in improving the accuracy and consistency of dose determination. Compared to previous codes of practice based on air kerma standards, this publication introduces small differences in the determined absorbed dose to water. Users without absorbed dose to water calibrations can refer to air kerma-based international codes of practice.
Range of Beam Qualities Covered
The publication covers the following ranges of radiation qualities:
Low-energy X-rays: Generating potentials up to 100 kV. Medium-energy X-rays: Generating potentials above 70 kV. 60Co gamma radiation. High-energy photons: Generated by electrons with energies below 25 MeV, with TPR20,10 values between approximately 0.6 and 0.8. Megavoltage photon beams generated with special accelerators or including magnetic resonance imaging techniques are not included. Electrons: Energy interval of 4–25 MeV with a half-value depth, R50, of 1.4–10 g·cm–2. Protons: Energy interval of 50 MeV to ~250 MeV with a practical range, Rp, of 0.25–37 g·cm–2.
Light ions: Atomic number Z between 2 (He) and 10 (Ne) with a practical range in water, Rp, of 2–30 g·cm–2 (for carbon ions, this corresponds to an energy range of 85–430 MeV/u).
Practical Use of the Code of Practice
The code of practice is designed for simple practical use. It is structured similarly to Ref. [10], with individual sections for each radiation type, each forming a self-contained code of practice with detailed procedures and worksheets. This minimizes the need to search for information in other sections. While this structure introduces some repetition, it enhances ease of use, especially for users with limited access to radiation types. The first four sections cover general concepts applicable to all radiation types, and the appendices provide complementary information. In the overlap region of 70–100 kV for low and medium energy kilovoltage X-rays, either method described in Sections 8 and 9 can be used.
Expression of Uncertainties
Uncertainties are evaluated according to ISO guidelines, expressed as relative standard uncertainties, and classified as Type A (statistical analysis) or Type B (based on other means). The uncertainty estimates in this publication are generally smaller than those in previous codes of practice due to greater confidence in absorbed dose to water determinations and a more rigorous uncertainty analysis.
Quantities and Symbols
The symbols used are largely consistent with Refs [9, 10] and the first edition of the code of practice. Table 1 provides a summary of relevant quantities and symbols, including:
Bw: Backscatter factor. cpl: Material-dependent scaling factor for ranges and depths in plastic phantoms. Dw,Q: Absorbed dose to water at the reference depth. Eo, Ez: Mean energy of an electron beam at the phantom surface and at depth z. hpl: Material-dependent charged particle fluence scaling factor. HVL: Half-value layer, a beam quality index for low and medium energy X-rays. Kair,Q: Air kerma at a reference point. ki: General correction factor for influence quantities. kelec: Calibration factor of an electrometer. kh: Factor to correct for humidity. kpol: Factor to correct for polarity. kQ,Qo: Factor to correct for the difference between reference and user beam qualities. ks: Factor to correct for incomplete charge collection.
of measurement of the chamber, Peff. For plane parallel ionization chambers, pdis is not required.
Peff: The effective point of measurement of an ionization chamber. For the standard calibration geometry, Peff is shifted from the centre of the chamber towards the source by a distance that depends on the type of beam and chamber. For plane parallel ionization chambers, Peff is usually assumed to be situated in the centre of the front surface of the air cavity.
pch: The overall perturbation factor for an ionization chamber for in- phantom measurements at a beam quality Q. It is equal to the product of various factors correcting for different effects, namely pcav, pcel, pdis and pwall.
pwall: A factor that corrects the response of an ionization chamber for the non-medium equivalence of the chamber wall and any waterproofing material.
Q: A general symbol indicating the quality of a radiation beam. A subscript ‘o’, that is Qo, indicates the reference quality.
Qint: Intermediate beam quality used to reduce the data required for managing the beam quality correction factors.
rdg: A value representing the reading of a dosimeter, in arbitrary units.
R50: Half-value depth in water, used as the beam quality index for electron beams. This is the depth in water at which the absorbed dose is 50% of its value at the absorbed dose maximum. Unit: g/cm2.
Rp: Practical range for electron, proton and ion beams. Unit: g/cm2.
Rres: Residual range for proton and ion beams. Unit: g/cm2.
rcyl: Cavity radius of a cylindrical ionization chamber.
SAD: Source–axis distance.
SCD: Source–chamber distance.
SOBP: Width of the spread-out Bragg peak in proton and heavier ion beams.
SSD: Source–surface distance.
sm,air: Stopping power ratio of medium to air, defined as the ratio of the mean restricted mass stopping power of material m and air, averaged over an electron spectrum.
TMR: Tissue-maximum ratio.
TPR20,10: Tissue-phantom ratio in water at depths of 20 g/cm2 and 10 g/ cm2, for a field size of 10 cm × 10 cm and an SCD of 100 cm, used as the beam quality index for high energy photon radiation.
uc: Combined standard uncertainty of a quantity.
Wair: Mean energy expended in dry air per ion pair formed.
zmax: Depth of maximum dose. Unit: g/cm2.
zref: Reference depth for in-phantom measurements. When specified at zref, the absorbed dose to water refers to Dw,Q at the intersection of the beam central axis with the plane defined by zref. Unit: g/cm2.
Important Considerations
Depths and ranges are expressed in units of g/cm2.
The difference between ND,air and ND,w is close to the value of the water to air stopping power ratio for 60Co gamma rays.
The reference point of a chamber is specified in each section for each type of chamber. It usually refers to the point of the chamber specified by a calibration document where the calibration coefficient applies.
