Characterization of the phantom material Virtual Water™ in high-energy photon and electron beams

The effect of electron backscatter and charge build up in media on beam current transformer signal for ultra-high dose rate (FLASH) electron beam monitoring

Objective.Beam current transformers (BCT) are promising detectors for real-time beam monitoring in ultra-high dose rate (UHDR) electron radiotherapy. However, previous studies have reported a significant sensitivity of the BCT signal to changes in source-to-surface distance (SSD), field size, and phantom material which have until now been attributed to the fluctuating levels of electrons backscattered within the BCT. The purpose of this study is to evaluate this hypothesis, with the goal of understanding and mitigating the variations in BCT signal due to changes in irradiation conditions.Approach.Monte Carlo simulations and experimental measurements were conducted with a UHDR-capable intra-operative electron linear accelerator to analyze the impact of backscattered electrons on BCT signal. The potential influence of charge accumulation in media as a mechanism affecting BCT signal perturbation was further investigated by examining the effects of phantom conductivity and electrical grounding. Finally, the effectiveness of Faraday shielding to mitigate BCT signal variations is evaluated.Main Results.Monte Carlo simulations indicated that the fraction of electrons backscattered in water and on the collimator plastic at 6 and 9 MeV is lower than 1%, suggesting that backscattered electrons alone cannot account for the observed BCT signal variations. However, our experimental measurements confirmed previous findings of BCT response variation up to 15% for different field diameters. A significant impact of phantom type on BCT response was also observed, with variations in BCT signal as high as 14.1% when comparing measurements in water and solid water. The introduction of a Faraday shield to our applicators effectively mitigated the dependencies of BCT signal on SSD, field size, and phantom material.Significance.Our results indicate that variations in BCT signal as a function of SSD, field size, and phantom material are likely driven by an electric field originating in dielectric materials exposed to the UHDR electron beam. Strategies such as Faraday shielding were shown to effectively prevent these electric fields from affecting BCT signal, enabling reliable BCT-based electron UHDR beam monitoring.

AAPM Task Group Report 306: Quality control and assurance for tomotherapy: An update to Task Group Report 148

Since the publication of AAPM Task Group (TG) 148 on quality assurance (QA) for helical tomotherapy, there have been many new developments on the tomotherapy platform involving treatment delivery, on-board imaging options, motion management, and treatment planning systems (TPSs). In response to a need for guidance on quality control (QC) and QA for these technologies, the AAPM Therapy Physics Committee commissioned TG 306 to review these changes and make recommendations related to these technology updates. The specific objectives of this TG were (1) to update, as needed, recommendations on tolerance limits, frequencies and QC/QA testing methodology in TG 148, (2) address the commissioning and necessary QA checks, as a supplement to Medical Physics Practice Guidelines (MPPG) with respect to tomotherapy TPS and (3) to provide risk-based recommendations on the new technology implemented clinically and treatment delivery workflow. Detailed recommendations on QA tests and their tolerance levels are provided for dynamic jaws, binary multileaf collimators, and Synchrony motion management. A subset of TPS commissioning and QA checks in MPPG 5.a. applicable to tomotherapy are recommended. In addition, failure mode and effects analysis has been conducted among TG members to obtain multi-institutional analysis on tomotherapy-related failure modes and their effect ranking.

Investigation of absolute dose calibration accuracy for TomoTherapy using real water

A systematic bias in TomoTherapy output calibration was reported by the Imaging and Radiation Oncology Core Houston (IROC‐H) after analyzing intensity‐modulated radiation therapy (IMRT) credentialing results from hundreds of TomoTherapy units. Multiple theories were developed to explain this observation. One theory was that the use of a solid water “cheese” phantom instead of real water in the calibration measurement was the culprit. A phantom filled with distilled water was built to investigate whether our TomoTherapy was miscalibrated due to the use of a solid water phantom. A miscalibration of −1.47% was detected on our TomoTherapy unit. It is found that despite following the vendor's updated recommendation on computed tomography (CT) number to density calibration, the cheese phantom was still mapped to a density of 1.028 g/cm³, rather than the 1.01 g/cm³ value reported in literature. When the density of the cheese phantom was modified to 1.01 g/cm³ in the treatment planning system, the measurement also indicated that our TomoTherapy machine was miscalibrated by −1.52%, agreeing with the real water phantom findings. Our single‐institution finding showed that the cheese phantom density assignment can introduce greater than 1% errors in the TomoTherapy absolute dose calibration. It is recommended that the absolute dose calibration for TomoTherapy be performed either in real water or in the cheese phantom with the density in TPS overridden as 1.01 g/cm³.

