L.A. De Prez

The new NMi orthovolt x-rays absorbed dose to water primary standard based on water calorimetry

A new water calorimeter for orthovolt x-rays has been developed at NMi. The purpose of this calorimeter is to provide calibrations in terms of absorbed dose to water. Four internationally accepted CCRI qualities (100-250 kV) have been characterized. Correction factors have been determined with measurements, Monte Carlo calculations and heat transport models. The results of the calculations have been validated against measurements. The absorbed dose to water calibration coefficient, N(Dw,WCM), with the new calorimeter has been compared to the N(Dw,NCS) based on an air-kerma calibration coefficient, using the current NCS-10 dosimetry protocol. A good agreement is found for all beam qualities where the total uncertainty (1 SD) has decreased from 2.5% for N(Dw,NCS) to better than 1.5% for N(Dw,WCM), using the new water calorimeter.A new water calorimeter for orthovolt x-rays has been developed at NMi. The purpose of this calorimeter is to provide calibrations in terms of absorbed dose to water. Four internationally accepted CCRI qualities (100–250 kV) have been characterized. Correction factors have been determined with measurements, Monte Carlo calculations and heat transport models. The results of the calculations have been validated against measurements. The absorbed dose to water calibration coefficient, NDw,WCM, with the new calorimeter has been compared to the NDw,NCS based on an air-kerma calibration coefficient, using the current NCS-10 dosimetry protocol. A good agreement is found for all beam qualities where the total uncertainty (1 SD) has decreased from 2.5% for NDw,NCS to better than 1.5% for NDw,WCM, using the new water calorimeter.

Comparison of the standards for absorbed dose to water of the VSL and the BIPM for 60Co γ-rays

An indirect comparison of the standards for the quantity absorbed dose to water was carried out between the Van Swinden Laboratorium (VSL) and the Bureau International des Poids et Mesures (BIPM) in September 2005. The comparison was based on the calibration coefficients determined for three NE2611A type ionization chambers under reference conditions in the 60Co beams of both institutes. The comparison result, reported as a ratio of the VSL and the BIPM evaluations, is 0.9926 with a relative standard uncertainty of 4.9 ? 10?3. This result replaces the previous indirect comparison result obtained in 2000 for the ongoing BIPM.RI(I)-K4 comparison. The degrees of equivalence for the VSL standard and the nine other national metrology institutes in this comparison are presented in the form of a matrix, together with those of a linked regional comparison. The opportunity was also taken to undertake a pilot study using the VSL water calorimeter at the BIPM. 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).

Application of an adapted Fano cavity test for Monte Carlo simulations in the presence of B-fields.

With the advent of MR guided radiotherapy the relevance of Monte Carlo radiation transport simulations in the presence of strong magnetic fields (B-fields) is increasing. While new tests are available to benchmark these simulation algorithms for internal consistency, their application to known codes such as EGSnrc, PENELOPE, and GEANT4 is yet to be provided. In this paper a method is provided to apply the Fano cavity test as a benchmark for a generic implementation of B-field effects in PENELOPE. In addition, it is investigated whether violation of the conditions for the Fano test can partially explain the change in the response of ionization chambers in the presence of strong B-fields. In the present paper it is shown that the condition of isotropy of the secondary particle field (Charged Particle Isotropy, CPI) is an essential requirement to apply the Fano test in the presence of B-fields. Simulations in PENELOPE are performed with (B  =  0.0 T) and (B  =  1.5 T) for cylindrical cavity geometry. The secondary particle field consists of electrons generated from a mono-energetic source (E  =  0.5-4.0 MeV) with a uniform source density and different angular distributions; isotropic, mono-directional, and Compton. In realistic photon fields the secondary radiation field has a non-isotropic angular distribution due to the Compton process. Based on the simulations for the Compton angular distribution (non-CPI), the response change of the cavity model in a uniform radiation field in the presence of B-fields is investigated. For the angular distributions that violate the CPI condition and B  =  1.5 T, the deviations from 1 are considerable, which emphasizes the requirement of CPI. For the isotropic angular distributions obeying this requirement, both the results for B  =  0.0 T and B  =  1.5 T shows deviations from the predictions for E  ⩾  1.5 MeV with values up to 1.0% for E  =  4.0 MeV. Nevertheless, due to the high correlation in the deviation for B  =  0.0 T and B  =  1.5 T, the accuracy of the PENELOPE code for the simulation of the change in detector response in the presence of B-fields is within 0.3%. The effect of the B-field on the detector response for non-isotropic angular distributions suggests that violation of CPI is a major contribution to the response change of ionization chambers in the presence of B-fields.

