Towards high spatial resolution tissue-equivalent dosimetry for microbeam radiation therapy using organic semiconductors.

Spatially fractionated ultra-high-dose-rate beams used during microbeam radiation therapy (MRT) have been shown to increase the differential response between normal and tumour tissue. Quality assurance of MRT requires a dosimeter that possesses tissue equivalence, high radiation tolerance and spatial resolution. This is currently an unsolved challenge. This work explored the use of a 500 nm thick organic semiconductor for MRT dosimetry on the Imaging and Medical Beamline at the Australian Synchrotron. Three beam filters were used to irradiate the device with peak energies of 48, 76 and 88 keV with respective dose rates of 3668, 500 and 209 Gy s-1. The response of the device stabilized to 30% efficiency after an irradiation dose of 30 kGy, with a 0.5% variation at doses of 35 kGy and higher. The calibration factor after pre-irradiation was determined to be 1.02 ±â€…0.005 µGy per count across all three X-ray energy spectra, demonstrating the unique advantage of using tissue-equivalent materials for dosimetry. The percentage depth dose curve was within ±5% of the PTW microDiamond detector. The broad beam was fractionated into 50 microbeams (50 µm FHWM and 400 µm centre-to-centre distance). For each beam filter, the FWHMs of all 50 microbeams were measured to be 51 ±â€…1.4, 53 ±â€…1.4 and 69 ±â€…1.9 µm, for the highest to lowest dose rate, respectively. The variation in response suggested the photodetector possessed dose-rate dependence. However, its ability to reconstruct the microbeam profile was affected by the presence of additional dose peaks adjacent to the one generated by the X-ray microbeam. Geant4 simulations proved that the additional peaks were due to optical photons generated in the barrier film coupled to the sensitive volume. The simulations also confirmed that the amplitude of the additional peak in comparison with the microbeam decreased for spectra with lower peak energies, as observed in the experimental data. The material packaging can be optimized during fabrication by solution processing onto a flexible substrate with a non-fluorescent barrier film. With these improvements, organic photodetectors show promising prospects as a cost-effective high spatial resolution tissue-equivalent flexible dosimeter for synchrotron radiation fields.

Synchrotron Microbeam Radiation Therapy for the Treatment of Lung Carcinoma: A Preclinical Study.

PURPOSE: In the past 3 decades, synchrotron microbeam radiation therapy (S-MRT) has been shown to achieve both good tumor control and normal tissue sparing in a range of preclinical animal models. However, the use of S-MRT for the treatment of lung tumors has not yet been investigated. This study is the first to evaluate the therapeutic efficacy of S-MRT for the treatment of lung carcinoma, using a new syngeneic and orthotopic mouse model. METHODS AND MATERIALS: Lewis Lung carcinoma-bearing mice were irradiated with 2 cross-fired arrays of S-MRT or synchrotron broad-beam (S-BB) radiation therapy. S-MRT consisted of 17 microbeams with a width of 50 µm and center-to-center spacing of 400 µm. Each microbeam delivered a peak entrance dose of 400 Gy whereas S-BB delivered a homogeneous entrance dose of 5.16 Gy (corresponding to the S-MRT valley dose). RESULTS: Both treatments prolonged the survival of mice relative to the untreated controls. However, mice in the S-MRT group developed severe pulmonary edema around the irradiated carcinomas and did not have improved survival relative to the S-BB group. Subsequent postmortem examination of tumor size revealed that the mice in the S-MRT group had notably smaller tumor volume compared with the S-BB group, despite the presence of edema. Mice that were sham-implanted did not display any decline in health after S-MRT, experiencing only mild and transient edema between 4 days and 3 months postirradiation which disappeared after 4 months. Finally, a parallel study investigating the lungs of healthy mice showed the complete absence of radiation-induced pulmonary fibrosis 6 months after S-MRT. CONCLUSIONS: S-MRT is a promising tool for the treatment of lung carcinoma, reducing tumor size compared with mice treated with S-BB and sparing healthy lungs from pulmonary fibrosis. Future experiments should focus on optimizing S-MRT parameters to minimize pulmonary edema and maximize the therapeutic ratio.