Evaluation of water-mimicking solid phantom materials for use in HDR and LDR brachytherapy dosimetry

In modern HDR or LDR brachytherapy with photon emitters, fast checks of the dose profiles generated in water or a water-equivalent phantom have to be available in the interest of patient safety. However, the commercially available brachytherapy photon sources cover a wide range of photon emission spectra, and the range of the in-phantom photon spectrum is further widened by Compton scattering, so that the achievement of water-mimicking properties of such phantoms involves high requirements on their atomic composition. In order to classify the degree of water equivalence of the numerous commercially available solid water-mimicking phantom materials and the energy ranges of their applicability, the radial profiles of the absorbed dose to water, D _w, have been calculated using Monte Carlo simulations in these materials and in water phantoms of the same dimensions. This study includes the HDR therapy sources Nucletron Flexisource Co-60 HDR (⁶⁰Co), Eckert und Ziegler BEBIG GmbH CSM-11 (¹³⁷Cs), Implant Sciences Corporation HDR Yb-169 Source 4140 (¹⁶⁹Yb) as well as the LDR therapy sources IsoRay Inc. Proxcelan CS-1 (¹³¹Cs), IsoAid Advantage I-125 IAI-125A (¹²⁵I), and IsoAid Advantage Pd-103 IAPd-103A (¹⁰³Pd). Thereby our previous comparison between phantom materials and water surrounding a Varian GammaMed Plus HDR therapy ¹⁹²Ir source (Schoenfeld et al 2015) has been complemented. Simulations were performed in cylindrical phantoms consisting of either water or the materials RW1, RW3, Solid Water, HE Solid Water, Virtual Water, Plastic Water DT, Plastic Water LR, Original Plastic Water (2015), Plastic Water (1995), Blue Water, polyethylene, polystyrene and PMMA. While for ¹⁹²Ir, ¹³⁷Cs and ⁶⁰Co most phantom materials can be regarded as water equivalent, for ¹⁶⁹Yb the materials Plastic Water LR, Plastic Water DT and RW1 appear as water equivalent. For the low-energy sources ¹⁰⁶Pd, ¹³¹Cs and ¹²⁵I, only Plastic Water LR can be classified as water equivalent.

Technical Note: On the use of cylindrical ionization chambers for electron beam reference dosimetry

PURPOSE

To investigate the use of cylindrical chambers for electron beam dosimetry independent of energy by studying the variability of relative ion chamber perturbation corrections, one of the main concerns for electron beam dosimetry with cylindrical chambers.

METHODS

Measurements are made with sets of cylindrical and plane-parallel reference-class chambers as a function of depth in water in 8 MeV and 18 MeV electron beams. The ratio of chamber readings for similar chambers is normalized in a high-energy electron beam and can be thought of as relative perturbation corrections. Data are plotted as a function of mean electron energy at depth for a range of depths close to the phantom surface to R₈₀ , the depth at which the ionization falls to 80% of its maximum value. Additional, similar measurements are made in a Virtual Water® phantom with cylindrical chambers at the reference depth in a 4 MeV electron beam.

RESULTS

The variability of relative ion chamber perturbation corrections for nominally identical cylindrical Farmer-type chambers is found to be less than 0.4%, no worse than plane-parallel chambers with similar specifications.

CONCLUSIONS

This work discusses several issues related to the use of plane-parallel ion chambers and suggests that reference-class cylindrical chambers may be appropriate for reference dosimetry of all electron beams. This would simplify the reference dosimetry procedure and improve accuracy of beam calibration.

