Measuring absorbed dose to water in medium energy x-ray beams–traceability to the primary standards available in Europe and future dosimetry protocols

Physica Medica Volume 31, Supplement 2, November 2015, Page e49, doi:10.1016/j.ejmp.2015.10.072
M. Pinto, M. Pimpinella, A.S. Guerra, B. Rapp, M. Denozière, L. Büermann, J. De Pooter, L. De Prez, G. Machula, J.-M. Bordy

Introduction: While radiotherapy dosimetry is traceable to absorbed dose to water (Dw) primary standards in high-energy photon, electron, proton, and ion beams, this is not the case for other radiotherapy modalities, including low- and medium-energy x-rays. Activities within workpackage 1 (WP1) of the EMRP HLT09 project “MetrExtRT” have been outlined to fill this gap for medium-energies with reference conditions as in the IAEA 398 protocol.

Results: Two new primary standards have been built and three existing primary standards available at National Metrology Institutes (NMI) have been advanced for the direct determination of DW in filtered medium-energy x-ray beams. These national standards are based on an extrapolation ionization chamber (MKEH, Hungary), a graphite calorimeter in a water phantom (ENEA-INMRI, Italy), and three water calorimeters (PTB, Germany; VSL, the Netherlands; LNE-LNHB, France), therefore advancing and enforcing the European metrological scenario for dosimetry directly traceable to DW standards.

A questionnaire was compiled at hospitals in the EU to assess their use of medium-energy x-ray qualities. Taking into account the results of this survey, the institutes have participated in the first EURAMET DW indirect-type comparison on medium energy x-rays (EURAMET.RI(I)-S13) in the CCRI radiation qualities in the range 100 kV–250 kV (Allisy-Roberts et al, Rapport BIPM, 2011). Three PTW TM30013 Farmer-type, water-proof ionization chambers were circulated among the participating NMIs between June 2014 and January 2015. Using 10 cm × 10 cm (or 10 cm circular) beams, the chambers were calibrated at a depth of 2 g cm−2 in terms of absorbed dose to water against the new DW primary standards, and in terms of air kerma against the national primary standards. Additionally, the three calibrated Farmer chambers were used for measurements of DW at the depth of 5 g cm−2, for the estimation of the radiation quality beam specifier DW, 5 cm/DW, 2 cm. Its value was compared to the Half Value Layer, for the same beams, in relation to x-ray spectral parameters like mean energy or mass-energy absorption coefficients. This analysis (in progress) can indicate the potentially most suitable beam quality specifier.

Conclusions: Five Dw primary standards for medium energy x-rays are now available in Europe, making dosimetry directly traceable to Dw standards feasible.

Acknowledgement: This work is supported by the EMRP joint research project MetrExtRT.

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