HDR brachytherapy well chamber calibration and stability evaluated over twenty years of clinical use.

PURPOSE: The purpose of the study was to establish, using a retrospective analysis of existing hospital records, the long-term stability and accuracy of a high-dose-rate brachytherapy well chamber. This should be assessed to determine reliability and appropriate calibration frequency. The accrual of long-term data that demonstrates the stability of our chamber may inform others of the performance they might expect from similar equipment. METHODS AND MATERIALS: We evaluated air kerma strength measurements made with the PTW 32002 (Nucletron 077.091) high-dose-rate well chamber on 72 192Ir sources over an 18-year period and the seven calibrations of that chamber which span a 27-year period. RESULTS: Consecutive air kerma strength measurements agreed within 0.01% on average. The chamber measurement agreed with the source specification within 0.02% on average, but was up to 1.4% during some calibration periods. The chamber calibration coefficient varied by a maximum of 5% over seven chamber calibration measurements. CONCLUSIONS: The constancy of the well chamber current compared with the source manufacturer suggests that our chamber has been stable to better than 2% over a period of 18 years. Although the chamber has received different calibration coefficients over time, these coefficients are within the combined uncertainties of any two calibrations and are consistent with the chamber being stable. The agreement we have observed between clinical measurements and the source manufacturer would justify an action level for further investigation of 1%, for this specific chamber.

Evaluation of the PTW microDiamond in edge-on orientation for dosimetry in small fields.

PURPOSE: The PTW microDiamond has an enhanced spatial resolution when operated in an edge-on orientation but is not typically utilized in this orientation due to the specifications of the IAEA TRS-483 code of practice for small field dosimetry. In this work the suitability of an edge-on orientation and advantages over the recommended face-on orientation will be presented. METHODS: The PTW microDiamond in both orientations was compared on a Varian TrueBeam linac for: machine output factor (OF), percentage depth dose (PDD), and beam profile measurements from 10 × 10 cm2 to a 0.5 × 0.5 cm2 field size for 6X and 6FFF beam energies in a water tank. A quantification of the stem effect was performed in edge-on orientation along with tissue to phantom ratio (TPR) measurements. An extensive angular dependence study for the two orientations was also undertaken within two custom PMMA plastic cylindrical phantoms. RESULTS: The OF of the PTW microDiamond in both orientations agrees within 1% down to the 2 × 2 cm2 field size. The edge-on orientation overresponds in the build-up region but provides improved penumbra and has a maximum observed stem effect of 1%. In the edge-on orientation there is an angular independent response with a maximum of 2% variation down to a 2 × 2 cm2 field. The PTW microDiamond in edge-on orientation for TPR measurements agreed to the CC01 ionization chamber within 1% for all field sizes. CONCLUSIONS: The microDiamond was shown to be suitable for small field dosimetry when operated in edge-on orientation. When edge-on, a significantly reduced angular dependence is observed with no significant stem effect, making it a more versatile QA instrument for rotational delivery techniques.

Evaluation of the IAEA-TRS 483 protocol for the dosimetry of small fields (square and stereotactic cones) using multiple detectors.

The IAEA TRS 483 protocol1 for the dosimetry of small static fields in radiotherapy was used to calculate output factors for the Elekta Synergy linac at the Australian Radiation Protection and Nuclear Safety Agency (ARPANSA). Small field output factors for both square and circular fields were measured using nine different detectors. The "corrected" output factors (ratio of detector readings multiplied by the appropriate correction factor from the protocol) showed better consistency compared to the "uncorrected" output factors (ratio of detector readings only), with the relative standard deviation decreasing by approximately 1% after the application of the relevant correction factors. Comparisons relative to an arbitrarily chosen PTW 60019 microDiamond detector showed a reduction of maximal variation for the corrected values of approximately 3%. A full uncertainty budget was prepared to analyze the consistency of the output factors. Agreement within uncertainties between all detectors and field sizes was found, except for the 15 mm circular field. The results of this study show that the application of IAEA TRS 4831 when measuring small fields will improve the consistency of small field measurements when using multiple detectors contained within the protocol.

Impact of magnetic fields on dose measurement with small ion chambers illustrated in high-resolution response maps.