Dosimetric properties of a Solid Water High Equivalency (SW557) phantom for megavoltage photon beams

The dosimetric properties of the recently developed SW557 phantom have been investigated by comparison with those of the existing SW457 phantom in megavoltage photon beams. The electron fluence ratio φ_pl^w, and chamber ionization ratio k_pl, of water to SW457 and water to SW557 for 4-15MV photons were calculated as a function of depth using Monte Carlo simulations, and compared with measured values. Values of φ_pl^w for SW457 were in the range of 1.004-1.014 for 4MV, and 1.014-1.018 for 15MV photons. The φ_pl^w for SW557 ranged from 1.005 to 1.008 for 4MV and from 1.010 to 1.015 for 15MV photons and the variation of φ_pl^w with depth for each beam energy was within ±0.5%. Values of k_pl were obtained with a PTW 30013 Farmer-type ionization chamber. The k_pl for SW457 ranged from 0.997 to 1.011 for 4-15MV photons. Values of k_pl for SW557 were almost unity for 4 and 6MV photons, while in the case of 10 and 15MV photons they were less than 1.006, excepting the build-up region. The measured and calculated k_pl values of water to SW557 were in the range of 0.997-1.002 and 1.000-1.006, respectively, for 4-15MV photons, at a depth of 10cm with a source-to-axis distance of 100cm. The measured and calculated k_pl values were in agreement within their uncertainty ranges. As a water-equivalent phantom, SW557 can be used with a dosimetric difference within±0.6%, for 4-15MV photons, and is more water-equivalent than SW457 in megavoltage photon beams.

Water equivalent phantom materials for ¹⁹²Ir brachytherapy

Several solid phantom materials have been tested regarding their suitability as water substitutes for dosimetric measurements in brachytherapy with (192)Ir as a typical high energy photon emitter. The radial variations of the spectral photon fluence, of the total, primary and scattered photon fluence and of the absorbed dose to water in the transversal plane of the tested cylindrical phantoms surrounding a centric and coaxially arranged Varian GammaMed afterloading (192)Ir brachytherapy source were Monte-Carlo simulated in EGSnrc. The degree of water equivalence of a phantom material was evaluated by comparing the radial dose-to-water profile in the phantom material with that in water. The phantom size was varied over a large range since it influences the dose contribution by scattered photons with energies diminished by single and multiple Compton scattering. Phantom axis distances up to 10 cm were considered as clinically relevant. Scattered photons with energies reaching down into the 25 keV region dominate the photon fluence at source distances exceeding 3.5 cm. The tested phantom materials showed significant differences in the degree of water equivalence. In phantoms with radii up to 10 cm, RW1, RW3, Solid Water, HE Solid Water, Virtual Water, Plastic Water DT, and Plastic Water LR phantoms show excellent water equivalence with dose deviations from a water phantom not exceeding 0.8%, while Original Plastic Water (as of 2015), Plastic Water (1995), Blue Water, polyethylene, and polystyrene show deviations up to 2.6%. For larger phantom radii up to 30 cm, the deviations for RW1, RW3, Solid Water, HE Solid Water, Virtual Water, Plastic Water DT, and Plastic Water LR remain below 1.4%, while Original Plastic Water (as of 2015), Plastic Water (1995), Blue Water, polyethylene, and polystyrene produce deviations up to 8.1%. PMMA plays a separate role, with deviations up to 4.3% for radii not exceeding 10 cm, but below 1% for radii up to 30 cm. As suggested by the results of the dose simulations and the values of the linear attenuation coefficient, μ, over a large energy range, the balanced content of inorganic additives in a phantom material is regarded as the key feature, providing water equivalence with regard to the attenuation of the primary photons, the release of low-energy photons by Compton scattering, and their attenuation by a combination of the photoelectric and Compton effects.

Evaluation of backscatter dose from internal lead shielding in clinical electron beams using EGSnrc Monte Carlo simulations

