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- What is the main purpose of IAEA Safety Standards?
The main purpose of IAEA Safety Standards is to establish or adopt standards of safety for the protection of health and minimization of danger to life and property, and to provide for the application of these standards.
- What is the primary quantity of interest in radiotherapy treatments, according to the document?
According to the document, the primary quantity of interest in radiotherapy treatments is absorbed dose to water.
- What is the document number for the 'Absorbed Dose Determination in External Beam Radiotherapy'?
The document number for the 'Absorbed Dose Determination in External Beam Radiotherapy' is Technical Reports Series No. 398 (TRS‑398).
- What are the benefits of using standards based on absorbed dose to water compared to air kerma based standards?
Standards based on absorbed dose to water provide a more robust system of primary standards than air kerma based standards, allow the use of a simple formalism, and offer the possibility of reducing the uncertainty in the dosimetry of radiotherapy beams.
- According to the document, what are some of the advances since the 2000s that prompted an update to TRS-398?
Advances since the 2000s that prompted an update to TRS-398 include the publication of new key data for measurement standards in the dosimetry of ionizing radiation, the development of new radiation detectors that are now commercially available, and the introduction of new radiotherapy technologies implementing megavoltage photon beams, protons, and heavier ions.
- Explain the role of the IAEA/WHO Network of Secondary Standards Dosimetry Laboratories in the context of TRS-398.
The IAEA/WHO Network of Secondary Standards Dosimetry Laboratories played a key role in the development and update of TRS-398. The Scientific Committee of the network initially recommended the creation of a new international code of practice using standards based on absorbed dose to water, leading to the original TRS-398 publication. Later, the same committee recommended an update to TRS-398 to account for advances in dosimetry and radiotherapy technologies.
- How can users of IAEA safety standards inform the IAEA of their experience in using the standards?
Users of IAEA safety standards can inform the IAEA of their experience in their use via the IAEA Internet site, by post to Vienna International Centre, PO Box 100, 1400 Vienna, Austria, or by email to Official.Mail@iaea.org.
- What is the relationship between absorbed dose to water and radiotherapy treatments?
Absorbed dose to water relates directly to the quantity of interest in radiotherapy treatments. This means that the amount of energy deposited per unit mass of water (which is similar to human tissue) is a crucial factor in determining the effectiveness and safety of the treatment. By basing dosimetry on absorbed dose to water, clinicians can more accurately target tumors while minimizing damage to surrounding healthy tissues.
- Explain why standards based on absorbed dose to water are considered more robust than air kerma based standards.
Standards based on absorbed dose to water are considered more robust because they are less sensitive to variations in beam quality and detector characteristics. Air kerma based standards rely on converting measurements in air to dose in water, which introduces uncertainties related to the conversion factors. Absorbed dose to water standards directly measure the dose in a medium that is more relevant to radiotherapy, reducing the reliance on these conversion factors and leading to a more accurate and reliable dosimetry system.
- Based on the provided text, discuss the potential impact of TRS-398 on the accuracy and consistency of radiotherapy treatments worldwide.
TRS-398 aims to provide a systematic and internationally unified approach to the calibration of ionization chambers and the determination of absorbed dose. By promoting the use of absorbed dose to water as the primary standard, it reduces uncertainties associated with older methods and ensures greater consistency in dosimetry practices across different institutions and countries. This, in turn, leads to more accurate and reliable radiotherapy treatments, improving patient outcomes and minimizing the risk of complications due to incorrect dose delivery. The document's emphasis on incorporating new data and technologies further enhances its relevance and ensures that radiotherapy practices remain up-to-date with the latest advancements.
- What is the primary objective of the document?
The objective of the document is to provide guidance on determining absorbed dose to water for radiation beams used in radiotherapy, specifically for low, medium, and high energy photon beams, electron beams, proton beams, and heavier ion beams. It is intended for users with calibrations traceable to a primary standards dosimetry laboratory.
- Describe the method for specifying beam quality for low energy kilovoltage X-ray beams.
