Comparison of absorbed-dose-to-water units for Co-60 and high-energy x-rays between PTB and LNE–LNHB

Abstract

During the Euramet project JRP7 ‘External Beam Cancer Therapy’, PTB and LNE–LNHB used primary standards to determine the absorbed dose to water under IMRT conditions (in small fields). PTB used a water calorimeter to determine the absorbed-dose-to-water references in 6 MV and 10 MV beams for field sizes of 10 cm × 10 cm and 3 cm × 3 cm while LNE–LNHB used graphite calorimeters in 6 MV and 12 MV beams for field sizes of 10 cm × 10 cm, 4 cm × 4 cm and 2 cm × 2 cm. The purpose of this study is to compare PTB and LNE–LNHB new absorbed-dose-to-water references. LNE–LNHB sent an Exradin A1SL ionization chamber traceable to its primary standard to the PTB for calibration in 60Co and in linac beams and PTB sent a PTW 31010 ionization chamber traceable to its primary standard to LNE–LNHB for calibration in 60Co and in linac beams. Calculated Sw,air will be used as beam quality specifier for the ionization chamber comparison at different field sizes. The standard uncertainties (k = 1) of PTB and LNE–LNHB calibration coefficients lie respectively between 0.25% (60Co) and 0.40% (linac) and between 0.29% and 0.46%. PTB and LNE–LNHB absorbed-dose-to-water references developed for this project, based respectively on water calorimetry and on graphite calorimetry, agree within 1.5 standard deviations for field size of 10 cm × 10 cm down to 2 cm × 2 cm and for beams of 6 MV to 10 MV.

New standards of absorbed dose to water under reference conditions by graphite calorimetry for60Co and high-energy x-rays at LNE-LNHB

“…The method and the results have been presented at the 'Conference on Advanced Metrology for Cancer Therapy' in Braunschweig (29 November 2011) [34]. The ARPANSA has independently developed a very similar method at the same time [23].…”

European research projects for metrology in Brachytherapy and External Beam Cancer Therapy

“…The results show that water and graphite calorimetry can be applied successfully to determine the k Q factors of ionization chambers in small radiation fields. Compared with the k Q factors under reference field size no dependence of the k Q factors on the field size within the standard uncertainties was found for the investigated chambers [25,27]. This indicates that the k Q factors given in the dosimetry protocols can also be used for dosimetry in field sizes down to 2 cm × 2 cm.…”

“…Using the graphite calorimeter the k Q factors for a 10 cm × 10 cm field size down to a 2 cm × 2 cm size for 6 MV (with and without a flattening filter) and 12 MV photon beams were determined for the PTW31010 and Exradin A1SL chambers. The relative uncertainty achieved with Exradin A1SL is less than 0.4% [27]. The results show that water and graphite calorimetry can be applied successfully to determine the k Q factors of ionization chambers in small radiation fields.…”

Small section graphite calorimeter (GR-10) at LNE-LNHB for measurements in small beams for IMRT

“…The product i k i of the corrective factors is mainly for the impurities and the vacuum gaps if the absorbed dose to the graphite was given in a homogeneous graphite material. The new approach [5] is to determine the absorbed dose to water from the mean absorbed dose in the core in the real geometry of the calorimeter, so the correction for vacuum gaps (the main source of uncertainty, evaluated to 0.15%) is taken into account by the graphite-water conversion factor. The benefit to the measurements of the thermal feedback is illustrated in section 4.…”

“…The absorbed dose to water, the quantity of interest, is determined from the value of the absorbed dose to graphite (at a point in a homogeneous phantom [1] or in the core of the calorimeter [5]) with Monte Carlo simulations benchmarked with transfer dosimeters.…”