Internal lead shielding is utilized during superficial electron beam treatments of the head and neck, such as lip carcinoma. Methods for predicting backscattered dose include the use of empirical equations or performing physical measurements. The accuracy of these empirical equations required verification for the local electron beams. In this study, a Monte Carlo model of a Siemens Artiste linac was developed for 6, 9, 12, and 15 MeV electron beams using the EGSnrc MC package. The model was verified against physical measurements to an accuracy of better than 2% and 2 mm. Multiple MC simulations of lead interfaces at different depths, corresponding to mean electron energies in the range of 0.2–14 MeV at the interfaces, were performed to calculate electron backscatter values. The simulated electron backscatter was compared with current empirical equations to ascertain their accuracy. The major finding was that the current set of backscatter equations does not accurately predict electron backscatter, particularly in the lower energies region. A new equation was derived which enables estimation of electron backscatter factor at any depth upstream from the interface for the local treatment machines. The derived equation agreed to within 1.5% of the MC simulated electron backscatter at the lead interface and upstream positions. Verification of the equation was performed by comparing to measurements of the electron backscatter factor using Gafchromic EBT2 film. These results show a mean value of 0.997±0.022 to 1σ of the predicted values of electron backscatter. The new empirical equation presented can accurately estimate electron backscatter factor from lead shielding in the range of 0.2 to 14 MeV for the local linacs.

PACS numbers: 87.53.Bn, 87.55.K‐, 87.56.bd

Effect of improved TLD dosimetry on the determination of dose rate constants for (125)I and (103)Pd brachytherapy seeds

PURPOSE

To more accurately account for the relative intrinsic energy dependence and relative absorbed-dose energy dependence of TLDs when used to measure dose rate constants (DRCs) for (125)I and (103)Pd brachytherapy seeds, to thereby establish revised "measured values" for all seeds and compare the revised values with Monte Carlo and consensus values.

METHODS

The relative absorbed-dose energy dependence, f(rel), for TLDs and the phantom correction, Pphant, are calculated for (125)I and (103)Pd seeds using the EGSnrc BrachyDose and DOSXYZnrc codes. The original energy dependence and phantom corrections applied to DRC measurements are replaced by calculated (f(rel))(-1) and Pphant values for 24 different seed models. By comparing the modified measured DRCs to the MC values, an appropriate relative intrinsic energy dependence, kbq (rel), is determined. The new Pphant values and relative absorbed-dose sensitivities, SAD (rel), calculated as the product of (f(rel))(-1) and (kbq (rel))(-1), are used to individually revise the measured DRCs for comparison with Monte Carlo calculated values and TG-43U1 or TG-43U1S1 consensus values.

RESULTS

In general, f(rel) is sensitive to the energy spectra and models of the brachytherapy seeds. Values may vary up to 8.4% among (125)I and (103)Pd seed models and common TLD shapes. Pphant values depend primarily on the isotope used. Deduced (kbq (rel))(-1) values are 1.074 ± 0.015 and 1.084 ± 0.026 for (125)I and (103)Pd seeds, respectively. For (1 mm)(3) chips, this implies an overall absorbed-dose sensitivity relative to (60)Co or 6 MV calibrations of 1.51 ± 1% and 1.47 ± 2% for (125)I and (103)Pd seeds, respectively, as opposed to the widely used value of 1.41. Values of Pphant calculated here have much lower statistical uncertainties than literature values, but systematic uncertainties from density and composition uncertainties are significant. Using these revised values with the literature's DRC measurements, the average discrepancies between revised measured values and Monte Carlo values are 1.2% and 0.2% for (125)I and (103)Pd seeds, respectively, compared to average discrepancies for the original measured values of 4.8%. On average, the revised measured values are 4.3% and 5.9% lower than the original measured values for (103)Pd and (125)I seeds, respectively. The average of revised DRCs and Monte Carlo values is 3.8% and 2.8% lower for (125)I and (103)Pd seeds, respectively, than the consensus values in TG-43U1 or TG-43U1S1.

CONCLUSIONS

This work shows that f(rel) is TLD shape and seed model dependent suggesting a need to update the generalized energy response dependence, i.e., relative absorbed-dose sensitivity, measured 25 years ago and applied often to DRC measurements of (125)I and (103)Pd brachytherapy seeds. The intrinsic energy dependence for LiF TLDs deduced here is consistent with previous dosimetry studies and emphasizes the need to revise the DRC consensus values reported by TG-43U1 or TG-43U1S1.

A unique approach to high-dose-rate vaginal mold brachytherapy of gynecologic malignancies

PURPOSE

Patients with cervical and vaginal cancer sometimes have a less straightforward approach for choice of brachytherapy treatment owing to the tumor's location and clinical presentation. The staff at Royal Brisbane & Women's Hospital in Queensland, Australia, is trying to solve this problem by the use of an old technique in a new approach called vaginal molds. With a patient-specific vaginal mold, the appearance of the applicator and the dose distribution can be customized to provide an optimal treatment for each patient.