Beam quality for low energy kilovoltage X-ray beams is typically specified using the half-value layer (HVL) in millimeters of aluminum (mm Al). The HVL is the thickness of aluminum required to attenuate the beam intensity by 50%.
- Explain the challenges and considerations involved in determining absorbed dose to water for proton beams compared to photon or electron beams.
Determining absorbed dose to water for proton beams presents unique challenges due to the Bragg peak, where the dose deposition is highly localized. Accurate knowledge of the beam energy and range is crucial. Furthermore, the kQ,Qₒ factors for proton beams are more complex to determine and may require Monte Carlo simulations. The stopping power ratios for protons also differ significantly from those for photons or electrons, requiring careful consideration.
- What accuracy in absorbed dose delivery to a target volume was considered necessary for tumor eradication according to the ICRU in 1976?
The ICRU concluded that an accuracy of ±5% in the delivery of an absorbed dose to a target volume was needed for the eradication of the primary tumor.
- What was the combined standard uncertainty proposed by Wambersie for dose delivery at the specification point?
Wambersie proposed 3.5% for the combined standard uncertainty of the dose delivery at the specification point.
- What is the main concern from the dosimetry point of view, in addition to the clinical requirement for dose delivery?
The main concern from the dosimetry point of view is the uncertainty in the dose delivered, which starts with the uncertainty of the beam calibration.
- What was the main update in the 1997 edition of the code of practice compared to the 1987 edition?
The main update in the 1997 edition of the code of practice was the updated dosimetry of photon beams, mainly kilovoltage X rays.
- According to the text, what is the largest contribution to the uncertainty during beam calibration?
According to the text, the largest contribution to the uncertainty during beam calibration arises from the different physical quantities involved and the large number of steps performed, yielding standard uncertainties of up to 4%.
- Explain why the conversion of NK to ND,air (or Ngas) introduces considerable uncertainty in the calibration of clinical beams.
The conversion of NK to ND,air (or Ngas) introduces considerable uncertainty because it involves several quantities and conversions performed by the user at the hospital. These include factors like km (correction factor for the lack of air equivalence of the chamber material) and katt (correction factor for the attenuation and scatter of photons in the chamber material), which are used in most air kerma based codes of practice and dosimetry protocols. Uncertainties in these factors contribute to the overall uncertainty.
- What are the advantages of calibrating therapy level dosimeters in terms of absorbed dose to water, according to Reich?
According to Reich, the advantages of calibrating therapy level dosimeters in terms of absorbed dose to water are that it uses the same quantity and experimental conditions as the user, simplifying the process and reducing uncertainties associated with conversions.
- Describe the evolution of absorbed dose standards at PSDLs, mentioning the initial methods and the current capabilities.
Initially, absorbed dose to graphite was measured using graphite calorimeters. Comparisons of these measurements were satisfactory, leading to the development of standards of absorbed dose to water. Procedures to determine absorbed dose to water using ionometry, chemical dosimetry, and water and graphite calorimetry have considerably improved. While water calorimetry allows direct determination, conversion and perturbation factors for other procedures are now well known. PSDLs provide ND,w calibrations at 60Co gamma ray beams, and some have extended these to high energy photon and electron beams and medium energy kilovoltage X ray beams.
- Explain the role of SSDLs in the calibration chain and discuss the limitations they face in providing calibration coefficients for high-energy photon and electron beams.
SSDLs transfer calibration coefficients from PSDLs or the BIPM to hospital users. For 60Co gamma ray beams, most SSDLs can provide users with a calibration coefficient in terms of absorbed dose to water. However, it is generally not feasible for SSDLs to supply experimentally determined calibration coefficients at high energy photon and electron beams because they often lack the necessary equipment and resources to perform these complex calibrations.
- How have dosimetry procedures developed for high energy photons and electrons been adapted for proton and heavier ion irradiation facilities?
Dosimetry in proton and heavier ion irradiation facilities is also based on the use of ionization chambers that are provided with calibrations in terms of absorbed dose to water, similar to the procedures developed for high
determining absorbed dose to water in various radiotherapy beams, including low, medium, and high energy and 60Co photon beams, electron beams, proton beams, and light ion beams.