PURPOSE: Dosimetry of ionizing radiation in the presence of strong magnetic fields is gaining increased relevance in light of advances for MRI-guided radiation therapy. While the impact of strong magnetic fields on the overall response of ionization chambers has been simulated and measured before, this work investigates the local impact of the magnetic field on dose response in an ion chamber. High-resolution 1D and 2D response maps have been created for two small clinical thimble ionization chambers, the PinPoint chambers 31006 and 31014 (Physikalisch Technische Werkstaetten Freiburg, Germany). METHODS: Working on the Imaging and Medical Beam Line of the Australian Synchrotron an intense kilovoltage radiation beam with very low divergence, collimated to 0.1 mm was used to scan the chambers by moving them on a 2D motion platform. Measured current and beam position were correlated to create the response maps. Small neodymium magnets were used to create a field of about 0.25 T. Chamber axis, magnetic field, and beam direction were perpendicular to each other. Measurements were performed with both orientations of the magnetic field as well as without it. Chamber biases of 5 and 250 V in both polarities were used. RESULTS: The local distribution of the response of small thimble-type ionization chambers was found to be impacted by a magnetic field. Depending on the orientation of the magnetic field, the chamber response near the stem was either enhanced or reduced with the response near the tip behaving the opposite way. Local changes were in the order of up to 40% compared to measurements without the magnetic field present. Bending of the central electrode was observed for the chamber with the steel electrode. The size of the volume of reduced collection near the guard electrode was impacted by the magnetic field. As the here investigated beam and field parameters differ from those of clinical systems, quantitatively different results would be expected for the latter. However, the gyroradii encountered here were similar to those of a 6-7 MV MRI linac with a 1.5 T magnet. CONCLUSIONS: Magnetic fields impact the performance of ionization chambers also on a local level. For practical measurements this might mean a change in the effective point of measurement, in addition to any global corrections. Further knowledge about the local response will help in selecting or constructing optimized chambers for use in magnetic fields.

Australasian recommendations for quality assurance in kilovoltage radiation therapy from the Kilovoltage Dosimetry Working Group of the Australasian College of Physical Scientists and Engineers in Medicine.

The Australasian College of Physical Scientists and Engineers in Medicine (ACPSEM) Radiation Oncology Specialty Group (ROSG) formed a series of working groups to develop recommendations for guidance of radiation oncology medical physics practice within the Australasian setting. These recommendations provide a standard for safe work practices and quality control. It is the responsibility of the medical physicist to ensure that locally available equipment and procedures are sufficiently sensitive to establish compliance. The recommendations are endorsed by the ROSG, have been subject to independent expert reviews and have also been approved by the ACPSEM Council. For the Australian audience, these recommendations should be read in conjunction with the Tripartite Radiation Oncology Practice Standards and should be read in conjunction with relevant national, state or territory legislation which take precedence over the ACPSEM publication Radiation Oncology Reform Implementation Committee (RORIC) Quality Working Group, RANZCR, 2011a; Kron et al. Clin Oncol 27(6):325-329, 2015; Radiation Oncology Reform Implementation Committee (RORIC) Quality Working Group, RANZCR, 2018a, b).

Survey of 5 mm small-field output factor measurements in Australia.

The Australian Radiation Protection and Nuclear Safety Agency (ARPANSA) held a comparison exercise in April 2016 where participants came to ARPANSA and measured the output factor of a nominal 5 mm cone attached to the ARPANSA Elekta Synergy (Elekta, Crawley, UK) linear accelerator. The goal of the exercise was to compare the consistency and methods used by independent medical physicists in measuring small-field output factors. ARPANSA provided a three-dimensional scanning tank for detector setup and positioning, but the participants were required to measure the output factor with their own detectors. No information regarding output factors previously measured was supplied to participants to make each result as independent as possible. Fifteen groups travelled to ARPANSA bringing a wide range of detectors and methods. A total of 30 measurements of the output factor were made. The standard deviation of the measurements (excluding one expected outlier from an uncorrected ionization chamber measurement) was 3.6%. The results provide an insight into the consistency of small-field dosimetry being performed in Australia and New Zealand at the present time.