METHODS AND MATERIALS

The technique used at the Royal Brisbane & Women's Hospital uses a flexible two-part putty, moulded to the shape of the vagina, in which standard catheters (flexible implant tubes) are incorporated, in a pattern designed to permit a dose distribution more conformal to the target volume.

RESULTS

The presented technique is efficient and improves the accuracy of a homogeneous target cover and sparing of organs at risk for vaginal mold brachytherapy treatments at our institution.

CONCLUSION

This technique offers a customizable option when traditional cylindrical- or dome-type applicators cannot be used, or provide inadequate dose coverage. Molds to match the patient anatomy can be created quickly, while allowing flexibility in positioning of catheters to achieve the desired dose distribution.

Addendum to the AAPM's TG-51 protocol for clinical reference dosimetry of high-energy photon beams

An addendum to the AAPM's TG-51 protocol for the determination of absorbed dose to water in megavoltage photon beams is presented. This addendum continues the procedure laid out in TG-51 but new kQ data for photon beams, based on Monte Carlo simulations, are presented and recommendations are given to improve the accuracy and consistency of the protocol's implementation. The components of the uncertainty budget in determining absorbed dose to water at the reference point are introduced and the magnitude of each component discussed. Finally, the consistency of experimental determination of ND,w coefficients is discussed. It is expected that the implementation of this addendum will be straightforward, assuming that the user is already familiar with TG-51. The changes introduced by this report are generally minor, although new recommendations could result in procedural changes for individual users. It is expected that the effort on the medical physicist's part to implement this addendum will not be significant and could be done as part of the annual linac calibration.

Dosimetric impact of density variations in Solid Water 457 water-equivalent slabs

The purpose of this study was to determine the dosimetric impact of density variations observed in water-equivalent solid slabs. Measurements were performed using two 30 cm × 30 cm water-equivalent slabs, one being 4 cm think and the other 5 cm thick. The location and extent of density variations were determined by computed tomography (CT) scans. Additional imaging measurements were made with an amorphous silicon megavoltage portal imaging device and an ultrasound unit. Dosimetric measurements were conducted with a 2D ion chamber array, and a scanned diode in water. Additional measurements and calculations were made of small rectilinear void inhomogeneities formed with water-equivalent slabs, using a 2D ion chamber array and the convolution superposition algorithm. Two general types of density variation features were observed on CT images: 1) regions of many centimeters across, but typically only a few millimeters thick, with electron densities a few percent lower than the bulk material, and 2) cylindrical regions roughly 0.2 cm in diameter and up to 20 cm long with electron densities up to 5% lower than the surrounding material. The density variations were not visible on kilovoltage, megavoltage or ultrasound images. The dosimetric impact of the density variations were not detectable to within 0.1% using the 2D ion chamber array or the scanning photon diode at distances 0.4 cm to 2 cm beyond the features. High-resolution dosimetric calculations using the convolution-superposition algorithm with density corrections enabled on CT-based datasets showed no discernable dosimetric impact. Calculations and measurements on simulated voids place the upper limit on possible dosimetric variations from observed density variations at much less than 0.6%. CT imaging of water-equivalent slabs may reveal density variations which are otherwise unobserved with kV, MV, or ultrasound imaging. No dosimetric impact from these features was measureable with an ion chamber array or scanned photon diode. Consequently, they were determined to be acceptable for all clinical use.

