PURPOSE:Graphite calorimeters with a core diameter larger than the beam can be used to establish dosimetric references in small fields. The dose-area product (DAP) measured can theoretically be linked to an absorbed dose at a point by the determination of a profile correction. This study aims at comparing the DAP-based protocol to the usual absorbed dose at a point protocol in a 2 cm diameter field for which both references exist.METHODS:Two calorimeters were used, respectively, with a sensitive volume of 0.6 cm (for the absorbed dose at a point measurement) and 3 cm diameter (for the DAP measurement). Profile correction was calculated from a 2D dose mapping using three detectors: a PinPoint chamber, a synthetic diamond, and EBT3 films. A specific protocol to read EBT3 films was implemented and the dose-rate and energy dependences were studied to assure a precise measurement, especially in the penumbra and out-of-field regions.RESULTS:EBT3 films were found independent on dose rates over the range studied but showed a strong under-response (18%) at low energies. Depending on the dosimeter used for calculating the profile correction, a deviation of 0.8% (PinPoint chamber), 0.9% (diamond), or 1.9% (EBT3 films) was observed between the calibration coefficient derived from DAP measurements and the one directly established in terms of absorbed dose to water at a point.CONCLUSIONS:The DAP method can currently be linked to the classical dosimetric reference system based in an absorbed dose at a point only with a confidence interval of 95% (k = 2). None of the detectors studied can be used to determine an absorbed dose to water at a point from a DAP measurement with an uncertainty smaller than 1.2%.

L'objet de cette étude est de proposer une solution pour répondre à la demande émise par les fabricants de dosimètres et les exploitants du secteur nucléaire pour disposer de faisceaux de photons de haute énergie (6 MeV à 9 MeV) afin de procéder à la caractérisation des dosimètres (test de type) en vue de leur mise sur le marché et de les étalonner.Les installations de production de rayonnements photoniques de haute énergie sont des installations « lourdes » et très rares (accélérateur de protons, réacteurs nucléaires type piles piscines...).L'utilisation d'un accélérateur médical permet de mutualiser les installations entre la radioprotection et la radiothérapie et de diminuer les coûts d'exploitation.Dans un premier temps, nous avons défini (par simulations de type Monte-Carlo) puis réalisé un ensemble cible de conversion électrons/photons-atténuateur-égalisateur, qui permet l'obtention d'un faisceau homogène de photons de haute énergie (énergie moyenne pondérée par la fluence égale à 6,17 MeV) pour la radioprotection à partir d'un faisceau d'électrons de 18 MeV, fourni par l'accélérateur linéaire médical du LNE-LNHB.Le faisceau ainsi obtenu est homogène en termes de kerma dans l'air sur une surface de (30 × 30) cm 2 à 1 m.Dans un deuxième temps, nous avons fabriqué, assemblé et caractérisé deux chambres d'ionisation à cavité en graphite pour réaliser les mesures ionométriques.Pour l'une de ces chambres, nous avons mesuré le volume de collection des charges permettant ainsi de l'utiliser en tant qu'étalon primaires.L'autre chambre d'ionisation étant un étalon de transfert, elle a été étalonnée dans un faisceau de photons issu d'une source de 60 Co et dans le faisceau de photons de haute énergie pour la radioprotection.Les mesures effectuées avec les chambres d'ionisation ont permis d'évaluer la valeur du débit de kerma dans l'air dans le faisceau de photons de haute énergie pour la radioprotection : celleci couvre une gamme entre 80 mGy•h -1 et 210 mGy•h -1 , ce qui est compatible avec les besoins dans ce domaine.Enfin, nous avons calculé à l'aide de simulations de type Monte-Carlo des coefficients de conversion du kerma dans l'air vers les équivalents de dose pour des énergies de photons discrètes de 10 keV à 22,4 MeV dans des configurations géométriques spécifiques et pour la distribution spectrale de la fluence produite sur le LINAC du LNE-LNHB.

