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This paper summarizes the progress on the construction of a graphite calorimeter for the measurement of absorbed dose to water Dw in medium-energy filtered x-ray qualities, as part of the EMRP- funded project "Metrology for radiotherapy using complex radiation fields" (MetrExtRT, http://radiotherapy- emrp.eu/). The design and the construction of a new primary standard for absorbed dose to water (Dw) is shown, together with a presentation of the preliminary estimates of conversion factors, calculated by Monte Carlo using the EGSnrc code, and heat-transport calculations resulting from COMSOL simulations. The vacuum-gap-insulated calorimeter core has a thickness of 2 mm and a diameter of 20 mm. The graphite calorimeter is embedded in a PMMA waterproof sleeve and can be positioned at the desired measurement depth in a water phantom. Once completed, the new calorimeter will participate in an international comparison with other existing standards of absorbed dose to water for medium energy x-rays.

This paper is intended to give information about the on-going work of the MetrExtRT research project concerned with the dosimetric metrology of modern external beam radiotherapy. It started on the 1st of June 2012 for duration of 36 months and covers the study medium- and high-energy X-rays, high-energy electrons, electronic brachytherapy and scanned proton and carbon ion beams. The project concerns all the steps of traceability from the primary standards through to verification of the dose in and around the tumour, namely: developing new primary standards of absorbed dose to water; studying new detectors and improving the knowledge of the characteristics of existing detectors, useable for quality control and in vivo dosimetry, and publishing guide lines of good practice on their use. MetrExtRT gathers 10 partners that are all National Metrology Institutes (NMI) or Designated Institutes (DI) as represented by the list of authors, two Researcher Excellence Grants (REG) for the development of passive gel dosimetry (Universite d'Auverge – France) and diamond detectors (Tor Vergata University - Italy) and up to now 12 collaborators from industry, oncology centres and universities.

Introduction According to the IAEA TRS 398, in kilovoltage X-ray beams the calibration factor in terms of absorbed dose to water ( N D , w , Q ) for an ionization chamber can be derived from its calibration factor in terms of air kerma ( N K , Q ) if the chamber overall correction factor, p Q , is known. However, literature p Q values are lacking, especially when referring to the reference depth of 2 g/cm 2 . Purpose To determine the p Q factor at 2 g/cm 2 depth using a new absorbed-dose-to-water ( D w ) primary standard based on an in-water-phantom graphite calorimeter recently established for medium-energy X-ray beams. Materials and methods N D , w , Q factors are directly determined against the D w primary standard. Using the same beam qualities, the N K , Q factors are also determined. Values of p Q are obtained by comparing the N D , w , Q factors with the analogous calibration factors derived by N K , Q using the IAEA TRS 398 formalism. Ratios of mean mass-energy absorption coefficients, water to air, are taken from literature. Results For a Farmer type ionization chamber, the p Q factor was respectively 1.002 and 1.012 for the 180 kV and the 250 kV medium-energy quality of the CCRI series, with a relative combined standard uncertainty of 2%. This figure is going to be reduced to around 1% with the ongoing improvements on the new D w primary standard. Conclusion The new D w primary standard based on graphite calorimetry, although affected by a larger uncertainty compared to primary standards based on water calorimetry, allows independent measurements of p Q factors for ionization chambers currently used in radiotherapy dosimetry.

Dosimetry in radiotherapy with ionization chambers calibrated in 60Co gamma beams in terms of absorbed dose to water, DW, can be performed if a factor conventionally denoted as kQ is known. The factor kQ depends on the beam quality and the chamber characteristics. Calculated values of the kQ factors for many types of ionization chamber have been recently published. In this work the experimental determination of the kQ factors for various ionization chambers was performed for 6 MV and 15 MV photon beams and for a 14 MeV electron beam. The kQ factors were determined by a procedure based on relative measurements performed with the ionization chamber and ferrous sulphate solution in 60Co gamma radiation and accelerator beams, respectively. The experimental kQ values are compared with the calculated values so far published. Theoretical and experimental kQ values are in fairly good agreement. The uncertainty in the experimental kQ factors determined in this work is less than about 1%, that is, appreciably smaller than the uncertainty of about 1.5% reported for the calculated values.

A chemical vapour deposition diamond detector fabricated at Rome ‘Tor Vergata’ University was investigated for its applicability as transfer dosimeter in radiotherapy photon beams with small field sizes. The detector consists of a single crystal diamond with a very small sensitive volume (0.004 mm3). The detector showed a measurement repeatability of 0.1% and a long term reproducibility of 0.4%. Monte Carlo simulations revealed a response dependence on the photon beam energy of about 2% from the 60Co quality to 10 MV photon beam. The calculated detector response was found to be independent of field size within 0.5% from 10 cm × 10 cm to 2 cm × 2 cm beam size for both 6 MV and 10 MV photon beams, increasing in smaller field sizes. Dw values obtained by the diamond detector were found to be in agreement with Dw values obtained by a small volume ionization chamber in photon beams with field size down to 2 cm × 2 cm.