Recommendations for clinical electron beam dosimetry: supplement to the recommendations of Task Group 25

The goal of Task Group 25 (TG-25) of the Radiation Therapy Committee of the American Association of.Physicists in Medicine (AAPM) was to provide a methodology and set of procedures for a medical physicist performing clinical electron beam dosimetry in the nominal energy range of 5-25 MeV. Specifically, the task group recommended procedures for acquiring basic information required for acceptance testing and treatment planning of new accelerators with therapeutic electron beams. Since the publication of the TG-25 report, significant advances have taken place in the field of electron beam dosimetry, the most significant being that primary standards laboratories around the world have shifted from calibration standards based on exposure or air kerma to standards based on absorbed dose to water. The AAPM has published a new calibration protocol, TG-51, for the calibration of high-energy photon and electron beams. The formalism and dosimetry procedures recommended in this protocol are based on the absorbed dose to water calibration coefficient of an ionization chamber at 60Co energy, N60Co(D,w), together with the theoretical beam quality conversion coefficient k(Q) for the determination of absorbed dose to water in high-energy photon and electron beams. Task Group 70 was charged to reassess and update the recommendations in TG-25 to bring them into alignment with report TG-51 and to recommend new methodologies and procedures that would allow the practicing medical physicist to initiate and continue a high quality program in clinical electron beam dosimetry. This TG-70 report is a supplement to the TG-25 report and enhances the TG-25 report by including new topics and topics that were not covered in depth in the TG-25 report. These topics include procedures for obtaining data to commission a treatment planning computer, determining dose in irregularly shaped electron fields, and commissioning of sophisticated special procedures using high-energy electron beams. The use of radiochromic film for electrons is addressed, and radiographic film that is no longer available has been replaced by film that is available. Realistic stopping-power data are incorporated when appropriate along with enhanced tables of electron fluence data. A larger list of clinical applications of electron beams is included in the full TG-70 report available at http://www.aapm.org/pubs/reports. Descriptions of the techniques in the clinical sections are not exhaustive but do describe key elements of the procedures and how to initiate these programs in the clinic. There have been no major changes since the TG-25 report relating to flatness and symmetry, surface dose, use of thermoluminescent dosimeters or diodes, virtual source position designation, air gap corrections, oblique incidence, or corrections for inhomogeneities. Thus these topics are not addressed in the TG-70 report.

Monte Carlo calculations of correction factors for plastic phantoms in clinical photon and electron beam dosimetry

The purpose of this study is to calculate correction factors for plastic water (PW) and plastic water diagnostic-therapy (PWDT) phantoms in clinical photon and electron beam dosimetry using the EGSnrc Monte Carlo code system. A water-to-plastic ionization conversion factor k(pl) for PW and PWDT was computed for several commonly used Farmer-type ionization chambers with different wall materials in the range of 4-18 MV photon beams. For electron beams, a depth-scaling factor c(pl) and a chamber-dependent fluence correction factor h(pl) for both phantoms were also calculated in combination with NACP-02 and Roos plane-parallel ionization chambers in the range of 4-18 MeV. The h(pl) values for the plane-parallel chambers were evaluated from the electron fluence correction factor phi(pl)w and wall correction factors P(wall,w) and P(wall,pl) for a combination of water or plastic materials. The calculated k(pl) and h(pl) values were verified by comparison with the measured values. A set of k(pl) values computed for the Farmer-type chambers was equal to unity within 0.5% for PW and PWDT in photon beams. The k(pl) values also agreed within their combined uncertainty with the measured data. For electron beams, the c(pl) values computed for PW and PWDT were from 0.998 to 1.000 and from 0.992 to 0.997, respectively, in the range of 4-18 MeV. The phi(pl)w values for PW and PWDT were from 0.998 to 1.001 and from 1.004 to 1.001, respectively, at a reference depth in the range of 4-18 MeV. The difference in P(wall) between water and plastic materials for the plane-parallel chambers was 0.8% at a maximum. Finally, h(pl) values evaluated for plastic materials were equal to unity within 0.6% for NACP-02 and Roos chambers. The h(pl) values also agreed within their combined uncertainty with the measured data. The absorbed dose to water from ionization chamber measurements in PW and PWDT plastic materials corresponds to that in water within 1%. Both phantoms can thus be used as a substitute for water for photon and electron dosimetry.

Medical reference dosimetry using EPR measurements of alanine: development of an improved method for clinical dose levels

BACKGROUND

Electron spin resonance (EPR) is used to determine the absorbed dose of alanine dosimeters exposed to clinical photon beams in a solid-water phantom. Alanine is potentially suitable for medical reference dosimetry, because of its near water equivalence over a wide energy spectrum, low signal fading, non-destructive measurement and small dosimeter size.