Spatial response of synthetic microDiamond and diode detectors measured with kilovoltage synchrotron radiation.

PURPOSE: To map the spatial response of four solid-state radiation detectors of types commonly used for radiotherapy dosimetry. METHODS: PTW model 60016 Diode P, 60017 Diode E, 60018 Diode SRS, and 60019 microDiamond detectors were radiographed using a high resolution conventional X-ray system. Their spatial response was then investigated using a 0.1 mm diameter beam of 95 keV average energy photons generated by a synchrotron. The detectors were scanned through the beam while their signal was recorded as a function of position, to map the response. These 2D response maps were created in both the end-on and side-on orientations. RESULTS: The results show the location and size of the active region. End-on, the active area was determined to be centrally located and within 0.2 mm of the manufacturer's specified diameter. The active areas of the 60016 Diode P, 60017 Diode E, 60018 Diode SRS detectors are uniform to within approximately 5%. The 60019 microDiamond showed local variations up to 30%. The extra-cameral signal in the microDiamond was calculated from the side-on scan to be approximately 8% of the signal from the active element. CONCLUSIONS: The spatial response of four solid-state detectors has been measured. The technique yielded information about the location and uniformity of the active area, and the extra-cameral signal, for the beam quality used.

Establishing IAEA TRS-457 diagnostic X-ray beam qualities at the Australian primary standard dosimetry laboratory.

This paper reports the process and findings in establishing the diagnostic beam qualities for the calibration of diagnostic dose meters in the Australian primary standard dosimetry laboratory at the Australian Radiation Protection and Nuclear Safety Agency, ARPANSA. A novel iterative method to achieve the RQR beam quality requirements described by IEC 61267 is presented. The correction factors for the primary standard free-air chamber and their uncertainties are reported. The calibration procedure with detailed uncertainty budget of a typical diagnostic ionisation chamber (model PTW 34060) is reported. Beam profile measurements and details of the use of an external monitor chamber (model PTW 34014) are also reported. The uncertainty in the ARPANSA calibration coefficients is estimated to be 1.2% at k = 2 for both RQR and RQA beam qualities. The laboratory is now able to provide calibration services for radiation detectors used in general X-ray radiography.

Commissioning of a PTW 34070 large-area plane-parallel ionization chamber for small field megavoltage photon dosimetry.

PURPOSE: This study investigates a large-area plane-parallel ionization chamber (LAC) for measurements of dose-area product in water (DAPw ) in megavoltage (MV) photon fields. METHODS: Uniformity of electrode separation of the LAC (PTW34070 Bragg Peak Chamber, sensitive volume diameter: 8.16 cm) was measured using high-resolution microCT. Signal dependence on angle α of beam incidence for square 6 MV fields of side length s = 20 cm and 1 cm was measured in air. Polarity and recombination effects were characterized in 6, 10, and 18 MV photons fields. To assess the lateral setup tolerance, scanned LAC profiles of a 1 × 1 cm2 field were acquired. A 6 MV calibration coefficient, ND,w,LAC , was determined in a field collimated by a 5 cm diameter stereotactic cone with known DAPw . Additional calibrations in 10 × 10 cm2 fields at 6, 10, and 18 MV were performed. RESULTS: Electrode separation is uniform and agrees with specifications. Volume-averaging leads to a signal increase proportional to ~1/cos(α) in small fields. Correction factors for polarity and recombination range between 0.9986 to 0.9996 and 1.0007 to 1.0024, respectively. Off-axis displacement by up to 0.5 cm did not change the measured signal in a 1 × 1 cm2 field. ND,w,LAC was 163.7 mGy cm-2 nC-1 and differs by +3.0% from the coefficient derived in the 10 × 10 cm2 6 MV field. Response in 10 and 18 MV fields increased by 1.0% and 2.7% compared to 6 MV. CONCLUSIONS: The LAC requires only small correction factors for DAPw measurements and shows little energy dependence. Lateral setup errors of 0.5 cm are tolerated in 1 × 1 cm2 fields, but beam incidence must be kept as close to normal as possible. Calibration in 10 × 10 fields is not recommended because of the LAC's over-response. The accuracy of relative point-dose measurements in the field's periphery is an important limiting factor for the accuracy of DAPw measurements.