TH-CD-BRA-03: Direct Measurement of Magnetic Field Correction Factors, KQB, for Application in Future Codes of Practice for Reference Dosimetry

PURPOSE With an MR-linac, radiation is delivered in the presence of a magnetic field. Modifications in the codes of practice (CoPs) for reference dosimetry are required to incorporate the effect of the magnetic field. METHODS In most CoPs the absorbed dose is determined using the well-known kQ formalism as the product of the calibration coefficient, the corrected electrometer reading and kQ, to account for the difference in beam quality. To keep a similar formalism a single correction factor is introduced which replaces kQ, and which corrects for beam quality and B-field, kQ,B. In this study we propose a method to determine kQ,B under reference conditions in the MRLinac without using a primary standard, as the product of:- the ratio between detector readings without and with B-field (kB),- the ratio between doses in the point of measurement with and without B-field (rho),- kQ in the absence of the B-field in the MRLinac beam (kQmrl0,Q0),The ratio of the readings, which covers the change in detector reading due to the different electron trajectories in the detector, was measured with a waterproof ionization chamber (IBA-FC65g) in a water phantom in the MRLinac without and with B-field. The change in dose-to-water in the point of measurement due to the B-field was determined with a Monte Carlo based TPS. RESULTS For the presented approach, the measured ratio of readings is 0.956, the calculated ratio of doses in the point of measurement is 0.995. Based on TPR20,10 measurements kQ was calculated as 0.989 using NCS-18. This yields a value of 0.9408 for kQ,B. CONCLUSION The presented approach to determine kQ,B agrees with a method based on primary standards within 0.4% with an uncertainty of 1% (1 std.uncert). It differs from a similar approach using a PMMA-phantom and an NE2571 chamber with 1.3%.

WE-G-17A-06: A Water Calorimeter for Use in MRI Linacs

PURPOSE At VSL, Dutch Metrology Institute, a new water calorimeter was developed with the purpose to replace the existing primary standard for absorbed dose to water in the Netherlands. The new water calorimeter is designed to be operable in medium- to high energy photon beams, electrons, protons as well as MRI integrated linear accelerators. VSL has operated a water calorimeter since 2001. This calorimeter formed the basis for the NCS-18 dosimetry protocol, which is commonly applied by medical physicists in the Netherlands and Belgium. METHODS The unit Gray is the unit of interest for measurement of the absorbed dose to water. Water calorimetry involves the measurement of a small temperature rise (0.24 mK/Gy) with an uncertainty of less than 1 μK/Gy at a temperature of 4 °C. Using extensive multi-physics simulations the new calorimeters thermal performance was simulated before it was constructed at the end of 2013. With the advent of radiotherapy treatment units incorporating MR imaging the performance of the thermistor temperature sensors were characterized in a 1.5 T magnetic field. RESULTS A change of thermistor resistance was observed of less than 0.004% as a Result of the magneto-resistance effect in a 1.5 T magnetic field. Although a magneto-resistance effect was detectable, the effect on the temperature response in the water calorimeter was found to be negligible. CONCLUSION With the realization of the new calorimeter operable in MRI linacs and designed for use in a variety of beam modalities, VSL is ready for accurate dosimetry in new advanced radiotherapy modalities. Due to the small form factor the calorimeter can be used on location in the actual therapy beam inside a 68 cm linac bore. This work was supported by EMRP grant HLT06. The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union.

Key comparison BIPM.RI(I)-K3 of the air-kerma standards of the VSL, Netherlands and the BIPM in medium-energy x-rays

A key comparison has been made between the air-kerma standards of the VSL, Netherlands and the BIPM in the medium- energy x-ray range. The results show the standards to be in agreement at the level of the combined standard uncertainty when account is taken of the effect of the diaphragm support for the BIPM standard. The results are analysed and presented in terms of degrees of equivalence, suitable for entry in the BIPM key comparison database.