D T Burns

Consistency in reference radiotherapy dosimetry: resolution of an apparent conundrum when (60)Co is the reference quality for charged-particle and photon beams.

Substantial changes in ion chamber perturbation correction factors in (60)Co γ-rays, suggested by recent Monte Carlo (MC) calculations, would cause a decrease of about 1.5% in the reference dosimetry of all types of charged particles (electrons, protons and heavier ions) based on calculated kQ values. It has gone largely unnoticed that the ratio of calibration coefficients ND, w, Co60 and NK, air, Co60 yields an experimental value of Fch, Co60 = (sw-air pch)Co60 through ND, air, Co60. Coefficients provided by the IAEA and traceable to the BIPM for 91 NE-2571 chambers result in an average Fch, Co60 which is compared with published (and new) MC simulations and with the value in IAEA TRS-398. It is shown that TRS-398 agrees within 0.12% with the experimental Fch, Co60. The 1.5% difference resulting from MC calculations (1.1% for the new simulations) cannot be justified using current fundamental data and BIPM standards if consistency in the entire dosimetry chain is sought. For photons, MC kQ factors are compared with TRS-398. Using the same uncertainty for Wair, the two sets of data overlap considerably. Experimental kQ values from standards laboratories lie between the two sets of calculated values, showing no preference for one set over the other. Observed chamber-to-chamber differences, that include the effect of waterproof sleeves (also seen for (60)Co), justify the recommendation in TRS-398 for kQ values specifically measured for the user chamber. Current developments on I-values for the stopping powers of water and graphite are presented. A weighted average Iwater = 78 ± 2 eV is obtained from published experimental and DRF-based values; this would decrease sw-air for all types of radiotherapy beams between 0.3% and 0.6%, and would consequently decrease the MC derived Fch, Co60. The implications of a recent proposal for Igraphite = 81 eV are analysed, resulting in a potential decrease of 0.7% in NK, air, Co60 which would raise the experimental Fch, Co60; this would result in an increase of about 0.8% in the current TRS-398 value when referred to the BIPM standards. MC derived Fch, Co60 using new stopping powers would then agree at a level of 0.1% with the experimental value, confirming the need for consistency in the dosimetry chain data. Should world average standards be used as reference, the figures would become +0.4% for TRS-398 and -0.3% for the MC calculation. Fch, Q calculated for megavoltage photons using new stopping powers would decrease by between 0.2% and 0.5%. When they enter as a ratios in kQ, differences with MC values based on current key data would be within 0.2% but their discrepancy with kQ experimental photon values remains unresolved. For protons the new data would require an increase in Wair, Q of about 0.6%, as this is inferred from a combination of calorimetry and ionometry. This consistent scenario would leave unchanged the current TRS-398 kQ (NE-2571) data for protons, as well as for ions heavier than protons unless new independent Wair, Q values become available. Also in these advanced radiotherapy modalities, the need for maintaining data consistency in an analysis that unavoidably must include the complete dosimetry chain is demonstrated.

Determination of the specific heat capacity of a graphite sample using absolute and differential methods

An experimental assembly has been constructed to measure the specific heat capacity of macroscopic graphite samples at room temperature. The same batch of graphite constitutes the core of a graphite calorimeter, which is currently being realized to measure the absorbed dose due to ionizing radiation. Two different experimental procedures have been applied. In the first method the specific heat capacity of graphite was measured directly, where its value is corrected for the influence of impurities. The second method, to our knowledge not previously applied to macroscopic samples, is based on a series of differential measurements where no correction for added impurities is needed. By its nature, the second method reduces systematic effects. The specific heat capacity of a particular graphite sample is determined to be 706.9 J K−1 kg−1 with a combined relative standard uncertainty of 9 parts in 104 at 295.15 K. The specific heat capacity of cyanoacrylate has also been determined.

International framework of traceability for radiation dosimetry quantities

An overview of the international framework that supports the dissemination of the dosimetry quantities for ionizing radiation is presented. The aim of international traceability is for confidence in such measurements around the world, which is particularly important for the equivalence of patient treatment regimes as required in international clinical trials for radiotherapy but also for fields as diverse as industrial processing, diagnostic medicine and radiation protection. This paper gives some explanation of the comparison system for national primary standards in the field of dosimetry and shows how these can support the claims for the calibration and measurement capabilities of national metrology institutes represented in the BIPM key comparison database.