High dose rate brachytherapy source measurement intercomparison.

This work presents a comparison of air kerma rate (AKR) measurements performed by multiple radiotherapy centres for a single HDR 192Ir source. Two separate groups (consisting of 15 centres) performed AKR measurements at one of two host centres in Australia. Each group travelled to one of the host centres and measured the AKR of a single 192Ir source using their own equipment and local protocols. Results were compared to the 192Ir source calibration certificate provided by the manufacturer by means of a ratio of measured to certified AKR. The comparisons showed remarkably consistent results with the maximum deviation in measurement from the decay-corrected source certificate value being 1.1%. The maximum percentage difference between any two measurements was less than 2%. The comparisons demonstrated the consistency of well-chambers used for 192Ir AKR measurements in Australia, despite the lack of a local calibration service, and served as a valuable focal point for the exchange of ideas and dosimetry methods.

Characterization of a synthetic single crystal diamond detector for dosimetry in spatially fractionated synchrotron x-ray fields.

PURPOSE: Modern radiotherapy modalities often use small or nonstandard fields to ensure highly localized and precise dose delivery, challenging conventional clinical dosimetry protocols. The emergence of preclinical spatially fractionated synchrotron radiotherapies with high dose-rate, sub-millimetric parallel kilovoltage x-ray beams, has pushed clinical dosimetry to its limit. A commercially available synthetic single crystal diamond detector designed for small field dosimetry has been characterized to assess its potential as a dosimeter for synchrotron microbeam and minibeam radiotherapy. METHODS: Experiments were carried out using a synthetic diamond detector on the imaging and medical beamline (IMBL) at the Australian Synchrotron. The energy dependence of the detector was characterized by cross-referencing with a calibrated ionization chamber in monoenergetic beams in the energy range 30-120 keV. The dose-rate dependence was measured in the range 1-700 Gy/s. Dosimetric quantities were measured in filtered white beams, with a weighted mean energy of 95 keV, in broadbeam and spatially fractionated geometries, and compared to reference dosimeters. RESULTS: The detector exhibits an energy dependence; however, beam quality correction factors (kQ) have been measured for energies in the range 30-120 keV. The kQ factor for the weighted mean energy of the IMBL radiotherapy spectrum, 95 keV, is 1.05 ± 0.09. The detector response is independent of dose-rate in the range 1-700 Gy/s. The percentage depth dose curves measured by the diamond detector were compared to ionization chambers and agreed to within 2%. Profile measurements of microbeam and minibeam arrays were performed. The beams are well resolved and the full width at halfmaximum agrees with the nominal width of the beams. The peak to valley dose ratio (PVDR) calculated from the profiles at various depths in water agrees within experimental error with PVDR calculations from Gafchromic film data. CONCLUSIONS: The synthetic diamond detector is now well characterized and will be used to develop an experimental dosimetry protocol for spatially fractionated synchrotron radiotherapy.

Maintaining the accuracy of the (60)Co calibration service at the ARPANSA post source replacement in 2010.

The Australian Radiation Protection and Nuclear Safety Agency (ARPANSA) maintains a (60)Co teletherapy source primarily for the calibration of therapy dosemeters. The source and encapsulating head were replaced in early 2010 with an Eldorado 78 head and new (60)Co source. In this article we present the results of ongoing accuracy and stability measurements since the replacement. A number of formal and informal indirect comparisons have been carried out with laboratories holding primary and secondary standards for (60)Co. ARPANSA chambers have also been calibrated at international primary standard laboratories allowing comparison of calibration coefficients and thus (60)Co absorbed dose standards. (60)Co calibration coefficients supplied by manufacturers of chambers were compared to those measured at the ARPANSA when this calibration was traceable to a primary standard. ARPANSA also participates in an annual international mailed dosimetry audit conducted by the International Atomic Energy Agency. The results thus far demonstrate that the absorbed doses to water delivered by the new ARPANSA (60)Co source are consistent with international doses within the stated uncertainties.