In this work, we present the results of the first part of a research project aimed at offering a complete response to dosimeters providers and nuclear physicists’ demands for high-energy (6 – 9 MeV) photon beams for radiation protection purposes. Classical facilities allowing the production of high-energy photonic radiation (proton accelerators, nuclear reactors) are very rare and need large investment for development and use. A novel solution is proposed, consisting in the use of a medical linear accelerator, allowing a significant decrease of all costs.Using Monte Carlo simulations (MCNP5 and PENELOPE codes), a specifically designed electron-photon conversion target allowing for obtaining a high energy photon beam (with an average energy weighted by fluence of about 6 MeV) has been built for radiation protection purposes. Due to the specific design of the target, this “realistic” radiation protection high-energy photon beam presents a uniform distribution of air kerma rate at a distance of 1 m, over a 30 × 30 cm2 surface. Two graphite cavity ionizing chambers for ionometric measurements have been built. For one of these chambers, the charge collection volume has been measured allowing for its use as a primary standard. The second ionizing chamber is used as a transfer standard; as such it has been calibrated in a 60Co beam, and in the high energy photon beam for radiation protection.The measurements with these ionizing chambers allowed for an evaluation of the air kerma rate in the LINAC based high-energy photon beam for radiation protection: the values cover a range between 36 mGy/h and 210 mGy/h, compatible with radiation protection purposes.Finally, using Monte Carlo simulations, conversion coefficients from air kerma to dose equivalent quantities have been calculated in the range between 10 keV and 22.4 MeV, for the spectral distribution of the fluence corresponding to the beam produced by the linear accelerator of the LNE-LNHB.

Introduction: The development of new treatment modalities such as IMRT and stereotactic radiotherapy has led to an increasing use of small beams with multiple incidence angles. Therefore the reference conditions of international protocols (e.g. IAEA TRS 398 and AAPM TG-51) are very far from treatment conditions. In previous EMRP project (External Beam Cancer Therapy), LNE-LNHB developed references in terms of absorbed dose to water for MV X-ray beams of field sizes down to 2 cm. In HLT09-MetrExtRT project, since the miniaturization of absolute dosimeters is not possible for smaller field sizes, LNE-LNHB has investigated the use of dose area product (DAP) as a substitute to the absorbed dose at a point.

To extend the dosimetric reference system to field sizes smaller than 2 cm × 2 cm, the LNE-LNHB laboratory is studying an approach based on a new dosimetric quantity named the dose-area product instead of the commonly used absorbed dose at a point. A graphite calorimeter and a plane parallel ion chamber with a sensitive surface of 3 cm diameter were designed and built for measurements in fields of 2, 1 and 0.75 cm diameter. The detector surface being larger than the beam section, most of the issues linked with absolute dose measurements at a point could be avoided. Calibration factors of the plane parallel ionization chamber were established in terms of dose-area product in water for small fields with an uncertainty smaller than 0.9%.

The EURAMET #1177 project, identified as EURAMET RI(I) - S9 comparison, was the first EURAMET wide scale supplementary comparison in the field of diagnostic radiology for air kerma area product, PKA, and air kerma, K. It was conducted with the goal of testing the measurement and calibration capabilities for PKA and K, as well as of supporting the relevant CMCs of the participating laboratories. Two commercial KAP meters and an ionization chamber were selected as transfer instruments and circulated between the 22 European participants. The measurements were performed from April 2011 until July 2012. The stability and the performance of the transfer instruments were tested by the pilot laboratory (IRCL/GAEC-EIM) and few other laboratories as well. The test results revealed that the energy (radiation quality), Q, irradiation area, A, and air kerma rate, K̇ dependences of response of the transfer KAP meters influence the comparison of the results when different measurement conditions were pertained and therefore, appropriate correction factors were obtained and applied to the reported calibration results of the laboratories, when necessary. The comparison reference values (CRVs) for each instrument were determined as the weighted mean of the calibration coefficients of the three participating primary laboratories. The relative standard uncertainty of the CRVs were in the range of (0.4 - 1.6)% depending on the transfer instruments and beam qualities. The comparison result as the ratio of the corrected calibration coefficient of participant and the respective CRV, and its uncertainty were calculated for all beam qualities and transfer instruments. The informative degrees of equivalence (DoE) were calculated for the refrence RQR 5 beam quality. In case of air kema area product measurements the results for the RADCAL PDC KAP meter were used. The 216 KAP meter calibration results of the two different transfer instruments in terms of air kerma area product were consistent within 5% except 40 results of 8 participants. The 103 air kerma calibration results were consistent within 1.7%, except 10 results of 4 participants. 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, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

The LNE-LNHB has developed two primary standards to determine the absorbed dose to water under reference conditions (for 10 cm × 10 cm) in 60 Co, 6 MV, 12 MV and 20 MV photon beams: a new graphite calorimeter and a water calorimeter.This first paper presents the results obtained with the graphite calorimeter and the new associated methodology.The associated relative standard uncertainty (k = 1) of absorbed dose to water is 0.25% for 60 Co and lies between 0.32% to 0.35% for MV x-ray beams.