The appropriate correction factors have been determined to enable plane-parallel ionization chambers to be calibrated using a 60Co photon beam with a known air kerma rate. These factors refer to different calibration conditions and to different types of plane-parallel chambers. The main purpose of these correction factors is to allow calibration procedures more widely practicable than those based on the use of an electron beam of sufficiently high energy. The condition required to apply the correction factors is that the characteristics of the chambers to be calibrated are the same as those reported for the chambers considered in the investigation. To this end the types of chambers investigated were among the most widely used plane-parallel chambers commercially available.

A new diamond detector to perform absorbed-dose- to-water measurement in radiotherapy photon beams with small field sizes is being developed in the framework of the EURAMET/EC FP7 project “External Beam Cancer Therapy”. An objective of the project is to obtain detectors capable of ensuring traceability of absorbed dose measurements in radiotherapy photon beams with field size down to 1 cm x 1 cm. To this end the CVD diamond detectors up to now developed were found not adequate. The present project requires detectors with high spatial resolution, good stability and low energy dependence. Diamond detectors were chosen for their high spatial resolution and good tissue equivalence (Z = 6). The detector studied in this work is based on a single crystal CVD diamond with dimensions 3 mm x 3 mm x 0.5 mm on which 0.2 μm electrodes were thermally evaporated. Particular care was addressed to ensure the tissue-equivalence of the detector by using appropriate materials. A thorough analysis of the priming procedure, stability and signal reproducibility was carried out in a Co-60 gamma beam at dose rates in the range from 0.3 Gy min− 1 to 1.38 Gy min− 1. Moreover the detector response was studied by Monte Carlo calculations as a function both of the beam quality, from Co-60 to 10 MV photon beams, and field size, from 10 cm x 10 cm to 1 cm x 1 cm. The perturbation due to the non-water equivalence of electrodes was also determined by Monte Carlo simulations.

This paper outlines the joint research project “Increasing cancer treatment efficacy using 3D brachytherapy” co-funded in the FP7, according to the iMERA-Plus Grant Agreement No. 217257 between the EC and EURAMET e. V. (European Association of National Metrology Institutes). The project brings together the capabilities of ten major European National Metrology Institutes in the ionizing radiation field and it is focused on the targeted programme “Health”. It aims at establishing across Europe a more accurate metrological basis in brachytherapy by developing methods for the direct measurement of the quantity absorbed dose to water in brachytherapy dosimetry with an uncertainty on the dose delivered to the target volume less than 5% (k=1) at clinical level. In fact, in the current brachytherapy practice, the procedures to determine the absorbed dose imparted to the patient are affected by an uncertainty that could reduce the treatment success. Most of this uncertainty is due to a lacking metrology. No absorbed-dose primary standards are so far available to assure direct traceability in dosimetry of brachytherapy sources. In order to optimize the brachytherapy treatments there is also a need for more accurate dosimetry with high spatial resolution. The present research project is expected to increase the accuracy of brachytherapy to a level comparable to that typical of radiotherapy with external accelerator beams.

Introduction, material and method A work package of the European metrological joint research project (JRP) entitled ‘‘Metrology for radiotherapy using complex radiation fields – MetrExRT” is aimed at studying radiation qualities, measurement conditions and beam quality index suitable for dosimeter calibration in terms of Dw in the conditions of the IAEA protocol 398 for medium-energy X-rays. A survey was undertaken among European end users and calibration labs to select the most relevant beam qualities for dosimeter calibrations and international comparison. It is also aimed at gathering information about the use of medium energy X-rays for therapy. Two electronic questionnaires were circulated, one for the end users, and one for the calibration labs (32 and 9 answers respectively). Results and Conclusions Most of the X-ray generators of the end users were bought before 2000. High voltages lie between 7.5 and 300 kV while for treatment the range is between 10 and 250 kV. Addition filtration is up to 1 mm Cu (150 kV). Dose rates lie between 0.3 and 25 Gy/min, about 80% of the dose rates are below 5 Gy/min and about 10% are below 0.5 Gy/min. Concerning therapeutic indications, 54% are for contact therapy (rectum; skin; eyes; gynecology; etc.), 11% for intra-operative (breast; etc.), benign diseases are also treated. The largest numbers of treatments per year for a given service were found for contact therapy and benign diseases (from 50 to more than 500). Radiation qualities for calibration are chosen mainly in ISO 4037 and IEC 61267 and 60731. The calibration lab facilities cover the range of high voltage used for treatments. Radiation qualities are similar to those mentioned above, as it might have been foreseen, CCRI qualities are more used by metrological labs. Absorbed dose to water for 60Co is mentioned by the end users. The dose rates available at calibration labs are low compare to most of those used for the treatments. This suggests that saturation effect has to be studied to introduce a correction factor if needed. A comparison of the treatment spectra with the ones used by calibration labs will be shown. The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union.

