D. V. Webb

A temporal method of avoiding the Cerenkov radiation generated in organic scintillator dosimeters by pulsed mega-voltage electron and photon beams.

The output signal of an organic scintillator probe consists of a scintillation signal and Cerenkov and fluorescence radiation (CFR) signal when the probe is exposed to a mega-voltage photon or electron beam. The CFR signal is usually unwanted because it comes from both the scintillator and light guide and so it is not proportional to the absorbed dose in the scintillator. A new organic scintillator detector system has been constructed for absorbed dose measurement in pulsed mega-voltage electron and photon beams that are commonly used in radiotherapy treatment, eliminating most of the CFR signal. The new detector system uses a long decay constant BC-444G (Bicron, Newbury, OH, USA) scintillator which gives a signal that can be time resolved from the prompt CFR signal so that the measured contribution of prompt signal is negligible. The response of the new scintillator detector system was compared with the measurements from a plastic scintillator detector that were corrected for the signal contribution from the CFR, and to appropriately corrected ion chamber measurements showing agreement in the 16 MeV electron beam used.

Dealing with Cerenkov radiation generated in organic scintillator dosimeters by bremsstrahlung beams

An organic scintillator detector system has been developed for radiotherapy bremsstrahlung dosimetry. The scintillators are connected to photodiodes by light pipes as the photodiodes must be removed and shielded from the incident radiation. The photodiodes see visible and near-visible light emissions from the scintillator as well as Cerenkov and fluorescence radiation that has been generated and trapped in the scintillator and light pipe. The Cerenkov and fluorescence radiation limits the accuracy of the dosimeter. This work examines a range of methods for diminishing the signal contribution of Cerenkov and fluorescence radiation while optimizing the scintillator signal. Three methods of achieving these goals have been used. They are: reflective coatings on the scintillator, long-wavelength-emitting scintillators used in conjunction with the photodiode, and absorptive filters placed between the light guide and photodiode. The contribution of the Cerenkov radiation to the light seen by the photodiode has been modelled and the model predictions have been tested using bremsstrahlung beams of peak energy between 13 and 20 MV, showing agreement with measurement.

Direct MC conversion of absorbed dose to graphite to absorbed dose to water for 6060Co radiation

The ARPANSA calibration service for (60)Co gamma rays is based on a primary standard graphite calorimeter that measures absorbed dose to graphite. Measurements with the calorimeter are converted to the absorbed dose to water using the calculation of the ratio of the absorbed dose in the calorimeter to the absorbed dose in a water phantom. ARPANSA has recently changed the basis of this calculation from a photon fluence scaling method to a direct Monte Carlo (MC) calculation. The MC conversion uses an EGSnrc model of the cobalt source that has been validated against water tank and graphite phantom measurements, a step that is required to quantify uncertainties in the underlying interaction coefficients in the MC code. A comparison with the Bureau International des Poids et Mesures (BIPM) as part of the key comparison BIPM.RI(I)-K4 showed an agreement of 0.9973 (53).

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.

The Australian radiation protection and nuclear safety agency megavoltage photon thermoluminescence dosimetry postal audit service 2007–2010

The Australian radiation protection and nuclear safety agency (ARPANSA) has continuously provided a level 1 mailed thermoluminescence dosimetry audit service for megavoltage photons since 2007. The purpose of the audit is to provide an independent verification of the reference dose output of a radiotherapy linear accelerator in a clinical environment. Photon beam quality measurements can also be made as part of the audit in addition to the output measurements. The results of all audits performed between 2007 and 2010 are presented. The average of all reference beam output measurements calculated as a clinically stated dose divided by an ARPANSA measured dose is 0.9993. The results of all beam quality measurements calculated as a clinically stated quality divided by an ARPANSA measured quality is 1.0087. Since 2011 the provision of all auditing services has been transferred from the Ionizing Radiation Standards section to the Australian Clinical Dosimetry Service (ACDS) which is currently housed within ARPANSA.

Shift in absorbed dose for megavoltage photons when changing to TRS-398 in Australia

Australian primary standards of air kerma and absorbed dose are realized in60Co gamma rays. To calibrate the megavoltage photon beams from linear accelerators, radiotherapy centres have their ionization chamber calibrated in a60Co beam and then use a protocol to transfer this calibration to the higher energy. The radiotherapy community is in the process of changing from the ACPSEM Protocol (Second Edition 1998) based on an air kerma calibration to the IAEA’s TRS-398 Code of Practice, based on an absorbed dose to water calibration. To evaluate the shift in absorbed dose resulting from the new protocol, the absorbed dose should be determined using both protocols and compared. We present a formula for this shift which can be used to check the result. To use this formula the centre needs to measure a displacement correction and know the ratio of the air kerma to absorbed dose to water calibration factors at60Co. We calculate the change they should expect by using the average ratio of the air kerma and absorbed dose to water calibration factors for NE2571 and NE2561 chambers, based on Australian standards, and by estimating the displacement correction from published depth dose data. We find the absorbed dose in a megavoltage photon beam to increase by between 0.1 and 0.6% for NE2571 chambers and between 0.7 and 1.1% for NE2561 chambers, for beams up to 35 MV. The dose measured using TRS-398 is always higher.

The relative response of NE2561 and NE2611A ionization chambers in megavoltage x-ray beams

The relative energy response of NE2561 and NE261 IA ionization chambers to megavoltage photon beams from the ARPANSA linac indicates significant differences between these two types of chamber. In 16 MV beams of TPR20(10) 0.779, differences of about 2% are observed. The results are expressed as ratios KQ of the beam quality correction factors kQ, where the kQ factor for each type of chamber is the ratio of the absorbed dose to water calibration factor ND, at the x-ray quality Q to that at 60Co. These results have implications for the use of generic kQ factors in dosimetry protocols and suggest that NE2561 and NE2611A ionization chambers cannot be assumed to be identical.

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 60Co 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 60Co 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 60Co. ARPANSA chambers have also been calibrated at international primary standard laboratories allowing comparison of calibration coefficients and thus 60Co absorbed dose standards. 60Co 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 60Co source are consistent with international doses within the stated uncertainties.

A comparison of Australian and Canadian calibration coefficients for air kerma and absorbed dose to water for60Co γ radiation

Australian and Canadian calibration coefficients for air kerma and absorbed dose to water for60Co gamma radiation have been compared using transfer standard ionization chambers of types NE 2561 and NE 2611A. Whilst the primary standards of air kerma are similar, both being thick-walled graphite cavity chambers but employing different methods to evaluate the Awall correction, the primary standards of absorbed dose to water are quite different. The Australian standard is based on measurements made with a graphite calorimeter, whereas the Canadian standard uses a sealed water calorimeter. The comparison result, expressed as a ratio of calibration coefficientsR=NARPANSA/NNRC, is 1.0006 with a combined standard uncertainty of 0.35% for the air kerma standards and 1.0052 with a combined standard uncertainty of 0.47% for the absorbed dose to water standards. This demonstrates the agreement of the Australian and Canadian radiation dosimetry standards. The results are also consistent with independent comparisons of each laboratory with the BIPM reference standards. A ‘trilateral’ analysis confirms the present determination of the relationship between the standards, within the 0.09% random component of the combined standard uncertainty for the three comparisons.