- What is the impact of the new data on measurement standards for air kerma standards for kilovoltage X ray and 60Co beams?
The changes are up to ~1% for air kerma standards for kilovoltage X ray and 60Co beams (also for some brachytherapy sources; e.g. 192Ir).
- Explain the discrepancy regarding photoelectric cross-sections between synchrotron radiation measurements and the NIST database.
Accurate synchrotron radiation measurements of μen/ρ for air at low energies (3–10 keV) have questioned the adequacy of the photoelectric cross‑sections in current use. The measurements showed better agreement with the older Hubbell compilation than with the values in the NIST database, which is based on the widely used XCOM computer code. However, other measurements have shown better agreement with the NIST values.
- Describe the two approaches to account for electron binding effects in Compton scattering cross-sections and their differences.
One approach uses an incoherent scattering function that corrects the Klein–Nishina expression as a multiplication factor. A more elaborate approach, used in NIST XCOM and LLNL EPDL, accounts for binding effects using incoherent scattering function values but does not account for Doppler broadening. A more realistic approach uses the relativistic impulse approximation, which accounts for both Doppler broadening and binding effects.
- What is the main reason for the shift towards calibrations based on absorbed dose to water in radiotherapy dosimetry?
The main reason is the reduction of uncertainty in determining the absorbed dose to water in radiotherapy beams. Calibrations in terms of absorbed dose to water account for individual chamber variations, unlike air kerma calibrations which rely on chamber-dependent conversion factors that assume all chambers of a given type are identical.
- Why is absorbed dose to water of main interest in radiotherapy?
Absorbed dose to water is of main interest in radiotherapy because it relates closely to the biological effects of radiation.
- What are the advantages of an international code of practice based on standards of absorbed dose to water?
The advantages include a closer relationship to biological effects, reduced uncertainty in dosimetry, a more robust system of primary standards based
on different physical principles, and a simplified formalism for dose determination.
- What is the significance of the ratio ND,w/NK for ionization chambers?
The ratio ND,w/NK (the ratio of 60Co calibration coefficients) is a useful indicator of the uniformity within a given type of chamber. Variations in this ratio demonstrate chamber-to-chamber differences, highlighting the need for individual chamber calibrations.
- What is the impact of new key data on measurement standards and ionization chamber calibrations?
The impact varies depending on the radiation modality and type of standard used. The new data, endorsed by CCRI and implemented in standards laboratories, affect the calibration of ionization chambers.
- What is the role of Monte Carlo simulation in modern radiation dosimetry?
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 in previous methods. Updated cross-sections and coefficients in Monte Carlo systems reflect the impact of new key data on reference dosimetry.
- What is a drawback of air kerma standards?
A substantial drawback is that all such standards are based on measurements with ionization chambers and are therefore subject to potential common errors. In addition, a factor related to the attenuation in the chamber wall entering the determination of air kerma has been found to vary by up to 0.7% for some primary standards.
- What is ND,w,Q?
ND,w,Q is the calibration coefficient in terms of absorbed dose to water for a dosimeter at a user beam quality Q.
- Are procedures based on air kerma calibrations still important?
Yes, procedures based on air kerma calibrations are still of importance in a number of radiotherapy applications and other areas of radiation medicine. Of particular interest is the dosimetry of kilovoltage X rays.
- Why is the formalism related to absorbed dose to water simpler than that related to air kerma?
The formalism related to absorbed dose to water is simpler because it starts from a calibration coefficient in terms of absorbed dose to water and applies correction factors for all influence quantities, reducing the possibility of errors in the determination of absorbed dose to water in the radiation beam. The air kerma formalism requires several coefficients,
beam quality Q. This correction is necessary because the chamber's response can vary depending on the energy spectrum of the radiation beam.
- Describe the practical implementation of this code of practice, emphasizing its user-friendliness.
The code of practice is designed for practical use by placing recommendations and data for each radiation type in individual, self- contained sections. This minimizes the need to search for information in other parts of the publication, making it easier for users to perform dose determinations for specific beam types. Unavoidable repetition of text is included to ensure each section is independent and complete.