Purpose: The use of a primary dosimeter larger than the radiation field gives access to the integral of dose over a specified surface normal to the beam. If a relative dose profile of the beam is well known, it is then possible to calculate the distribution of the absorbed dose at any point on the considered surface. This study aims at validating the use of EBT3 gafchromic films for the measurement of 2D dose distribution and integrals of dose in small fields for such use. Methods: New EBT3 films have been fully characterized: the response versus energy, dose-rate and dose has been investigated. Profiles measured in circular field with a diameter of 20 mm have been compared to the ones measured with a diamond detector developed at CEA/LIST/LCD. The ratio of dose area products measured with EBT3 on a 6 mm and 30 mm diameter surface has been compared to the ratio measured with primary dosimeters (calorimeters) and calculated with Monte Carlo simulations. Results: There was no significant difference between the dose-calibration curves in a 6 MV and a 60Co beam. Deviation was within uncertainty bars when the dose rate inside a pulse was divided by a factor of 80 in the 6 MV photon beam. Profiles in small fields are in good agreement with the diamond profiles. Dose area product ratios obtained with EBT3, calorimeters and simulations are within 1%. Conclusion: EBT3 films are good candidates for the measurement of relative dose distribution in small fields as long as the average of several films is considered. They can be used in association with primary measurements to determinate dosimetric references in small fields and to transfer them to the end user.

Introduction: The calibration in small fields of a dosimeter which sensitive volume is larger than the field is possible thanks to the use of a new quantity of interest in radiotherapy: the dose-area product. If a relative dose profile of the beam is well known, it is then possible to calculate the absorbed dose at a point. Because of their good spatial resolution and nearly water equivalence, EBT3 films were chosen to get a 2D dose distribution in several small fields.

Purpose:To establish dosimetric references of absorbed dose in water in radiation fields smaller than 2 cm used in radiotherapy thanks to a new methodology based on the use of dosimeters larger than the field size.Methods:A new graphite calorimeter was constructed with a large sensitive volume (diameter of the core: 30 mm). This primary dosimeter was fully characterized and compared to previous LNE‐LNHB graphite calorimeters in a 60Co large field. A specially designed graphite parallel‐plate ionization chamber with a 30 mm collecting electrode was also assembled and tested. Measurements were then conducted in two 6 MV small circular fields of 2 cm and 1 cm diameter respectively, using the new concept of dose‐area product instead of punctual dose commonly used in radiotherapy.Results:The dose rate established in a large 60Co field with the new calorimeter is in agreement within 0.4% with previous calorimeters. The ionization chamber shows good characteristics except for a 0.06% drift per hour in water. The ratio of calorimetric against ionometric measurements in the 2 cm diameter field is 1.1% higher than the one in the 1 cm diameter field (with respectively 0.30% and 1.03% type A uncertainty for each field).Conclusion:Results presented here highlight the possibility of measuring dose‐area products in small fields with a graphite calorimeter and a parallel‐plate ionization chamber. Measurements in a 0.75 cm diameter field are already underway to confirm the trend observed in the 2 cm and 1 cm diameter fields. The last step to establish precise dosimetric references in small fields is to calculate correction factors thanks to Monte Carlo simulations.