The factor Kwall to correct for photon attenuation and scatter in the wall of ionization chambers for 60Co air-kerma measurement has been traditionally determined by a procedure based on a linear extrapolation of the chamber current to zero wall thickness. Monte Carlo calculations by Rogers and Bielajew (1990 Phys. Med. Biol. 35 1065-78) provided evidence, mostly for chambers of cylindrical and spherical geometry, of appreciable deviations between the calculated values of Kwall and those obtained by the traditional extrapolation procedure. In the present work an experimental method other than the traditional extrapolation procedure was used to determine the Kwall factor. In this method the dependence of the ionization current in a cylindrical chamber was analysed as a function of an effective wall thickness in place of the physical (radial) wall thickness traditionally considered in this type of measurement. To this end the chamber wall was ideally divided into distinct regions and for each region an effective thickness to which the chamber current correlates was determined. A Monte Carlo calculation of attenuation and scatter effects in the different regions of the chamber wall was also made to compare calculation to measurement results. The Kwall values experimentally determined in this work agree within 0.2% with the Monte Carlo calculation. The agreement between these independent methods and the appreciable deviation (up to about 1%) between the results of both these methods and those obtained by the traditional extrapolation procedure support the conclusion that the two independent methods providing comparable results are correct and the traditional extrapolation procedure is likely to be wrong. The numerical results of the present study refer to a cylindrical cavity chamber like that adopted as the Italian national air-kerma standard at INMRI-ENEA (Italy). The method used in this study applies, however, to any other chamber of the same type.

1 CEA, LIST, DM2I, Laboratoire National Henri Becquerel LNE/LNHB, F-91191 Gif sur Yvette, France 2 Denmarks Tekniske Universitet, DTU, Bygning 101, DK-2800 Kongens Lyngby, Denmark 3 Physikalisch-Technische Bundesanstalt, PTB, Bundesallee 100, D-38116 Braunschweig, Germany 4 Universte d’Auvergne, UdA, France 5 VSL B.V., Thijsseweg 11, NL-2629 JA Delft, Netherlands 6 Slovak Institute of Metrology, SMU, Karloveska 63, SK-842 55 Bratislava, Slovakia, 7 National Physical Laboratory, NPL, GB-TW11 0LW Teddington, Middlesex, United Kingdom, 8 Radiation and Nuclear Safety Authority, STUK, Laippatie 4, FI-00880 Helsinki, Finland, 9 Magyar Kereskedelmi Engedelyezesi Hivatal, MKEH, Hatosag 1534 Budapest, P.f.: 919 Hungary 10 Istituto Nazionale di Metrologia delle Radiazioni Ionizzanti, ENEA CR Casaccia, Via Anguillarese, 301, 00123 Roma, Italy 11 Cesky Metrologicky Institut Brno, CMI, Okruzni 30, 63800 Brno, Czech Republic 12 Universita degli Studi di Roma Tor Vergata,Dipartimento di Ingegneria Industriale, Via del Politecnico 1, 00133 Roma, Italy

Purpose: To investigate the dosimetric properties of synthetic single crystal diamond based Schottky diodes under irradiation with therapeutic electron beams from linear accelerators. Methods: A single crystal diamond detector was fabricated and tested under 6, 8, 10, 12, and 15 MeV electron beams. The detector performances were evaluated using three types of commercial detectors as reference dosimeters: an Advanced Markus plane parallel ionization chamber, a Semiflex cylindrical ionization chamber, and a p-type silicon detector. Preirradiation, linearity with dose, dose rate dependence, output factors, lateral field profiles, and percentage depth dose profiles were investigated and discussed. Results: During preirradiation the diamond detector signal shows a weak decrease within 0.7% with respect to the plateau value and a final signal stability of 0.1% (1σ) is observed after about 5 Gy. A good linear behavior of the detector response as a function of the delivered dose is observed with deviations below ±0.3% in the dose range from 0.02 to 10 Gy. In addition, the detector response is dose rate independent, with deviations below 0.3% in the investigated dose rate range from 0.17 to 5.45 Gy/min. Percentage depth dose curves obtained from the diamond detector are in good agreement with the ones from the reference dosimeters. Lateral beam profile measurements show an overall good agreement among detectors, taking into account their respective geometrical features. The spatial resolution of solid state detectors is confirmed to be better than that of ionization chambers, being the one from the diamond detector comparable to that of the silicon diode. A good agreement within experimental uncertainties was also found in terms of output factor measurements between the diamond detector and reference dosimeters. Conclusions: The observed dosimetric properties indicate that the tested diamond detector is a suitable candidate for clinical electron beam dosimetry. © 2013 American Association of Physicists in Medicine .[ http://dx.doi.org/10.1118/1.4774360]