The International Measurement System
The International Measurement System ensures consistency in radiation dosimetry by providing calibrated radiation instruments traceable to primary standards.
Key Organizations
BIPM: Established by the Metre Convention, it serves as the international center for metrology, ensuring worldwide uniformity.
PSDLs: Primary Standards Dosimetry Laboratories develop primary standards for radiation measurements.
IAEA/WHO SSDL Network: Eases the demand for calibration by calibrating secondary standards.
Traceability
User reference instruments are traceable to primary standards through direct calibration in a PSDL or, more commonly, in an SSDL with a direct link to BIPM, a PSDL, or the IAEA/WHO SSDL Network.
Standards and Calibration in Dosimetry
Primary and Secondary Standards Laboratories
Primary Standards Dosimetry Laboratories (PSDLs) develop, maintain, and improve primary standards in radiation dosimetry. Secondary Standards Dosimetry Laboratories (SSDLs) provide calibration services using at least
review by regional and inter-regional organizations before being available in the KCDB.
The IAEA/WHO Network of Secondary Standards
Dosimetry Laboratories
The IAEA and WHO established a network of SSDLs in 1976 to improve accuracy in radiation dosimetry. This network provides traceability to the International System of Units (SI) for national dosimetry standards, mainly for countries not in the Metre Convention. By 2020, the network included 87 SSDLs in 72 IAEA Member States, with over half in developing countries. The network also includes collaborating organizations and affiliated members, such as BIPM, national PSDLs, the ICRU, and other international organizations. The IAEA verifies that SSDL services follow internationally accepted metrological standards. This involves disseminating dosimeter calibrations from BIPM or PSDLs through the IAEA to the SSDLs. Bilateral comparisons and dose quality audits are implemented by the IAEA to ensure standards disseminated to users remain within required accuracy levels. A principal goal of the SSDL network in radiotherapy dosimetry is to ensure dose delivery to patients is within internationally accepted accuracy levels. This is achieved by ensuring SSDL instrument calibrations are within stated uncertainties, promoting SSDL participation in quality assurance programs, supporting dosimetry quality audits in radiotherapy centers, and assisting in calibrating radiotherapy equipment in hospitals.
Primary Standards for Absorbed Dose to Water
There are three basic methods sufficiently accurate to form the basis of primary standards for absorbed dose to water. PSDLs have developed experimental approaches to establish these standards. Many PSDLs maintain a primary standard for absorbed dose to water operating in 60Co gamma radiation, and some maintain standards at other radiation qualities. Primary standards operating in 60Co gamma radiation or in photon and electron beams produced by accelerators are based on:
Graphite Calorimeter: Several PSDLs use a graphite calorimeter to determine the absorbed dose to graphite in a graphite phantom. Conversion to absorbed dose to water is performed using the photon fluence scaling theorem, cavity ionization theory, or Monte Carlo calculations. Water Calorimeter: The water calorimeter offers a more direct determination of the absorbed dose to water in a water phantom. The sealed water system consists of a small glass vessel containing high purity water and thermistor detectors. Water purity is important to avoid exothermic or endothermic chemical reactions. Fricke Standard: The Fricke standard determines the response of a known volume of Fricke solution to the total absorption of an electron beam. The total absorbed energy is determined and related to the change in absorbance of the Fricke solution. The use of the Fricke
standard has diminished with the increasing adoption of water calorimetry. Ionization Chamber: The ionization chamber primary standard consists of an air-filled graphite cavity chamber with known cavity volume, designed to fulfill the requirements of a Bragg-Gray detector. The chamber is placed in a water phantom, and the absorbed dose to water is derived from the mean specific energy imparted to the air of the cavity.
Until approximately 2015, absolute measurements for determining absorbed dose to water in kilovoltage X-ray beams were based almost exclusively on extrapolation ionization chambers. Water or graphite calorimetry is now used at a number of PSDLs for the 100–250 kV X-ray range, and BIPM has developed a primary standard based on the free air ionization chamber. These standards enable calibrations in terms of absorbed dose to water for SSDL and user instruments in these X-ray beams. There has been no significant development of primary standards for absorbed dose to water for X-ray beam energies below 100 kV.
BIPM Comparisons of Primary Standards
Comparisons of primary standards for absorbed dose to water in 60Co gamma radiation have been conducted at BIPM since the 1990s. BIPM has a longer history of comparisons of air kerma primary standards. The results of these comparisons are available online in BIPM KCDB.
Comparisons in terms of absorbed dose to water for 60Co are registered as comparison series BIPM.RI(I)‑K4. The agreement is within the standard uncertainty stated by each PSDL.
Comparisons of air kerma primary standards for 60Co gamma radiation, registered as BIPM.RI(I)‑K1, show a similar distribution despite smaller uncertainties. The air kerma primary standards of all PSDLs are graphite cavity ionization chambers, and the associated correction factors are strongly correlated.
The standards for absorbed dose to water use different methods that have uncorrelated, or only weakly correlated, uncertainties. This constitutes a system that is more robust and less susceptible to systematic influences.
Since 2009, BIPM has operated a travelling primary standard for absorbed dose to water, a graphite calorimeter and dose conversion system. Between 2009 and 2016, this standard was used to make direct comparisons in the accelerator photon beams of PSDLs. Since 2017, comparisons have been made in reference beams maintained by BIPM at an external accelerator facility.
The results of these comparisons are registered as BIPM.RI(I)‑K6. Results for photon beams with TPR20,10 = 0.63–0.71 (as of July 2020) are available. The uncertainties are generally larger than those for 60Co gamma radiation, and agreement is within the stated expanded uncertainties (k = 2).