- Explain the significance of TPR20,10 in the context of high energy photons.
TPR20,10, the tissue phantom ratio, is used as a beam quality index for high energy photons. It is the ratio of the dose at a depth of 20 cm to the dose at a depth of 10 cm in a water phantom, both measured along the central axis of the beam. It provides information about the penetrating power of the photon beam.
- How are uncertainties expressed in this international code of practice, and how do they compare to previous codes?
Uncertainties are expressed as relative standard uncertainties, classified into Type A and Type B. Compared to previous codes, the uncertainty values given in this publication are generally smaller due to greater confidence in absorbed dose to water determinations and a more rigorous analysis of uncertainties according to ISO guidelines.
- What considerations should be taken into account when using this code of practice for proton and light ion beams?
When using this code of practice for proton and light ion beams, one must consider the practical range (Rp) in water, which is specified for each ion type. Additionally, material-dependent scaling factors (cpl and hpl) are used to convert ranges and depths measured in plastic phantoms into equivalent values in water, accounting for differences in charged particle fluence between plastic and water.
- What is the purpose of the International Measurement System in radiation metrology?
The International Measurement System provides a framework for consistency in radiation dosimetry by disseminating calibrated radiation instruments that are traceable to primary standards. This ensures worldwide uniformity in radiation measurements.
- Explain the difference between a primary standard and a secondary standard in the context of standards laboratories.
A primary standard is an instrument of the highest metrological quality that determines a quantity from its definition without reference to other standards. Its accuracy is verified by comparison with standards maintained by other institutes participating in the International Measurement System. A secondary standard has established precision and long-term stability and has a calibration traceable to a primary standard.
- What is the role of the BIPM in radiation dosimetry?
BIPM (Bureau International des Poids et Mesures) serves as the international center for metrology. It aims to ensure worldwide uniformity in matters relating to metrology. In radiation dosimetry, BIPM, along with PSDLs, develops primary standards for radiation measurements and calibrates the secondary standards of SSDLs.
- Define the calibration coefficient ND,w and its significance in dosimetry.
ND,w is the calibration coefficient in terms of absorbed dose to water for a 60Co beam. It represents the absorbed dose to water at the reference depth zref in the absence of the chamber, when the quality is taken as the reference beam quality Qo. It is crucial for accurate dose determination and avoiding errors in patient dose delivery.
- Explain the purpose of perturbation factors like pcav, pcel, pdis, and pwall in ionization chamber measurements.
Perturbation factors correct the response of an ionization chamber for effects related to the presence of the chamber in the phantom. pcav corrects for the air cavity effect, pcel corrects for the central electrode effect, pdis accounts for the effect of replacing a volume of water with the detector cavity, and pwall corrects for the non-medium equivalence of the chamber wall and waterproofing material. These factors are multiplied together to obtain the overall perturbation factor, pch.
- What is the significance of the stopping power ratio sm,air in high energy radiotherapy beams?
The stopping power ratio sm,air is the ratio of the mean restricted mass stopping power of material m and air, averaged over an electron spectrum. It is used to relate the dose measured in air to the dose absorbed in the medium (e.g., water). It is a crucial factor in converting ionization measurements to absorbed dose in the patient.
- Describe the function of Secondary Standards Dosimetry Laboratories (SSDLs) within the International Measurement System.
SSDLs calibrate the reference instruments of users, ensuring traceability to primary standards. They receive calibrations from PSDLs, BIPM, or the IAEA/WHO SSDL Network. They bridge the gap between primary standards and the clinical setting, providing a crucial link for accurate dosimetry in radiotherapy.
- Name three basic methods that are sufficiently accurate to form the basis of primary standards for absorbed dose to water.
The three basic methods are: calorimetry, chemical dosimetry, and ionization dosimetry.
- Describe how a graphite calorimeter is used to determine absorbed dose to water.