Les references actuelles, pour les rayons X de moyenne energie en radiotherapie, sont etablies au LNHB en termes de kerma dans l’air. La dose absorbee dans l’eau, grandeur d’interet pour la radiotherapie, est deduite de ces valeurs par transfert conformement aux protocoles internationaux. Ce travail de these a permit d’etablir les references en termes de dose absorbee dans l’eau dans les conditions de reference des protocoles en utilisant la methode de calorimetrie dans l’eau. La calorimetrie est la mesure de la dose absorbee a partir de l’elevation de temperature. Un « calorimetre-eau » a ete concu et realise afin d’effectuer des mesures a 2 cm de profondeur : conditions de reference definies par le protocole AIEA TRS-398. Les debits de dose absorbee dans l’eau ainsi determines ont ete compares aux valeurs issues de l’application des protocoles fondes sur le kerma dans l’air. Un ecart maximum inferieur a 2.1 % a ete trouve par rapport a la calorimetrie. L’incertitude type associee aux valeurs calorimetriques etant inferieure a 0.8 % et celle associee aux valeurs issues des protocoles etant de l’ordre de 3.0 %, les resultats sont compatibles aux incertitudes des methodes pres. Grâce a ces nouvelles references, la determination de la dose absorbee dans l’eau dans ce type de faisceau pourra desormais etre realisee en appliquant le protocole AIEA TRS-398, conduisant ainsi a une forte reduction des incertitudes (facteur 3 par rapport au protocole AIEA TRS-277). Actuellement, aucun autre laboratoire primaire ne possede un tel instrument permettant l’etablissement direct de ces references dans les conditions recommandees par les protocoles.

Nowadays, the absorbed dose to water for kilovoltage x-ray beams is determined from standards in terms of air-kerma by application of international dosimetry protocols. New standards in terms of absorbed dose to water has just been established for these beams at the LNE-LNHB, using water calorimetry, at a depth of 2 cm in water in accordance with protocols. The aim of this study is to compare these new standards in terms of absorbed dose to water, to the dose values calculated from the application of four international protocols based on air-kerma standards (IAEA TRS-277, AAPM TG-61, IPEMB and NCS-10). The acceleration potentials of the six beams studied are between 80 and 300 kV with half-value layers between 3.01 mm of aluminum and 3.40 mm of copper. A difference between the two methods smaller than 2.1% was reported. The standard uncertainty of water calorimetry being below 0.8%, and the one associated with the values from protocols being around 2.5%, the results are in good agreement. The calibration coefficients of some ionization chambers in terms of absorbed dose to water, established by application of calorimetry and air-kerma based dosimetry protocols, were also compared. The best agreement with the calibration coefficients established by water calorimetry was found for those established with the AAPM TG-61 protocol.

Water calorimeters are used to establish absorbed dose standards in several national metrology laboratories involved in ionizing radiation dosimetry. These calorimeters have been first used in high-energy photons of 60Co or accelerator beams, where the depth of measurement in water is large (5 or 10 cm). The LNE-LNHB laboratory has developed a specific calorimeter which makes measurements at low depth in water (down to 0.5 cm) easier, in order to fulfil the reference conditions required by the international dosimetry protocols for medium-energy x-rays. This new calorimeter was first used to measure the absorbed dose rate in water at a depth of 2 cm for six medium-energy x-ray reference beams with a tube potential from 80 to 300 kV. The relative combined standard uncertainty obtained on the absorbed dose rate to water is lower than 0.8%. An overview of the design of the calorimeter is given, followed by a detailed description of the calculation of the correction factors and the calorimetric measurements.

Within the Euramet JRP7 project External Beam Cancer Therapy, a work package was dedicated to the primary standards for IMRT ( intensity modulated radiation therapy). The French national metrology laboratory for ionizing radiations, LNE-LNHB, was involved in determining absorbed dose to water based on graphite calorimeters in 6MV and 12MV beams for field sizes of 10 cm x 10 cm, 4 cm x 4 cm and 2 cm x 2 cm.The existing GR-09 graphite calorimeter has been successfully used for the beam sizes of 10 cm x 10 cm and 4 cm x 4 cm whereas it was not small enough to perform measurements in the 2 cm x 2 cm beam size. Therefore, during the project a small section graphite calorimeter, GR-10, has been developed.This work deals with the design, construction and tests of this new graphite calorimeter.

Introduction: Currently, the references for medium energy radiotherapy X-rays beams are established in terms of air kerma. Absorbed dose to water, which is the quantity of interest, is achieved via transfer dosimetric techniques in accordance with international protocols. The use of water calorimetry enables benefiting, through primary measurements of absorbed dose to water, of a direct and more reliable reference with a reduced uncertainty.