MATERIAL AND METHODS

A Bruker EMX-micro EPR spectrometer with a rectangular cavity and a measurement time of two minutes per dosimeter was used for reading of irradiated alanine dosimeters. Under these conditions a new algorithm based on scaling of known spectra was developed to extract the alanine signal.

RESULTS

The dose accuracy, including calibration uncertainty, is less than 2% (k=1) above 4 Gy (n=4). The measurement uncertainty is fairly constant in absolute terms (approximately 30 mGy) and the relative uncertainty therefore rises for dose measurements below 4 Gy. Typical reproducibility is <1% (k=1) above 10 Gy and <2% between 4 and 10 Gy. Below 4 Gy the uncertainty is higher. A depth dose curve measurement was performed in a solid-water phantom irradiated to a dose of 20 Gy at the maximum dose point (d(max)) in 6 and 18 MV photon beams. The typical difference between the dose measured with alanine in solid water and the dose measured with an ion chamber in a water tank was about 1%. A difference of 2% between 6 and 18 MV was found, possibly due to non-water equivalence of the applied phantom.

DISCUSSION

Compared to previously published methods the proposed algorithm can be applied without normalisation of phase shifts caused by changes in the g-value of the cavity. The study shows that alanine dosimetry is a suitable candidate for medical reference dosimetry especially for quality control applications.

Solid water phantom heat conduction: Heating and cooling rates

Solid water is often the phantom material of choice for dosimetry procedures in radiotherapy high-energy X-ray and electron beam radiation calibration and quality assurance. This note investigates variation in heat conduction that can occur for a common commercially available solid water stack phantom when a temperature differential occurs between the phantom and ambient temperature. These variations in temperature can then affect radiation measurements and thus the accuracy of radiation dosimetry. In this manuscript, we aim to investigate the variations in temperature which can occur in radiation measurement incorporated (RMI) solid water phantoms, their thermal properties and the effects on radiation dosimetry which can occur because of temperature differentials. Results have shown that the rate of temperature change at a phantom center is a complex function but appears relatively proportional to the surface area of the phantom in normal clinical usage. It is also dependent on the thermal conductivity of any material in contact with the phantom; and the nature of the phantom construction, i.e., the number and thickness of slices within the phantom. A thermal time constant of approximately 20 min was measured for a 2-cm solid water phantom slice when located on a steel workbench in comparison to 60 min when located on a wooden workbench (linac couch insert). It is found that for larger solid water stack phantoms, a transient (within 1 degrees C) thermal equilibrium exists at the center for up to 2 h, before the temperature begins to change. This is assumed to be due to the insulating properties of multiple slices within the stack, whereby very small air spaces are introduced inhibiting the heat conduction through the phantom material. It is therefore recommended that the solid water/phantom material is kept within the treatment room for closest thermal accuracy conditions or at least placed within the room approximately 10 h before dosimetry measurements. If these options are not available, a standard linear interpolation method for calculation of temperature should be used to minimize uncertainty of temperature measurements.

An empirical method for the determination of wall perturbation factors for parallel-plate chambers in high-energy electron beams

The calibration of ion chambers in high-energy electron beams in terms of absorbed dose to water at the National Physical Laboratory requires knowledge of the ratio of perturbation factors in graphite and water phantoms. During a review of data required for the NPL calibration procedure an empirical model was developed to calculate the perturbation due to the rear wall, pwall, of a well-guarded ion chamber in a high-energy electron beam. The overall uncertainty in this method is estimated to be 0.4%, which is the lowest value reported to date. The model reproduces measured data at the 0.1% level or better and indicates that the NACP ion chamber has a nonzero perturbation factor in electron beams due to backscatter from the rear wall. The effect is small (<0.5%) at high energies (R50>4 cm, E0>10 MeV) but becomes large at low energies-up to 1.4% at E0=4 MeV (R50=1.2 cm). The model indicates that there is a nonzero correction for the NACP chamber in both a graphite and water phantom and that material adjacent to the air cavity has a significant effect on the measured ionization. These values are consistent with previous measurements and recent Monte Carlo calculations. The model could be used in the design of ion chambers and in the estimation of corrections for non-homogeneous systems, especially in the absence of accurate Monte Carlo simulations.