A graphite calorimeter is used to determine the absorbed dose to graphite in a graphite phantom. The conversion to absorbed dose to water at the reference point in a water phantom can be performed in different ways, for example by application of the photon fluence scaling theorem, by measurements based on the cavity ionization theory or by direct Monte Carlo calculations.
- Why is water purity important in a water calorimeter?
Water purity is important because the presence of impurities can result in exothermic or endothermic chemical reactions that modify the relationship between absorbed dose and temperature rise – the so called ‘heat defect’.
- Explain the role of the IAEA in the SSDL network.
As the organizer of the network, the IAEA has the responsibility to verify that the services provided by the member SSDLs follow internationally accepted metrological standards (including traceability for instruments used in radiation protection and diagnostic radiology). The first step in this process is the dissemination of dosimeter calibrations from BIPM or PSDLs through the IAEA to the SSDLs. Subsequently, bilateral comparisons and dose quality audits are implemented by the IAEA for the SSDLs to ensure that the standards disseminated to users remain within the levels of accuracy required by the International Measurement System.
- What is the Fricke standard and how is it used to determine absorbed dose to water?
The Fricke standard for absorbed dose to water determines the response of a known volume of Fricke solution to the total absorption of an electron beam in the volume. Knowing the electron energy, the beam current and the mass of Fricke solution, the total absorbed energy can be determined and related to the change in absorbance of the Fricke solution as measured spectrophotometrically.
- Describe the process by which calibration and measurement capabilities are reviewed before being made available in the KCDB.
Before being made available in the KCDB, the calibration and measurement capabilities undergo a rigorous review process, firstly by the appropriate regional metrology organization and subsequently through an inter‑regional review. Through this process, users can have confidence in the results and the stated uncertainties of the measurement services offered by
participating national metrology institutes and international organizations throughout the world.
- Explain the significance of comparisons of primary standards for absorbed dose to water in 60Co gamma radiation carried out at BIPM within the CIPM MRA framework.
The comparisons of primary standards for absorbed dose to water in 60Co gamma radiation carried out at BIPM within the CIPM MRA framework are significant because they establish the degree of equivalence between national primary standards and an agreed international reference value (the BIPM primary standard). The results, available in the BIPM KCDB, demonstrate the agreement between PSDLs and provide confidence in the accuracy and traceability of dosimetry measurements worldwide. This ensures that measurements are consistent and reliable across different countries and laboratories, which is crucial for applications like radiotherapy where accurate dose delivery is essential.
- What is the purpose of BIPM comparisons of standards for absorbed dose to water and air kerma?
The purpose of BIPM comparisons is to ensure the consistency and accuracy of primary standards maintained by Primary Standard Dosimetry Laboratories (PSDLs) worldwide. These comparisons help to identify and address any discrepancies in measurements, ultimately improving the reliability of radiation dosimetry.
- What are the reference conditions for calibrations in terms of absorbed dose to water?
Reference conditions for calibrations in terms of absorbed dose to water include the geometrical arrangement (distance and depth), the field size, the material and dimensions of the irradiated phantom, and the ambient temperature, pressure, and relative humidity. These conditions are defined to ensure the calibration coefficient is valid without further correction factors.
- Explain the concept of influence quantities in the context of dosimeter calibration and provide examples.
Influence quantities are factors that affect the measurement but are not the subject of the measurement itself. They can arise from the dosimeter (e.g., aging, zero drift), the environment (e.g., pressure, temperature), or the radiation field (e.g., beam quality, dose rate). Examples include air pressure, humidity, dose rate in 60Co gamma radiation, and polarization voltage. Correction factors are applied to account for the effect of these influence quantities.
- Describe the equation used to determine absorbed dose to water when a dosimeter is used in a beam with a different quality than the one used for calibration.
Standards for absorbed dose to water use different methods that have uncorrelated, or only weakly correlated, uncertainties. In contrast, air kerma primary standards rely on graphite cavity ionization chambers, and the associated correction factors are strongly correlated, making them more susceptible to systematic errors.
- How often do PSDLs typically make new comparisons with BIPM, and what is the maximum agreed-upon time between consecutive comparisons?