Direct calibration in megavoltage photon beams using Monte Carlo conversion factor: validation and clinical implications.

The Australian Radiation Protection and Nuclear Safety Agency (ARPANSA) has established a method for ionisation chamber calibrations using megavoltage photon reference beams. The new method will reduce the calibration uncertainty compared to a (60)Co calibration combined with the TRS-398 energy correction factor. The calibration method employs a graphite calorimeter and a Monte Carlo (MC) conversion factor to convert the absolute dose to graphite to absorbed dose to water. EGSnrc is used to model the linac head and doses in the calorimeter and water phantom. The linac model is validated by comparing measured and modelled PDDs and profiles. The relative standard uncertainties in the calibration factors at the ARPANSA beam qualities were found to be 0.47% at 6 MV, 0.51% at 10 MV and 0.46% for the 18 MV beam. A comparison with the Bureau International des Poids et Mesures (BIPM) as part of the key comparison BIPM.RI(I)-K6 gave results of 0.9965(55), 0.9924(60) and 0.9932(59) for the 6, 10 and 18 MV beams, respectively, with all beams within 1σ of the participant average. The measured kQ values for an NE2571 Farmer chamber were found to be lower than those in TRS-398 but are consistent with published measured and modelled values. Users can expect a shift in the calibration factor at user energies of an NE2571 chamber between 0.4-1.1% across the range of calibration energies compared to the current calibration method.

Long term OSLD reader stability in the ACDS level one audit.

The Australian Clinical Dosimetry Service (ACDS) has demonstrated the capacity to perform a basic dosimetry audit on all radiotherapy clinics across Australia. During the ACDS's three and a half year trial the majority of the audits were performed using optically stimulated luminescence dosimeters (OSLD) mailed to facilities for exposure to a reference dose, and then returned to the ACDS for analysis. This technical note investigates the stability of the readout process under the large workload of the national dosimetry audit. The OSLD readout uncertainty contributes to the uncertainty of several terms of the dose calculation equation and is a major source of uncertainty in the audit. The standard deviation of four OSLD readouts was initially established at 0.6 %. Measurements over 13 audit batches--each batch containing 200-400 OSLDs--showed variability (0.5-0.9 %) in the readout standard deviation. These shifts have not yet necessitated a change to the audit scoring levels. However, a standard deviation in OSLD readouts greater than 0.9 % will change the audit scoring levels. We identified mechanical wear on the OSLD readout adapter as a cause of variability in readout uncertainty, however, we cannot rule out other causes. Additionally we observed large fluctuations in the distribution of element correction factors (ECF) for OSLD batches. We conclude that the variability in the width of the ECF distribution from one batch to another is not caused by variability in readout uncertainty, but rather by variations in the OSLD stock.

Remote auditing of radiotherapy facilities using optically stimulated luminescence dosimeters.

PURPOSE: On 1 July 2012, the Australian Clinical Dosimetry Service (ACDS) released its Optically Stimulated Luminescent Dosimeter (OSLD) Level I audit, replacing the previous TLD based audit. The aim of this work is to present the results from this new service and the complete uncertainty analysis on which the audit tolerances are based. METHODS: The audit release was preceded by a rigorous evaluation of the InLight® nanoDot OSLD system from Landauer (Landauer, Inc., Glenwood, IL). Energy dependence, signal fading from multiple irradiations, batch variation, reader variation, and dose response factors were identified and quantified for each individual OSLD. The detectors are mailed to the facility in small PMMA blocks, based on the design of the existing Radiological Physics Centre audit. Modeling and measurement were used to determine a factor that could convert the dose measured in the PMMA block, to dose in water for the facility's reference conditions. This factor is dependent on the beam spectrum. The TPR20,10 was used as the beam quality index to determine the specific block factor for a beam being audited. The audit tolerance was defined using a rigorous uncertainty calculation. The audit outcome is then determined using a scientifically based two tiered action level approach. Audit outcomes within two standard deviations were defined as Pass (Optimal Level), within three standard deviations as Pass (Action Level), and outside of three standard deviations the outcome is Fail (Out of Tolerance). RESULTS: To-date the ACDS has audited 108 photon beams with TLD and 162 photon beams with OSLD. The TLD audit results had an average deviation from ACDS of 0.0% and a standard deviation of 1.8%. The OSLD audit results had an average deviation of -0.2% and a standard deviation of 1.4%. The relative combined standard uncertainty was calculated to be 1.3% (1σ). Pass (Optimal Level) was reduced to ≤2.6% (2σ), and Fail (Out of Tolerance) was reduced to >3.9% (3σ) for the new OSLD audit. Previously with the TLD audit the Pass (Optimal Level) and Fail (Out of Tolerance) were set at ≤4.0% (2σ) and >6.0% (3σ). CONCLUSIONS: The calculated standard uncertainty of 1.3% at one standard deviation is consistent with the measured standard deviation of 1.4% from the audits and confirming the suitability of the uncertainty budget derived audit tolerances. The OSLD audit shows greater accuracy than the previous TLD audit, justifying the reduction in audit tolerances. In the TLD audit, all outcomes were Pass (Optimal Level) suggesting that the tolerances were too conservative. In the OSLD audit 94% of the audits have resulted in Pass (Optimal level) and 6% of the audits have resulted in Pass (Action Level). All Pass (Action level) results have been resolved with a repeat OSLD audit, or an on-site ion chamber measurement.