The primary standard of absorbed dose to water established at ENEA for the Co-60 gamma-ray quality is based on a graphite calorimeter and an ionometric transfer system. This standard was recently improved after a more accurate assessment of some perturbation effects in the calorimeter and a modification of the water phantom shape and size. The conversion procedure requires two corresponding depths, one in graphite and one in water, where the radiation energy spectra must be the same. The energy spectra at the corresponding points were determined by a Monte Carlo simulation in water and graphite scaled phantoms. A thorough study of the calorimeter gap effect corrections was also made with regard to their dependence on depth and field size. A comparison between the ionization chamber calibration procedures based on the standards of absorbed dose to water and of air kerma was also made, confirming the consistency of the two methods.

Eight national standards for absorbed dose to water in 60Co gamma radiation at the dose levels used in radiation processing have been compared over the range from 1 kGy to 30 kGy using the alanine dosimeters of the NIST and the NPL as the transfer dosimeters. The comparison was organized by the Bureau International des Poids et Mesures, who also participated at the lowest dose level using their radiotherapy-level standard for the same quantity. The national standards are in general agreement within the standard uncertainties, which are in the range from 1 to 2 parts in 102. Evidence of a dose rate effect is presented and discussed briefly. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCRI Section I, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

A comparison of the standards for absorbed dose to water of the Istituto Nazionale di Metrologia delle Radiazioni Ionizzanti of the Ente per le Nuove Tecnologie, l'Energia e l'Ambiente, Italy (ENEA-INMRI), and of the Bureau International des Poids et Mesures (BIPM) has been made in 60Co gamma radiation under the auspices of the key comparison BIPM.RI(I)-K4. The comparison result, based on the calibration coefficients for three transfer standards and expressed as a ratio of the ENEA and the BIPM standards for absorbed dose to water, is 0.9999 (0.0044). The present 2007 result replaces the earlier ENEA value in this key comparison. The degrees of equivalence between the ENEA and the other participants in this comparison have been calculated and the results are given in the form of a matrix for the ten national metrology institutes (NMIs) that have published results in this ongoing comparison for absorbed dose to water. A graphical presentation is also given. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCRI Section I, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

International audience; In the current brachytherapy practice, the procedures to determine the absorbed dose imparted to the patient are affected by an uncertainty higher than in radiotherapy with external beams. That could reduce the success of brachytherapy treatments. Most of the uncertainty is due to a lacking metrology: no absorbed-dose primary standards are so far available to assure direct traceability in dosimetry of brachytherapy sources. This paper outlines the project “Increasing cancer treatment efficacy using 3D brachytherapy”, co-funded in the framework of the project iMERA-Plus according to the Grant Agreement No. 217257 between the European Commission and the European Association of National Metrology Institutes. The aim of the project is to develop methods for the direct measurement of the absorbed dose to water and to extend the use of this reference quantity to brachytherapy dosimetry with an uncertainty on the dose delivered to the target volume less than 5% (k=1) at clinical level. The present project will potentially increase the accuracy and safety of brachytherapy to a level comparable to that typical of radiotherapy with external accelerator beams.

The uncertainty associated with TL measurements has been related to the heating phase of the annealing and of the reading cycles, in particular, the need for stability of the ovens and the reproducibility of the heating rate during TL readout is pointed out. The cooling conditions are another element of importance in the annealing cycle of some TL materials. Therefore, sensitivity variations, in terms of cooling rate from high temperature to room temperature, have been investigated in BeO, LiF and CaF2:Dy loose detectors. Fast cooling in LiF but slow cooling in BeO markedly increases sensitivity, while CaF2:Dy is unaffected by cooling rate. The different behaviour of these TL materials could be explained by their different structures. The effect described is found not to be permanent, i.e. TL materials 'remember' only the last cooling cycle, since the crystalline structure of the detectors is not modified by thermal procedures carried out for annealing. To ensure reliability of the dosimetric data, extreme reproducibility of the entire annealing cycle is required.