PSDLs typically make new comparisons with BIPM every 10 years, although a maximum of 15 years between consecutive comparisons is agreed upon within CCRI.
- What is the purpose of introducing an intermediate quality (Qint) in dosimetry?
The intermediate quality (Qint) is introduced to simplify the data requirements for determining beam quality correction factors (kQ). It transforms the required factor kQ,Qo into a ratio of two factors, kQ,Qint and kQo,Qint, reducing the need for extensive two-dimensional tables of kQ,Qo values for each chamber type. No measurements are made at Qint; it's simply a calculation tool.
- In electron dosimetry, why is an electron beam preferred over 60Co gamma radiation as the intermediate quality (Qint)?
In electron dosimetry, an electron beam is preferred as the intermediate quality (Qint) because some chamber types designed for electron beams lack reliable kQ data for 60Co gamma radiation. Furthermore, using an electron beam as Qint reduces the uncertainty of the factors kQ,Qint and kQo,Qint, especially if Qint is closer in energy and of the same beam type as Q and Qo.
- What is R50 and why is it chosen to be 7.5 g/cm2 as the intermediate quality in electron beams?
R50 is the beam quality index in electron beams. In this international code of practice, R50 = 7.5 g/cm2 is chosen as the intermediate quality (Qint) because it is near the middle of the range of electron beam energies typically used in clinical practice. This choice helps to minimize the uncertainty in the kQ,Qint and kQo,Qint factors.
- Describe one method for obtaining the calibration coefficient of a plane parallel chamber at the electron beam quality Qo.
One method for obtaining the calibration coefficient of a plane parallel chamber at the electron beam quality Qo is by cross-calibration in the clinical electron beam of quality Qo against a reference chamber with a known calibration coefficient ND,w,ref,Qo for that quality. The process of cross-calibration in the clinic is described in Section 4.5.
- Explain how Equation (5) demonstrates the use of 60Co as an intermediate quality.
Equation (5) demonstrates the use of 60Co as the intermediate quality in evaluating the factor k18 MV,6 MV that converts calibration coefficients from a 6 MV photon beam to an 18 MV photon beam. It uses existing kQ factors based on 60Co, even though no user or calibration measurements are made in 60Co gamma radiation. It breaks down the conversion into two steps using 60Co as a reference point for calculation.
- How can the calibration coefficient for a proton beam be derived from a calibration in a 6 MV photon beam?
The calibration coefficient ND,w,3 g/cm2 for a proton beam with residual range Rres = 3 g/cm2 can be derived from a calibration with coefficient ND,w,6 MV in a 6 MV photon beam using the equation: ND,w,3 g/cm2 = (k MV / k3 g/cm2) * ND,w,6 MV. Values for k3 g/cm2 and k6 MV are provided in Sections 10.7.2 and 6.5.1, respectively.
- What is a limitation of using generic kQ,Qo values for ionization chambers?
A limitation of using generic kQ,Qo values is that they are common to a specific ionization chamber type and, as such, cannot account for chamber- to-chamber differences in response with energy. These values are based on averages and may not accurately reflect the behavior of a specific individual chamber.
- What are the two approaches for providing ND,w,Q calibrations at multiple beam qualities?
The two approaches are: (a) Providing a series of ND,w,Q calibrations at beam qualities Q, allowing the user to select a reference quality Qo and derive kQ,Qo values by normalizing to ND,w,Qo. (b) Providing a calibration coefficient ND,w,Qo at a reference quality Qo, along with a series of directly measured kQ,Qo factors for the user chamber at beam qualities Q.
- What is the main advantage of having standards laboratories with radiation sources operating at different beam qualities?
The main advantage is that the individual chamber response in a water phantom irradiated by various beam types and qualities is intrinsically taken into account. This allows for more accurate and chamber-specific calibration coefficients and beam quality correction factors.
- Describe one method for obtaining generic calculated values for beam quality correction factors.
One method for obtaining generic calculated values is using detailed Monte Carlo simulations of a given chamber type. This technique simulates the interaction of radiation with the chamber and surrounding materials to determine the chamber's response at different beam qualities.