Re-evaluation of the BIPM international standard for air kerma in 60Co gamma radiation

A re-evaluation of the BIPM standard for air kerma in 60Co radiation has been made. The changes to the air-kerma rate determination arise from four sources: (i) the results of Monte Carlo calculations of correction factors for the standard; (ii) a re-evaluation of the correction factor for saturation; (iii) a new evaluation of the air volume of the standard using an experimental chamber of variable volume; (iv) the adoption of a new reference beam at the BIPM. The combined effect of these changes is an increase in the BIPM determination of air-kerma rate by the factor 1.0054 and a reduction of the relative standard uncertainty of this determination to 1.5 parts in 103. A full uncertainty budget is presented. The new standard is effective from 1 November 2007.

Re-evaluation of the BIPM international standards for air kerma in x-rays

A re-evaluation of the BIPM standards for air kerma in low- and medium-energy x-rays has been made. The changes to the air-kerma rate determination arise from three sources: (i) the results of Monte Carlo calculations of diaphragm correction factors for the standards, (ii) a measurement of the effect of scatter from the diaphragm support and (iii) a re-evaluation of the electron-loss corrections for the medium-energy standard. Scatter from the diaphragm support explains a long-standing trend observed in the results of international comparisons for medium-energy x-rays. The new standards were implemented on 1 September 2009.

Summary of the BIPM.RI(I)-K1 comparison for air kerma in 60Co gamma radiation

International comparisons of air kerma in 137Cs gamma radiation beams have been made at the Bureau International des Poids et Mesures (BIPM) since 1994. Twelve national metrology institutes have taken part, seven of which have repeated the comparison over the intervening years. The key comparison reference value (KCRV) is taken as the BIPM evaluation, each comparison result being the ratio of the national metrology institute (NMI) evaluation to that of the BIPM standard under the same reference conditions. The degrees of equivalence between each NMI and the KCRV and a graphical presentation are given using the most recent published result for eleven NMIs. 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).

A new method to determine ratios of electron stopping powers to an improved accuracy

A new method is presented to determine the ratio of electron stopping powers which is effective in the transfer of absorbed dose from one medium to another. The method involves an accurate measurement of the electron range in each of the media combined with a full Monte Carlo simulation of each experimental geometry. For the specific case of graphite and water, the uncertainty attainable is estimated to be around +/- 0.5% at the 95% confidence level, which is approximately a factor of three better than the best methods currently in use.

Comparison of the standards for absorbed dose to water of the NRC and the BIPM for accelerator photon beams

A comparison of the dosimetry for high-energy accelerator photon beams was carried out between the National Research Council of Canada (NRC) and the Bureau International des Poids et Mesures (BIPM) in June 2009. The comparison was based on the determination of absorbed dose to water for three radiation qualities. The comparison result, reported as a ratio of the NRC and the BIPM evaluations, is 0.997 at 6 MV, 1.001 at 10 MV and 0.994 at 25 MV, each with a relative standard uncertainty of 6???10?3. This result is the first of the ongoing BIPM.RI(I)-K6 comparison. 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).

Air-kerma determination using a variable-volume cavity ionization chamber standard

A graphite-walled cavity ionization chamber of modular design and variable volume has been used to determine the air-kerma rate in the reference 60Co field at the BIPM. The chamber can be configured in five sizes. High-accuracy mechanical measurements of the volume of the air cavity were made for each configuration using a co-ordinate measuring machine. Ionization current measurements were made for each configuration and corrected for the effects of ion recombination and diffusion, stem scatter and chamber orientation. Monte Carlo calculations of cavity dose were made to evaluate the correction factors kwall and kan. A reproducibility of the ionization current per mass of 1.5 parts in 10(4) was achieved on the repeated assembly of each configuration. The results show an air-kerma rate determination that increases with volume, the total change being around 8 parts in 10(4). When analysed differentially, the air-kerma rate relative to the BIPM standard is Kdiff/KBIPM = 1.0026(6). A detailed uncertainty budget is presented. Possible reasons for the observed behaviour are discussed that might have consequences for all existing standards for air-kerma.

Key comparison BIPM.RI(I)-K2 of the air-kerma standards of the ENEA-INMRI, Italy and the BIPM in low-energy x-rays

A key comparison has been made between the air-kerma standards of the ENEA-INMRI, Italy and the BIPM in the low-energy x-ray range. The results show the standards to be in agreement at the level of the standard uncertainty for the comparison of 1.9 parts in 10 3 . No significant trend with radiation quality is observed. The results are analysed and presented in terms of degrees of equivalence, suitable for entry in the BIPM key comparison database.