Australasian brachytherapy audit: results of the 'end-to-end' dosimetry pilot study.

INTRODUCTION: We present the results of a pilot study to test the feasibility of a brachytherapy dosimetry audit. METHODS: The feasibility study was conducted at seven sites from four Australian states in both public and private centres. A purpose-built cylindrical water phantom was imaged using the local imaging protocol and a treatment plan was generated to deliver 1 Gy to the central (1 of 3) thermoluminescent dosimeter (TLD) from six dwell positions. RESULTS: All centres completed the audit, consisting of three consecutive irradiations, within a 2-h time period, with the exception of one centre that uses a pulsed dose rate brachytherapy unit. All TLD results were within 4.5% of the predicted value, with the exception of one subset where the dwell position step size was incorrectly applied. CONCLUSIONS: While the limited data collected in the study demonstrated considerable heterogeneity in clinical practice, the study proved a brachytherapy dosimetry audit to be feasible. Future studies should include verification of source strength using a Standard Dosimetry Laboratory calibrated chamber, a phantom that more closely mimics the clinical situation, a more comprehensive review of safety and quality assurance (QA) procedures including source dwell time and position accuracy, and a review of patient treatment QA procedures such as applicator position verification.

Comparison of TLD calibration methods for 192Ir dosimetry.

For the purpose of dose measurement using a high-dose rate (192)Ir source, four methods of thermoluminescent dosimeter (TLD) calibration were investigated. Three of the four calibration methods used the (192)Ir source. Dwell times were calculated to deliver 1 Gy to the TLDs irradiated either in air or water. Dwell time calculations were confirmed by direct measurement using an ionization chamber. The fourth method of calibration used 6 MV photons from a medical linear accelerator, and an energy correction factor was applied to account for the difference in sensitivity of the TLDs in (192)Ir and 6 MV. The results of the four TLD calibration methods are presented in terms of the results of a brachytherapy audit where seven Australian centers irradiated three sets of TLDs in a water phantom. The results were in agreement within estimated uncertainties when the TLDs were calibrated with the (192)Ir source. Calibrating TLDs in a phantom similar to that used for the audit proved to be the most practical method and provided the greatest confidence in measured dose. When calibrated using 6 MV photons, the TLD results were consistently higher than the (192)Ir-calibrated TLDs, suggesting this method does not fully correct for the response of the TLDs when irradiated in the audit phantom.

The 2009 survey of therapy equipment and dosimetry practices in Australian radiotherapy centres.

The results of a recent survey of Australian radiotherapy centres are presented. A total of 56 treatment centres were identified, operating 139 linacs, 26 kV units and 25 remote afterloaders. Of these centres, 51 participated in the survey (91%). Results are presented which summarize the beam qualities, dosimetry protocols, ion chambers and treatment modalities in clinical use. The results provide a snapshot of the equipment and practices used in radiotherapy in Australia in 2009.