- For what types of radiation beams is a cylindrical ionization chamber suitable?
A cylindrical ionization chamber is suitable for radiotherapy beams of low and medium energy X rays above 70 kV, 60Co gamma radiation, high energy photon beams, electron beams with energy above ~9 MeV, and therapeutic proton and heavier ion beams.
- For what electron energies is the use of plane parallel chambers mandatory?
The use of plane parallel chambers is mandatory for all electron energies, and below 8 MeV.
- Describe the specifications for reference class ionization chambers regarding leakage and polarity effect.
For reference class ionization chambers, the leakage should be less than 0.1% of the chamber reading, and the polarity effect should be less than 0.4% of the chamber reading, with the polarity energy dependence being less than 0.3% between the energies of the 60Co beam and 10 MV photon beam.
- Explain the recombination correction requirements for reference class ionization chambers in pulsed and continuous beams.
For pulsed beams, a plot of 1/MQ (charge reading) versus 1/V (voltage) should be linear at least for practical values of V. For continuous beams, the plot of 1/MQ versus 1/V2 should be linear, describing the effect of general recombination. The difference in the initial recombination correction obtained with opposite polarities should be <0.1%.
- What is the recommended range of polarizing voltages for ionization chambers used in radiotherapy dosimetry?
The recommended range of polarizing voltages should be 50–400 V for ionization chambers used for radiotherapy dosimetry.
- What material is water NOT equivalent to when constructing phantoms for reference dosimetry, except in the case of low energy X- rays?
Solid phantoms in slab form, such as polystyrene, PMMA, and certain water equivalent plastics such as Solid Water, Plastic Water and Virtual Water may not be used for reference dosimetry, except in the case of low energy X rays, where a PMMA phantom is permitted.
- What is the typical thickness range for the entrance window of a plane parallel ionization chamber designed for measuring low energy X-rays?
The plane parallel chamber has to have a thin entrance window (ideally with a thickness in the range 2–3 mg/cm2) to provide full buildup of the
primary beam and filter out secondary electrons generated in beam limiting devices.
- What is the acceptable variation in the electrometer's response over one year (long term stability)?
The variation in the response should not exceed ±0.2% over one year (long term stability).
- Describe the circumstances under which the leakage current of an ionization chamber may exceed 0.01% of the ionization current produced by a typical radiotherapy beam.
In limited cases (e.g. small volume chambers in low dose rate beams) the leakage current may exceed this limit. In such circumstances, the leakage current has to be evaluated carefully and a correction needs to be applied to the raw ionization chamber reading.
- Explain why it is important to reverse the polarity of the polarizing voltage when using an ionization chamber.
It should be possible to reverse the polarity of the polarizing voltage, so that the polarity effect of the ionization chamber can be determined, and to vary the voltage in order to determine the collection efficiency.
- What considerations should be taken into account when using plastic phantoms for routine quality assurance measurements?
This should include a determination of the mean thickness and density of each slab, as well as the thickness variation over a single slab, and an investigation by radiograph/CT scan for bubbles or voids in the plastic. The relationship between the dosimeter readings in plastic and water has to be established for the user beam. This involves a careful comparison of measurements performed in plastic with measurements carried out in water. Periodic checks at reasonable intervals might be also needed to assure the validity and consistency of the original comparison result. Ionization chamber measurements in plastic water substitute phantoms are prone to effects such as charge storage and temperature inhomogeneities, and it needs to be verified that these effects have no impact on the measurement. Plastics usually have low thermal conductivity; the dosimeter temperature needs to be determined by direct measurement at the position of the detector and/or by allowing sufficient time for the establishment of thermal equilibrium with the room.
- Explain the purpose of using a waterproof sleeve with an ionization chamber and why it is necessary.
Unless the ionization chamber is designed so that it can be inserted directly into water, it has to be used with a waterproof sleeve.
- Describe the requirements for the phantom size when measuring absorbed dose for medium energy X-rays and photon, electron, proton and heavier ion beams.
